# When will they collide?

At $$t = 0$$ , 2 balls $$b_{1}$$ and $$b_{2}$$ are dropped from rest from heights of $$10 m$$ and $$40 m$$ respectively.

Also at $$t = 0$$ , a very large heavy plank starts to move upward with speed $$5~m/s$$.

Find the time $$T$$ (in sec) at which the balls collide for the first time.

$$\textbf{NOTE-}$$

Motion takes place in vertical plane

$$g = 10 ~m/s^{2}$$

Assume all collisions as $$elastic$$.

The plank still moves with speed $$5~m/s$$ after collision with the ball $$b_{1}$$

The mass of the balls are negligible as compared to that of the plank.

The balls are to be considered point objects.

Collision takes negligible time

×