gravity and time

Has anyone considered that gravity and time are inversely related?  Since it seems that time seems to come to a virtual stop beyond the event horizon of a black hole, maybe that's because whatever 'causes' time is pushed out of the way by gravity.  The same might also explain the relativistic effects regarding the passage of times on objects approaching the speed of light.  I mean, physics class always told us that the ground pressing back up on us was the opposite of gravity.  But what if that ground just happened to be in the way, because that is the space it occupies, and it's the particle/wave/whatever that creates time that gravity actually displaces?

Regards,
Tony 

Big Bang and the conservation of Mass

Harilal asked:
“During the big bang it is supposed that the small particles expanded in a very very small interval of time and there was nothing before that. If it is so, then how can it in be good agreement with law of conservation of mass?”
Ans:
After the Big Bang, the universe expanded rapidly. The big bang theory is not a theory which is completely resolved. There are many questions to be answered.
Further, mass alone is not conserved, it is mass+energy  which is conserved.
Energy can be converted into matter and vice versa. E=mc2
Dark Energy and Dark Matter are two widely discussed concepts in association with the Big Bang. These correspond to the mass (matter) and energy which we cannot account for by calculations and theory.
Further, BIGBANG is not the only possible cause of the Birth of Universe. There are many other possibilities.
Big Bang theory originated after the discovery that the universe is expanding. SO if we think in reverse direction, we can imagine that the entire universe was confined to an infinitesimally small (Compared to the present size of the universe) space. This idea started the theoritization of the Big Bang.
Please visit the following links for further reference:
http://www.aip.org/history/cosmology/ideas/bigbang.htm
http://www.aip.org/history/cosmology/ideas/journey.htm
http://en.wikipedia.org/wiki/Big_Bang
http://www.aip.org/history/cosmology/ideas/expanding.htm
 
(Comments and better answers are expected from visitors and experts visiting the site)














Gauss Theorem - Electrostatics

DIVYA asked:
“WHAT IS THE  MAGNITUDE OF ELECTRIC  FIELD IN THE  GAUSSIAN SURFACE OF  A CUBE,AT ITS FACE ,AT ITS CENTRE, AT ITS CORNERS OR AT ANY OTHER POINT INSIDE THE CUBE. ALSO TELL ME THE THE WAY TO KNOW IT.”
Ans:
According to Gauss Theorem, the total electric flux through a closed surface,\oint_S \mathbf{E} \cdot \mathrm{d}\mathbf{A} = \frac{Q}{\varepsilon_0},
The Gaussian surface is an imaginary surface. So, for calculating the electric field at a point using Gauss theorem, we have to imagine a Gaussian surface symmetric with the given charge distribution.
I have assumed that there is a point charge Q at the centre of the cube.
At its face (at a point on the face on the line radially outwards from the point charge at the centre of the cube and perpendicular to the face)
The distance is equal to half the length of side of the cube (Say L).
Therefore the electric field,  

 
On the corner,
Calculate the distance from the centre of the cube to its corner and replace (L/2) in the above equation with that distance.










Time Period of a Simple Pendulum in Space

Mary Beth asked:
“if you set up a pendulum aboard an orbiting space vehicle, would the period be less than, the same as, or greater than it would be in our lab? explain.”
Ans:
In a space vehicle orbiting around the earth, the apparent weight is zero as the entire gravitational pull is being utilized in providing the centripetal force required for revolving. This is called weightlessness in space.
i.e; in effect, the acceleration due to gravity is zero which make the time period of the satellite infinity. The pendulum will not oscillate at all.
__________________________________________________________
Tailpiece:
However, in manned space vehicles, there is an option to simulate gravity by making the system rotate at a particular rate so as to make the inhabitants feel as if there is weight and gravity.

Pendulum on Mount Everest !

Michelle Guzman Asked:
If you set a pendulum atop Mt. Everest, would the period be less than, greater than or equal to the period you would get in the lab?  Explain.
Ans:
As you know, the acceleration due to gravity decreases as we go upwards from surface of earth.
The time period of a simple pendulum is given by
Where l is the length of the pendulum, and g is the acceleration due to gravity.
From the equation, we can conclude that, the Time period of the pendulum increases (i.e. it goes slower)  if the pendulum is set up on top of Mount Everest.
For Creative and Critical Thinkers
Here we have not considered the change in temperature between the laboratory conditions and the temperature on top of Mount Everest which is very low. A decrease in temperature would decrease the length of the pendulum (Unless it is made up of such a substance which does not change its length with change in  temperature) and will also decrease the time period of the pendulum, making it move faster.
My first answer is correct only if the laboratory conditions are the same as on top of Mt. Everest or vice versa, except for the difference in altitude (height)
Ask Physics


A question from Kinetic energy

Atul asked:
A lorry and a car moving with the same KE are brought to rest having same retarding force .which one of them should come at rest at shorter distance.

Ans:
According to Work Kinetic Energy theorem
Both should stop travelling the same distance
(This was a quick answer without a deep thought. If the readers have a diffrent opinion, please comment)

A question from Gravitation.

Prince Patel Asked:
“Sun have high gravitational energy ,then why planets of solar system doesn’t colloid with sun?”
Answer:
“The planets are moving around the sun and the gravitational force is used in providing the necessary centripetal force (The force required by a body to move in a circular path). As long as the planets are in motion, they will not move towards the sun. A similar explanation hold good for moon going around earth also.”

