A girl standing on a stationary lift (open from above) throws a ball u...
When the lift starts moving upwards with uniform speed of 5 ms−1, there is no change of relative velocity of ball w.r.t. girl. Hence, even in this case the ball will return to the girl’s hands in the same time i.e 10s.
A girl standing on a stationary lift (open from above) throws a ball u...
-1, what will be the motion of the ball as observed by the girl?
From the girl's perspective, the ball will move upwards with an initial velocity of 50 m s-1. However, since the lift is moving upwards with a uniform speed of 5 m s-1, the velocity of the ball relative to the ground will be the sum of the velocity of the lift and the initial velocity of the ball.
Let's assume that the positive direction is upwards. Then, the velocity of the ball relative to the ground can be calculated as:
v_ball,ground = v_ball,girl + v_lift
where
v_ball,girl = 50 m s-1 (initial velocity of the ball as observed by the girl)
v_lift = 5 m s-1 (velocity of the lift)
Therefore,
v_ball,ground = 50 m s-1 + 5 m s-1 = 55 m s-1
So, as observed by someone on the ground, the ball will move upwards with an initial velocity of 55 m s-1. Its motion will be affected by gravity, so it will eventually slow down, reach its maximum height, and then fall back down to the ground. However, the motion of the lift will not affect the motion of the ball in the vertical direction, since the lift is moving uniformly and not accelerating.