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2014-10-20T11:51:01+05:30

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Velocity of the ball just before hitting the ground : = v
v²  = u² + 2 g s
v² = 0 + 2 *10 * 20  = 400 
v = 20 meter/sec  This is directed downwards. So  v = -20 m/sec

Let the velocity of the ball be V after bouncing back.
0 = V² - 2 g S
0 = V² - 2 * 10 * 5 
V = 10 m/sec.  It is directed upwards. So  V = +10 m/s

 The change in momentum during the collision =  mV - mv = m(V-v)
            =  1 kg * (10 - (-20) ) m/sec  = 30 kg-m/sec

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