A question from photoelectric effect

Jyothir Asked:
In photoelectric effect, what happens to the photon after it collides elastically with the electron ?

Ans:
The photon is completely absorbed by one electron, the energy of the electron provides the work function which is equal to the binding energy of the electron and the balance energy appears as the electrons Kinetic energy.
As Einstein's Photo electric equation suggests
KE = h (v-vo)


HYSTERISIS

Namrata Asked:
What is hysteresis? I only know its related to the frequency.

Ans:
Hysterisis mens “Lagging Behind”

Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect
The lack of retraceability of the magnetization curve is the property called hysteresis

Magnetic hysteresis


When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.
The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.
If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence (RETENTIVITY).
If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The 'thickness' of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material.



Archimedes Screw and transmutation

1. Is there possibility to transport people upstairs by mechanism of Archimedes' screw?
2. Is there possibility to transmutate Carbon Dioxide to Helium? At what energy expand it is possible if any?

Asked Jacez

Do you know the answer have any comments please respond

Will mass change with heating?

If we heat a metal for a certain period and at a certain heat it radiates EM waves (light). If we perpetuates to heat it for a long time will the mass of the metal body gets affected?

Eswar asked

Answer:

Small changes in Temperature will not change the mass. But if heated at high temperature for long time, the answer is yes, unless the loss of mass by other means is not prevented.

I'll explain more soon. By the time you can have some more research and google it. Search "change in mass on heating".

Your search will possibly lead to the conclusion that mass does not change with temperature. But it is not a one word answer as per your question.


Refer

http://www.mikecurtis.org.uk/heating.htm


Potential energy of a heavenly body

What is the maximum value of potential energy that can be possessed by a heavenly body? Give the general expression for potential energy of an object  near the surface of earth.

Asked Diptimayee


Answer:

Gravitational potential energy is energy an object possesses because of its position in a gravitational field.


However, for objects near the earth the acceleration of gravity g can be considered to be approximately constant and the expression for potential energy relative to the Earth's surface becomes



Sound for class 9th CBSE

I want to know about explanation about the chapter sound for class 9th chapter cbsc
Asked Farooq

Answer: Dear Farooq, please specify your questions or atleast the area of doubt.

Interference from a split laser beam without using slits

Is it possible to obtain interference from a split laser beam without using slits??I.E Take a laser beam split it into two then combine them to give distructive or constructive interference..

Regards,
Geo

Answer:
Double slit is just one of the various methods to produce interference fringes. Other popular methods are
1. By using a Lloyd's Single Mirror and 2. using Fresnel's Biprism. Go through the following links for more information.

https://www.youtube.com/watch?v=73Htg2T2tuY
http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Interference/text/Lloyds_mirror/index.html

2. Fresnel's biprism interference
http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Interference/text/Fresnel_biprism/index.html

The following file briefs about the various methods of producing interference fringes.
http://www.physics.wisc.edu/undergrads/courses/spring2012/202/phy202_lect28_handout.pdf

HYSTERISIS

Namrata Asked:
What is hysteresis? I only know its related to the frequency.

Ans:
Hysterisis mens “Lagging Behind”

Hysteresis phenomena occur in magnetic materials, (as well as in the elastic, electric, and magnetic behavior of materials,) in which a lag occurs between the application and the removal of a field (or force) and its subsequent effect
The lack of retraceability of the magnetization curve is the property called hysteresis

Magnetic hysteresis


When an external magnetic field is applied to a ferromagnetic substance, the atomic dipoles align themselves with the external field. Even when the external field is removed, part of the alignment will be retained: the material has become magnetized.
The relationship between magnetic field strength (H)(EXTERNAL) and magnetic flux density (B) (ACQUIRED) is not linear in such materials. If the relationship between the two is plotted for increasing levels of field strength, it will follow a curve up to a point where further increases in magnetic field strength will result in no further change in flux density. This condition is called magnetic saturation.
If the magnetic field is now reduced linearly, the plotted relationship will follow a different curve back towards zero field strength at which point it will be offset from the original curve by an amount called the remanent flux density or remanence (RETENTIVITY).
If this relationship is plotted for all strengths of applied magnetic field the result is a sort of S- shaped loop. (See the diagram) The 'thickness' of the middle bit of the S describes the amount of hysteresis, related to the coercivity of the material.



A numerical from Projectile motion, friction and more

Hale asked:

One side of the roof of a building slopes up at 33.5°. A roofer kicks a round, flat rock that has been thrown onto the roof by a neighborhood child. The rock slides straight up the incline with an initial speed of 15.0 m/s. The coefficient of kinetic friction between the rock and the roof is 0.430. The rock slides 10.0 m up the roof to its peak. It crosses the ridge and goes into free fall, following a parabolic trajectory above the far side of the roof, with negligible air resistance. Determine the maximum height the rock reaches above the point where it was kicked.

(The question is left for visitors to answer.)

An Idea for Cycle Rickshaw

Sumit Asks:

Sir,
   While seeing a rickshaw puller struggling to pull heavy weight,I wondered if the load carried by him can be reduced for carrying same weight people.Since the load carried by him is dependent on the horizontal component of frictional force (i.e,µ.mgsinØ) ,cant we apply a spring in the rickshaw so that the effective weight will decrease and hence the load carried by him?

