You can apply simple formula...
calculate the acceleration 'a'... by
(square of final velocity)-(square of initial velocity)=2x a x distance
a=20x20/(2x40)
a=5
balance the force
uk x mass x g = mass x a
uk=a/g
uk=5/10=0.5
you can't assume light ray to be stable...
since it is the basis of relativity that a photon moves wid the velocity which is actually 299792458 m/s irrespective of the speed of the observer...
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see there are various ways to control volume but the best way can be explained as follow,,,,
source of production is same hence volume is controlled using amplifiers to increase or decrease the extent of sound ,,we call it as volume
a surface to be chosen as Gaussian surface where all of the electric field lines are perpendicular to that surface...
remember.. flux=ExA cos(angle bw area vector of the surface and electric field lines)
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