Black Hole again

Hello,
I understand that because of time dilation that we could never see something actually entering a black hole.  That is to say, from our earth view, time would appear to stop at or near the event horizon and observed objects would appear to move slower and slower until they seemed to be standing still (to be clear, I certainly understand that the object does not actually halt its motion, and will continue to accelerate and fall into the hole).  I understand (laymans level) the basic concept of time dilation, and that it is how the speed of light remains constant for all observers, but the idea of something appearing to halt its motion is pretty mind-blowing...I picture (when it becomes technically possible to actually see one) an event horizon cluttered with incoming matter but that matter never desending (according to our observation) into the black hole.   That seems far fetched to me, so I am sure I'm missing something.

By the way, I am in my sixties, and this question has vexed me to decades, so any light (time-dilated or not) that you can shed would be really appreciated!!

Al

Regards,
Alan Negin

Electrostatic Equilibrium

Two identical pith balls,each carrying a charge q,are suspended from a common point by two strings of equal length l.find the mass of each ball if the angle between them is 2x in equilibrium.

Regards,
Ayash Mohanty

Alternating current frequency

if an alternating current of frequency 50 hz is flowing through a conducting wire then how many times the current becomes zero in 1 second

Asked Aniketh

Image formed by a transparent sphere

A transparent sphere of radius R and refractive index of u is kept in air. At what distance from the surface of sphere should a point object be placed so as to form real image at same distance from sphere?

Regards,
shivendu

[R / (u-1)] - R

Try yourself with lensmakers formula
The optic centre is at the centre of the sphere. SO the surface is already at a distance R from the optic centre.

Defying Gravity?!

I have once come across a materiel / substance / compound / or something that keeps objects cut off from the earth's gravitational pull.  In other words this will keep the object suspended in the air and will not allow it to touch the ground.  Hence we can say that this is a barrier between the object and the earth's gravity.  Can you please give me the name of this material / substance / compound / gas / or what ever it is?

Regards,
Parveen

Magnetic levitation is one phenomenon which can do this.

Impulse

A Hammer weighing 3kg strikes the head of a nail with a speed of 2m/s and drives it by 1cm into the wall. find the impulse?
Asked Sree

Impulse is the product of force an time. Impulse of a force is also equal to the change in momentum produce. (Impulse Momentum Principle)

Here change in momentum is equal to 3kg x 2m/s = 6 kg m/s. 
(if we assume that the hammer does not bounce off after striking the nail)

Is acceleration have a scalar quantity. if not why?

Acceleration is a vector quantity and therefore acceleration due to gravity too is vector.
Acceleration due to gravity has magnitude as well as direction. The direction of acceleration due to gravity is towards the centre of earth.

Don't get confused! We are not PSYCHICS .. it's Physics here....!

We get many questions here which are intended to be asked to a psychic reader. But we are not those. We deal with Physics here ... often referred to as the true science explaining the various natural phenomena using cause effect relationship and more.

Sound distance speed

1. A man while beating a drum at the rate 5 per second walks toward a cliff till echo of beating dissappears completely. He walks at the rate of 8 km/hr. Calculate the distance of man from cliff in the beginning if he walked for 5 minute(Velocity of sound in air=350m/s).

2. A source emitting sound of frequency 180 Hz is placed in front of a wall at a distance of 2m from it. A detector is also placed in front of the wall at the same distance from it. Find the minimum distance between the source and the detector for which the detector detects a maximum of sound. Take the speed of sound in air=360m/s.

Regards,

Nilanjana Gupta

dimension of mass

if force, velocity, and time are taken as fundamental units then how can be find out the dimension of mass

Asked anupama at www.plustwophysics.com

potential gradient problem

in electric field, a 6.75uc charge experiences 2.5 N force , when placed at distance of 5m from the origin. then potential gradient at this point? GIVE FULL SOLUTION.

Regards,
vaibhav s. ugale