How can the electric current in a circuit be kept continuous?

Susmita asked:

  1. What is Current?
  2. How can electric current in a circuit be kept continuous?

 

Ans:

The word current means flow.

Electric Current refers to charges in motion. The most common way of producing current is by applying a potential difference across the terminals of a conductor. Then the free electrons in the conductor drifts towards the positive terminal.

The current in the circuit can be kept continuous by providing a constant potential difference between two points of the circuit. This is usually done by connecting the ends of the circuit to the terminals of an electrochemical cell (which converts chemical energy to electrical energy) or a dynamo (which converts mechanical energy to electrical energy)

Please refer the following links for more information

Why earth moves in an elliptical orbit instead of circular one?

Zeenath PV asked:

why earth moves in an elliptical orbit instead of circular one?

Ans:

image Please note that ELLIPSE is the more general form for an orbit and CIRCLE is a speacial case of ellipse.

So, it is more general way of saying that the orbits are elliptical. If it were circular, we would have more trouble explaining why they are perfect circles.

The situation where the Sun, for instance, would be exactly in the center of a perfectly circular planetary orbit couldn't happen in our Solar System since there are other planets that would gravitationally affect the orbit and cause it to be immediately non-circular by pulling the object to one side or the other.  Any other stable orbit type than a perfectly circular one turns out to be an elliptical orbit.

Read more at : http://www.astro-tom.com/technical_data/why_elliptical.htm

How earths tilt causes seasonal change?

Zeenath PV asked: how earths tilt causes seasonal change?
Answer:
image The climate on earth is mainly determined by the distance from sun and how much sunlight (and heat radiations) is received by earth. The relative duration of day and night as well as the climate is greatly dependant on the tilt of the earth’s axis with respect to its plane of orbit around sun and the elliptical shape of earth
Please follow these links to read more:
  1. http://www.msnbc.msn.com/id/3077384/
  2. http://en.wikipedia.org/wiki/Season
  3. http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980211f.html
  4. http://www.eoearth.org/article/Causes_of_climate_change

A Question from Electric Flux and Gauss Theorem

Zeenath asked:
How can you prove that dS in φ =E.dS is dS Cos θ if the area is tilted at an angle ? I need the mathematical Steps.

Ans;
Dear Zeenath,
Electric Flux is defined as the total no of field lines passing normal to the surface. So while calculating, we need to consider the area of the surface normal to the field lines only. That is why we take the dot product of E and ds, where ds is the area vector (not just the area: Area vector is a vector whose magnitude is equal to the area and dirtected normal to the surface.)
Then by definition of dot product,
dφ =E.dS = EdS cos θ
which gives the component of E and the component of area vector in the direction of E (When area vector is in the direction of E, the actual component of area is perpendicular to E)
Hope the matter is clear.
Mathew Abraham

Mass Energy conversion and the Big Bang

Ajesh Asked:

Respected sir, in the famous theory of Albert Einstein which is of mass energy equivalence  he stated a formula E = mc2 .
sir does this mean that mass can be converted in to energy or the opposite will happen .
it is even stated in the big bang theory  that the universe was created with respect to the  above theory is that so?

 

Answer:

The mass energy conversion and the equivalence is a widely discussed topic in relation to the creation of the universe or the Big Bang.

There is some energy missing in calculation when we consider the equivalence; this is called dark energy.

Please read the following links for more details.

http://csep10.phys.utk.edu/astr162/lect/cosmology/hotbb.html

http://www.big-bang-theory.com/

http://ssscott.tripod.com/BigBang.html

Cosmological constant

A constant introduced by Einstein  (1917) into the equations of general relativity to allow a steady state cosmological solution to the Einstein field equations. The constant was introduced before the concept of the Big Bang had been conceived, so an expanding or contracting universe  was regarded as physically implausible, leading Einstein to add as a "fudge factor." In theory, the constant can be derived from quantum field theory, but the derivation has not yet been performed. Einstein's cosmological constant is equivalent to a vacuum energy density, which means it can be put on the left hand side of Einstein's equations with the geometry (as Einstein did), or on the right hand side with the stress-energy, both forms being mathematically equivalent.

 

The value of in our present universe is not known, and may be zero, although there is some evidence for a nonzero value; a precise determination of this number will be one of the primary goals of observational cosmology in the near future.

The value of the cosmological constant is an empirical issue which will ultimately be settled by observation; meanwhile, physicists would like to develop an understanding of why the energy density of the vacuum has this value, whether it is zero or not. There are many effects which contribute to the total vacuum energy,
including the potential energy of scalar fields and the energy in “vacuum fluctuations” as predicted by quantum mechanics, as well as any fundamental cosmological constant.

image If the recent observational suggestions of a nonzero are confirmed, we will be faced with the additional task of inventing a theory which sets the vacuum energy to a very small value without setting it precisely to zero. In this case we may distinguish between a “true” vacuum which would be the state of lowest possible energy which simply happens to be nonzero, and a “false” vacuum, which would be a metastable state different from the actual state of lowest energy (which might well have = 0). Such a state could eventually decay into the true vacuum, although its lifetime could be much larger than the current age of the universe. A
final possibility is that the vacuum energy is changing with time — a dynamical cosmological “constant”. This alternative, which is sometimes called “quintessence”, would also be compatible with a true vacuum energy which was ultimately zero, although it appears to require a certain amount of fine-tuning to make it work.
No matter which of these possibilities, if any, is true, the ramifications of an accelerating universe for fundamental physics would be truly profound.

