A sphere A is thrown up with velocity 20m/sec from the ground. Sphere B of same mass is dropped from a height of 80 m simultaneously, at time t = 0 so that both collide in air and stick together. Find the time (in second) after which the combined mass will fall on the ground. (Calculate from t = 0)`(g = 10m//"sec"^2)`

Answer

4

Explanation

The motion of the centre of mass is shown in the figure
`s = ut + 1/2 g t^2 ," "-40 = 10t = 5t^2`
or `5t^2 – 10t – 40 = 0`
or `5t^2 - 20t + 10t - 40 = 0`
or `5t (t - 4) + 10(t - 4) = 0" "` Hence, t = 4sec.