Question

The time at which they collide after the projection of the first ball is

• A. $3.5s$
• B. $6.5s$
• C. $4.5s$
• D. $4.0s$

Answer 1

The correct option is B $6.5s$

Position of first stone above the ground after time $t$ is:

$h_1=ut-\frac{1}{2}g{t}^2$

$=49t-4.9t^2$

Position of second stone above the ground after time $t$ is :

$h_2=u(t-3)-\frac{1}{2}g(t-3{)}^2$

$=49(t-3)-4.9(t-3{)}^2$

We need to find time when $h_1=h_2$
$⟹t=6.5s$