Read More at

  1.  http://relativity.livingreviews.org/Articles/lrr-2001-1/
  2. http://www.astro.ucla.edu/~wright/cosmo_constant.html
  3. http://www.scholarpedia.org/article/Cosmological_constant

Effect of Relativity on Gravitation

Davide (from Australia) Asks: I wonder, If we take atoms travelling with speeds close to that of light, there will be an increase in mass (according to the theory of relativity).  As the mass increases, the gravitational force should also increase.  Will this increase in gravity affect the surrounding space? If an atoms travels with 99% the speed of light, what will be its increase in mass?
(The question was reformatted to suit the language)
Answer: I must congratulate you first for thinking deep and differently. The relativistic increase in mass is given by the equation The increase in mass increases the gravity, but the force of gravity is governed by the value of the universal constant of gravitaton, which is very low (6.67 x 10-11). For the second part of your question, the increase in mass can be calculated to be about 7 times from the equation mentioned above.
(Please post your further doubts on this topic as comment to this post so that all related ideas come together.)

Download CBSE Class IX and X Science Sample Question Paper for Term I (SA1)

CBSE has published sample papers in all subjects for Class IX and X. You can download Science Sample Papers for Class IX and X here. The question paper contains BluPrint and Marking scheme too. Practising this with a view to score maximum marks as per the marking scheme will be a great help for the students to perform well in exams. CBSE Class X Science Sample Paper CBSE Class IX Science Sample Paper

Doubt from HC Verma's Concepts of Physics Volume I

If any one can explain step wise calculation of Example no. 17 of Chapter 2 of concepts of Physics by H C Verma Vol. 1. Thanks in advance Ans: Click here for Hint Solutions to the entire problems from concepts of Physics by HC Verma available at http://sites.google.com/site/physicsclassroomonline as well as www.plustwophysics.com

What is impact parameter?

Impact parameter is the perpendicular distance between the velocity vectors of two objects (considered to take part in a collision) when they are at infinite distance (at such a distance that there is no interaction between the two particles)

In Rutherford’s alpha scattering experiment, the gold nucleus is at rest and therefore, in this case

Impact parameter is the perpendicular distance between the gold nucleus and the the direction of the velocity vector of alpha particle when the alpha particle is at infinite distance (i.e; the force of interaction between the gold nucleus and the alpha particle is negligibly small)

Why ordinary ammeter cannot measure AC?

Ac changes its direction frequently and therefore the ordinary ammeter which is just a galvanometer with a shunt, tends to respond to the magnitude and direction of the instantaneous values of current and in effect it will not show any deflection.
Ac is measured using hot wire ammeter, which is based on the heating effect of current

MAGNETIC HILLS & GRAVITY HILLS

AMRITHA ASKED "Sir, is it true that in magnetic hill in Ladhak defies gravity? If so, Why?" ANSWER: The “magnetic hill” is located on the Leh-Kargil-Batalik national highway, about 30 km from Leh, at a height of 14,000 feet above sea level. On its eastern side, flows the Indus, which originates in Tibet and goes to Pakistan. The Indian Army maintains a sikh Gurudwara near the hill where Guru Nanak Dev, meditated in the 17th century.[citation needed] Due to both the Gurudwara and the gravity hill, the area has become a popular sightseeing destination. The hill is alleged to have magnetic properties strong enough to pull cars uphill and force passing aircraft to increase their altitude in order to escape magnetic interference; in reality, the effect is an optical illusion created by the gravity hill. The slope of gravity hills is an optical illusion, although sites are often accompanied by claims that magnetic or even supernatural forces are at work. The most important factor contributing to the illusion is a completely or mostly obstructed horizon; without a horizon, judging the slope of a surface is difficult as a reliable reference is missing. Objects one would normally assume to be more-or-less perpendicular to the ground (such as trees) may actually be leaning, offsetting the visual reference. The illusion is similar to the well-known Ames room, in which balls can also appear to roll against gravity. SOURCE: WIKIPEDIA

Devices using permanent magnets and electromagnets

Galvin Pinto asked;

  1. List the devices where permanent magnets & electromagnets are used.
  2. Sketch the pattern of the field lines around a current carrying:
    • Straight Conductor

    • Loop of wire

    • Solenoid

    Ans:

    1. Permanent Magnets are used in
      • Laboratory magnets
      • loud speakers
      • microphones
      • cyclotron
      • generators
      • and many more
    2. Pattern of the field lines around a current carrying

    Straight Conductor

    image

    Loop of wire

    image

    Solenoid

    image

    A Question from Magnetic Effects of Current

    Gavin Pinto asked

    “A current carrying wire is placed above a magnetic needle. What do you observe? If you want to increase this effect, what will you do? If you want to reverse the effect, what will you do?”

