Tue May 22, 2012 By: Shourya Mukherjee

A ball thrown vertically upwards with speed of ‘u’ from top of a tower reaches the ground in 9 seconds. Another ball thrown vertically downwards from the same position with a speed of ‘u’, takes 4 seconds to reach the ground. Calculate the value of ‘u’.(take g=10m/s^2) Please explain clearly.

Expert Reply
Fri June 01, 2012

As the ball thrown vertically up will come back to the point of projection with the same speed, Both the balls will moves downward with same initial velocity, so both will hit the ground with same speed   -v2 = u2 + 2gh

the ball projected up will take (2u/g) time more to reach the ground than the stone projected downwards.

 2u/g = 5

u = 5 * 10/ 2 = 25 m/s


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