Question

A ball is dropped from the top of a tower of height $100m$. Simultaneously,another ball was thrown upward from the bottom of the tower with a speed of $50m/s0(g=10m/$s^2$)$. These two balls would cross each other after a time:

• A. 1 second
• B. 2 seconds
• C. 3 seconds
• D. 4 seconds

Answer 1

The correct option is B 2 seconds

Suppose they will cross each other after time t then the sum of the value of their displacement should be equal to height of tower.

Now displacement of first ball in time t is $g{t}^2/2$

and for second ball it will be $ut-g{t}^2/2$

adding them we will get $ut$ which should be equal to $100meter$

so $t=2second.$