     

    Ans: When a current carrying conductor is placed above a magnetic needle, the magnetic needle is deflected and tends to align itself perpendicular to the conductor. The effect can be increased by increasing the strength of current through the wire or by decreasing the distance.

    However, the maximum deflection which can be produced is 90 degree, when the magnetic field produced by the current carrying wire is very much greater than the earth’s magnetic field.

    Gauss`s law

    Nagendra chowdary asked : -

    “A circular ring of radius r made of a non conducting material is placed with its axis is parallel to a uniform electric field if the ring rotated by 180 degrees Does the flux changes? A Q charge is uniformly distributed on a thin spherical shell If a point charge is brought near it what is the field at the center?Is u r answer depends on whether it is conducting or non conducting?”

    Questions from Properties of gases

    Senarita asked:
    we know that temperature has a properties to make some thing bigger in volume.we say that gas has no particular volume or shape.where we place a gas it takes the shape of that container.my question is- a gas of mass 'm' has a particular number of atoms or molecules.if the gas is not getting any temperature then how it is occupying all the volume of a bigger container.i mean how the inner space of two atoms or two molecules of that particular gas is increasing when i am transferring it to a bigger container.(say,every condition is constant.nothing is changed.)please answer me.
    “can you give me some name of sites where i can get explanations on hydrostatics or the topics of physics.i am now in class 11.”
    The visitors are requested to respond

    A Question from Electromagnetic Waves

    Rajat asked: - we know that electric and magnetic field are associated with charged particle but when light travells,it produces electric and magnetic field both but it consests of photons,which is a uncharged particle.why it did so?


    If any visitor wants to answer, please use the commenting system

    Wave travelling from one medium to another

    Iqbal Lucky asked
    "when a wave enters from air to water, its wavelength changes but frequency remains unchanged. why"
    Answer Velocity of a wave = frequency x time period V = ν λ When a wave travels from one medium to another, its velocity changes and so also is its wavelength, but frequency; which is the number of vibrations produced per second is independent of the medium, remains constant. However, their is a phenomenon called Doppler Effect where the apparent frequency (observed frequency) is different from actual frequency and is caused by the relative movement between the source and observer (listener) Got a better answer? Please post it as comment.

    Physical Quantity

    Prabesh wants to know more on Physical quantities:
    Physical Quantity is a quantity which can be measured. eg; length, mass, force, time etc
    (The quantities which cannot be measured are abstract. Eg; Love, anger intelligence)

    Elliptical Orbits

    Sreeda asks:
    1. Why do planets move in elliptical paths and not in circular ones?
    Answer: Ellipse is a more general shape than a circle. Circle is a special case of ellipse when its semimajor axis and the semiminor axis are of equal lengths. The shape of orbit depends on various factors. Johannes Kepler in the 1600s was the first to start discussing the shape of planetary orbits, formulating a series of laws to explain their shape and characteristics. Isaac Newton and Albert Einstein contributed additional research and theories. Shape of a planet's orbit is affected by the gravitational force of sun as well as other planets. Also Remember that the sun is also in motion, which makes elliptical orbit an easy one than a perfectly circular one. [Comments solicited from esteemed visitors]

    Resolution of Vectors

    From: Vakas
    Subject: Resolution with geometry
    Message Body:
    Dear sir,
    I m not able to understand that when we resolute mg in a inclined plane that plane having angle theta,how does the vertical component goes through inclined surface & cos comp opp to N.
    explain with geometry so that i don't get confused in future in determining the two comps is right direction.
    Ans:
    In a right angled triangle,

    and

    When mg is resolved into mutually perpendicular directions, one parallel to plane and the other perpendicular to the plane (opposite to the Normal reaction), the component which is adjacent to θ is the cos component and the one away from θ is the sine component.
    Students find difficulty ,mainly in identifying the angle .
    See the diagram below and try to analyze
    resolution

    Questions from Properties of gases

    Senarita asked:
    “we know that temperature has a properties to make some thing bigger in volume.we say that gas has no particular volume or shape.where we place a gas it takes the shape of that container.my question is- a gas of mass 'm' has a particular number of atoms or molecules.if the gas is not getting any temperature then how it is occupying all the volume of a bigger container.i mean how the inner space of two atoms or two molecules of that particular gas is increasing when i am transferring it to a bigger container.(say,every condition is constant.nothing is changed.)please answer me.”
    can you give me some name of sites where i can get explanations on hydrostatics or the topics of physics.i am now in class 11
    Master will be answering the question shortly. The visitors are requested to respond

    A Question from Electromagnetic Waves

    Rajat asked: - we know that electric and magnetic field are associated with charged particle but when light travells,it produces electric and magnetic field both but it consests of photons,which is a uncharged particle.why it did so? Master will soon answer this. By the time, if any visitor wants to answer, please use the commenting system

    A question from kinematics - motion in one dimension

    Natasha Sehgal Asked:
    "A stone is thrown in a vertically upward direction with a velocity of 5m/s. If the acceleration of the stone during its motion is 10m/s^2 in the downward direction , what will be the height attained by the stone and how much time will it take to reach there?"
    Answer: Take, u=5m/s a= -10 m/s2 v=0 at the topmost position S= Hmax = ? The eqn of motion to use v2 - u2 = 2aS Which gives H = 1.25 m From the eqn v=u + at t= 0.5 s

    Find time of death of patient with differential equations

    Mr. N.P. Sahani asked:
    "The differential equation and solution for finding the time of death of a patient"

    Answer:

    See that the question is to find the time of death (Not predicting the time of death. I thought that when I read it first.)

