Question

A ball falls from a height such that it strikes the floor of lift at 10m/s.if lift is moving in the upward direction with a velocity 1m/s, then velocity with which the ball rebound after elastic collision will be.

Answer 1

Let's take a coordinate system moving vertically downwards at a speed of 1 m/sec, consequently, the lift in this system will be in a stationary position (v_ball=11 m/sec). According to the law of conservation of momentum (where m_lift≫m_ball):

m_ball v_ball=-m_ball^' v_ball^'

v_ball^'=-v_ball

v_ball^'=-11 m/sec

Let's return to the original coordinate system

v_ball^'=-10 m/sec