# 1) A ball is released from the top of a tower. The ratio of work done by force of gravity in first, second and third second of motion of the ball is {answer should come 1:3:5}2) The kinetic energy of a body of mass 2 kg and momentum of 2 Ns is:{answer should come 1 joule}3)A block of mass m is suspended by a light thread from an elevator. The elevator is accelerating upward with uniform acceleration a. The work done by tension on the block during t second is (u = 0) : {diagram is given} {answer should come }

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by alwaz

2015-01-13T11:25:40+05:30

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Gravity Force = m g
distance traveled in t seconds = s = u t + 1/2 g t² = 1/2 g t²
total distance traveled at t = 1, t= 2, t  =3 sec. are g/2, 2 g, 9g/2
distance traveled in 1st, 2nd and 3rd second are = g/2, (2g-g/2), (9g/2-2g)
= g/2 , 3g/2,  5g/2
work done during 1st, 2nd, 3rd seconds are = m g²/2,  mg²*3/2 , mg² 5/2
ratio = 1 : 3 : 5
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KE = p²/2m = 2²/2*2 = 1 J
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T - m g =  m a
T = m (g+a)
work done by Tension = W = T * displacement = T * [1/2 a t² ]
W1  = m (g+a) a t² / 2

just for clarity,  Work done by gravity = W2 = - m g [1/2 a t² ]

Net work done on block and actual change in its energy = W1 + W2 = m a² t² /2