    I can give some links which will be helpful in this connection

    1. http://www.exploreforensics.co.uk/estimating-the-time-of-death.html
    2. http://en.wikipedia.org/wiki/Thermal_death_time
    3. http://autarkaw.wordpress.com/2008/07/21/time-of-death-a-classic-ode-problem/
    4. http://people.uncw.edu/lugo/mcp/DIFF_EQ/deproj/death/death.htm
    5. http://mathforum.org/library/drmath/view/53409.html
    6. http://www.scribd.com/doc/21514655/Newton-s-Law-of-Cooling-Finding-the-Time-of-Death-of-a-John-Doe

    Resistance and Resistivity

    Simran Asked
    "Two wires A and B of equal lengths,different cross-sectional areas and made of same metal a)name the property which is same for both the wires. b)name the property which is different for both the wires"
    Answer:
    Since both the wires are of same metal, the resistivity will be same for both whereas since the dimensions (Area of cross section here) are different, the resistance will be different

    Potential Difference

    Pawan Asked
    "What is meant by potential difference?"
    Answer:
    "Potential Difference between two points is the difference in Potential between the two points and is equal to the work done per unit charge ion carrying a positive test charge from one point to another against the electric field and without any acceleration"

    Limitations of Homogenity

    Paraser asked
    "What are the limitations of homogenity?"
    The question is acutually"What are the limitations of dimensional analysis?" The dimensional analysis is based on the principle of homogenity of dimensions. The question is already discussed earlier  HERE

    Geothermal Energy

    p.tejaswini asked
    "Why is geothermal energy an alternative source of nuclear energy?"
    There is some problem with the doubt you asked. It is correct to say "Geothermal energy and Nuclear energy are alternative sources of energy" Sun is the ultimate source of energy for earth. The energy of fuels, wind energy, water energy, food - all these are derived from sun. But Geothermal energy and nuclear energy are not derived from solar energy. GeoThermal Energy Links for Reference http://www.renewableenergyworld.com/rea/tech/geothermal-energy http://www.geos.iitb.ac.in/geothermalindia/pubs/geoweb.htm See a diagram of Geothermal plant [caption id="" align="alignleft" width="525" caption="Geo Thermal Energy"]Geo Thermal Energy[/caption] The centre of the Earth is around 6000 degrees Celsius - easily hot enough to melt rock. Even a few kilometres down, the temperature can be over 250 degrees Celsius if the Earth's crust is thin. In general, the temperature rises one degree Celsius for every 30 - 50 metres you go down, but this does vary depending on location In volcanic areas, molten rock can be very close to the surface. Sometimes we can use that heat. Geothermal energy has been used for thousands of years in some countries for cooking and heating. The name "geothermal" comes from two Greek words: "geo" means "Earth" and "thermal" means "heat". Read More on Geothermal Energy Here

    End of Universe ; Vishnu says

    last day i saw a prog in natgeo
    "How universe ends?"

    it presented 2 theories
    one says
    the universe expands expands expands
    everything splits and expands
    at last
    molecules splits to atoms
    and no more molecules exist
    only isolated atoms
    it will be cold and dead
    thats the death of universe
    then somehow an atom gets excited
    it collides with another
    thus a point of hier gravity created
    everything pulled into it
    and everything joins
    then another
    BIG BANG
    thats one theory
    the next says
    universe expands to a limit
    then its energy runs out
    and falls back
    thus contraction of universe
    contracts contracts contracts
    to a
    Singularity
    then another
    BIG BANG
    new universe
    these are the 2 theories they speak of
    the first one got more weightage
    since the expansion of universe is accelerating

    second one happens only if it decelerates

    it is determined by the amount of dark matter and dark energy we have
    if dark matter is more, then the contraction theory comes true.else the expansion.

    Electronic Regulator

    Manishankar asked: how an electronic circuit works as a regulator? In an ordinary regulator, the current in the circuit is controlled by increasing or decreasing the resistance. But in an electronic regulator, the control is taken over by electronic circuits. There are different forms of electronic regulators available. Some use transistors, some use operational amplifiers etc. In electronic regulators, the voltage output is changes by electronic means (as in a transistor amplifier). Visit http://en.wikipedia.org/wiki/Voltage_regulator#Electronic_voltage_regulators for more details

    IISc Launched A New UG Programme For Future Researchers

    Indian Institute of Science (IISc), Bangalore is all set to launch a new four-year undergraduate (UG) Bachelor of Science (BS) programme in the coming academic session in 2011. The main aim of the course is to prepare young minds for research and doctoral studies in the upcoming areas of science and technology. Besides motivating students to take up higher research, mentoring and tutorials, the programme will introduce rigorous specializations in fourth, fifth, and sixth semesters. The students will be allowed to choose electives from other streams as well during this period. In the seventh and eighth semesters, students will work on a research-oriented project supervised by a faculty member. Admissions to the programme will be based on national entrance examinations like IIT JEE, AIEEE, AIPMT, and Kishore Vaigyanik Protsahan Yojana.

