Question

A ball is dropped from the top of a tower 100 m high. Simultaneously, another ball is thrown upward with a speed of 50m/sec. After what time do they cross each other?

Answer 1

Answer:

2 sec

Explanation:

they should cross after 2 seconds.

as the top ball which was dropped ( initial velocity=0) will gain velocity at the rate g m/s^2

the lower one thrown with u = 50 m/s will start loosing its velocity

by g m/s^2

so even if g was absent the top ball will remain at 100m and lower one has to travel 100m in 2 seconds to meet .

If one writes the equations of motion

then suppose they meet at height h from ground

after t seconds of throw.

in t secs the top one will travel 100 - h = (1/2) g .t^2

and the lower one will travel

h = 50.t - (1/2) g.t^2 = 50.t - { 100 - h }

so t= 100/50 =2 s