    CLASS 11 - HALF YEARLY EXAMINATION SAMPLE PAPER -1

    HALF YEARLY EXAMINATION SAMPLE PAPER

    (QUESTIONS ARE FROM THE PART 1 NCERT TEXT BOOK)

    CLASS: XI SUB: PHYSICS MM: 70 TIME: 3hrs.

    BLUE PRJNT

    Sl.No

    Unit

    VSA(1)

    SA I (2)

    SA II(3)

    LA (5)

    Total

    1

    Physical World& Measurement

    2(2)

    4(2)

    3(1)

    -------------

    9(5)

    2

    Kinematics

    1(1)

    4(2)

    6(2)

    5(1)

    16(6)

    3

    Laws of Motion

    1(1)

    2(1)

    6(2)

    -----------

    9(4)

    4

    Work,Energy and Power

    1(1)

    4(2)

    3(1)

    5(1)

    13(5)

    5

    Motion of System of Particles&Rigid Body

    2(2)

    4(2)

    3(1)

    5(1)

    14(6)

    6

    Gravitation

    1(1)

    2(1)

    6(2)

    ----------------

    9(4)

    Total

     

    8(8)

    20(10)

    27(9)

    15(3)

    70(30)

    General instructions:

    (i) All questions are compulsory.

    (ii) There are 30 questions in total. Questions 1 to 8 carry one mark each, questions 9to18carry two marks each, questions 19to27carry three marks each and questions 28to30 carry five marks each.

    (iii) There is no overall choice. However, an internal choice has been provided in one question of two marks, in one question of three marks and all three questions of five marks each. You have to attempt only one of the given choice in such questions

    1. Name the fundamental force which is responsible for the stability of the nucleus. (1)

    2. State the no. of significant figures in the following

    a) 3.56 x 10-34 b) 0.00005302 (1)

    3. A pebble of mass 0.05kg is thrown vertically upwards. Given the direction and magnitude of the net force on the pebble,

    (a) During its upward motion

    (b) During its downward motion (1)

    4. A body is initially at rest. It undergoes one-dimensional motion with constant acceleration. Show that the power delivered to it at time‘t’ is proportional to‘t’. (1)

    5. What is the moment of inertia of a rod of mass M, length l about an axis perpendicular to it through one end? (1)

    6. Give the location of the centre of mass of a ring. Does the centre of mass of a body necessarily lie inside the body? (1)

    7. Draw the position-time graph of an object moving with negative velocity. (1)

    8. While considering the motion of an object under the gravitational influence of another object, which of the following quantities are conserved. (a) angular momentum (b) linear momentum (c) total mechanical energy (1)

    9. The resistance R=V/I where V= (100 ±5) V, I= (10±0.2). Find the percentage error in R. (2)

    10. Check the dimensional consistency of the equationclip_image002[1]. (2)

    11. Fig shows the x-t plot of one dimensional motion of a particle. Is it correct to say from the graph that the particle moves in a straight line for t<0 and in a parabolic path for t>0? If not suggest a suitable physical context for this graph.clip_image004[1] (2)

    12. The position of particle is given by r=3t î + 2t2ĵ + 5ǩ, where‘t’ is in seconds and the coefficients have proper units for ‘r’ to be in meters. Find V (t) and a (t) of the particle. (2)

    13. What is impulse of a force, how it is related to momentum. (2)

    14. The potential energy function for a particle executing linear simple harmonic motion is given by V(x)=kx2/2, where k is force constant of the oscillator. For k=0.5Nm-1 the graph of V(x) versus ‘x’ is as shown in figure. Show that a particle of total energy 1J moving under this potential must ‘turn back’ when it reaches ±2m.

    OR clip_image006[1]

    A body constrained to move along the Z-axis of a co-ordinate system is subject to a constant force F given by clip_image008[1]clip_image010[1].What is the work done by this force in moving the body through a distance of 4m along the Z-axis? (2)

    15. Prove work-energy theorem for a constant force. (2)

    16. Define centre of gravity of a rigid body. (2)

    17. What are the factors on which the moment of inertia of a body depends? (2)

    18. Does the escape speed of a body from the earth depend on a) mass of the body b0 the height of the location of the body from where the body is launched? Justify . (2)

    19. State Kepler’s laws of planetary motion. (3)

    20. Explain the variation of acceleration due to gravity with a) altitude b)depth (3)

    21. State i) theorem of parallel axes ii)theorem of perpendicular axes (3)

    22. A pump on the ground floor of a building can pump water to fill a tank of volume30m3 in 15minutes.If the tank is 40m above the ground, and the efficiency of the pump is 30â„…, how much electric power is consumed by the pump? (3)

    23. Explain why a) a horse cannot pull a cart and run in empty space b) a cricketer moves his hands backwards while holding catch. (3)

    24. a) State Newton’s second law of motion.

    b) A bullet of mass 0.04 kg moving with a speed of 90ms-1 enters a heavy wooden block and is stopped after a distance of 60cm.What is the average resistive force exerted by the block on the bullet? (3)

    25. Two bodies are thrown with the same initial velocity at angles α and ( 90o-α) with the horizontal. What will be the ratio of i) maximum heights attained by them and ii) of horizontal ranges.

    OR

    What is a projectile? Show that the path of the projectile is parabolic. (3)

    26. Explain any two uses of dimensional equations. Write any two of its limitations. (3)

    27. A woman starts from her home at 9:00 am walks with a speed of 5km h-1 on a straight road up to her office 2.5 km away, stays at the office up to 5:00 pm and returns home by an auto with a speed of 25km h-1.Choose suitable scales and plot the x-t graph of her motion. (3)

    28. a) What is uniform circular motion? Find the magnitude and direction of centripetal acceleration in case of uniform circular motion of an object.

    OR

    a) Find the magnitude and direction of the resultant of two vectors A and B in terms of their magnitudes and angle θ between them.

    b) A motor boat is racing towards north at 25kmh-1 and the water current in that region is 10kmh-1 in the direction of 60o east of south. Find the resultant velocity of the boat. (5)

    29. Obtain an expression for the potential energy of a spring and discuss the nature of its variation. Show that the total energy of the spring is constant in all positions. Show graphically the variation of PE, KE and TE with distance from the mean position.

    OR

    a) Answer the following questions with reasons:

    i) In an elastic collision of two billiard balls, is the total kinetic energy is conserved during the short time of collision of the balls (i.e when they are in contact)?

    ii) Is the total momentum is conserved during the short time of an elastic collision of two balls.

    b) Give a brief account of elastic collision in two dimensions. (5)

    30. a) Three bodies a ring ,a solid cylinder and a solid sphere roll down the same inclined plane without slipping. They start from rest. The radii of the bodies are identical. Which of the bodies reaches the ground with maximum velocity?

    b) Given moment of inertia of a disc about any of its diameters to be MR2/4, find its moment of inertia about an axis normal to the disc and passing through a point on its edge.

    c) Find the moment of the sphere about a tangent to the sphere, given moment of inertia of the sphere about any of its diameters to be 2MR2/5.

    OR

    a) Establish a relation between (i) torque and moment of inertia of a rigid body

    (ii) Angular momentum and moment of inertia of a rigid body

    b) A solid cylinder of mass20kg rotates about its axis with angular speed 100rad/s. The radius of the cylinder is 0.25m.What is the kinetic energy associated with rotation of the cylinder? What is the magnitude of angular momentum of the cylinder about its axis? (5)

    ******************************

    Physics Problem Link

    Try to solve this Problem Kinematics Problem

    How to get A in physics?

    Nischal Singh Dangol Asked:
    How to get A in physics?
    Answer: Whatever the syllabus or course, an A grade means "Excellent work indicating a clear mastery of the subject material" This requires evidently a basic understanding of the subject, thorough with the fundamentals and the ability to apply the principles to solve problems creatively. One should be able to make suitable connections between concepts logically to solve the problem in hand. (Remember, Physics is often referred to as a Problem Solving Discipline) A grade in Physics comes naturally if you start loving the subject and take active interest in applying the concepts of Physics to solve problems (in subject as well as in day to day life situations) Also important :
    • Maintaining good rapport is also important in getting an A grade.
    • Attend all lectures/classes and never bunk
    • Do your assignments sincerely and submit in time
    Wish you all the best. Got some more suggestions? Use the commenting system to post your ideas.

    Wave travelling from one medium to another ...

    Iqbal Lucky asked
    "when a wave enters from air to water, its wavelength changes but frequency remains unchanged. why"
    Answer Velocity of a wave = frequency x time period V = ν λ When a wave travels from one medium to another, its velocity changes and so also is its wavelength, but frequency; which is the number of vibrations produced per second is independent of the medium, remains constant. However, their is a phenomenon called Doppler Effect where the apparent frequency (observed frequency) is different from actual frequency and is caused by the relative movement between the source and observer (listener) Got a better answer? Please post it as comment.

    Limitations of Dimensional Analysis

    Ashmeeta Bhattarai asked:
    "What are limitations of principle of homogeneity of dimensional analysis?"
    The dimensional analysis has the following limitations
    1. It fails while using it to derive a relation among physical quantities, if there are more than 3 unknown variables on which a given physical quantity depends
    2. It does not tell whether a given Physical quantity is a scalar or a vector.
    3. It does not tell us the value of constants involved
    4. It does not always tell us the exact FORM of a relation
    5. It cannot be used for deriving logarithmic, trigonometric or exponential relations
    6. A dimensionally correct equation may not always be the correct relation. (Because there are more than one physical quantity having the same dimensions)
    Know some more? Write them as comments to this post

    Rain and Man problems - How to solve?

    Shashank Asked:
    "How to solve rain and man problems from motion in one dimension?"
    Answer: The problem is solved using principles of vector addition. There is a simple logic to remember
    A/C = A/B x B/C
    If we represent diagrammatically (in the form of vectors) the velocity of rain wrt ground, velocity of man wrt ground and velocity of rain wrt man, then we can write Vr/m = V r/g + Vg/m = V r/g - Vm/g = V r/g + (- Vm/g) Where (- Vm/g) is the negative vector of Vg/m I will be adding illustrations soon.

    Why connecting wires are not heated up in a circuit with electric heater?

    Why connecting wires are not heated up in a circuit with electric heater? Only the heating element gets heated up, not the wires which connect them to the supply though same current is passed through them. Why?

    We are shifting the website to a new platform. Please bear with us

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