Consider an object colliding elastically with an inclined wall (with much bigger mass than the object) as in picuture. The wall is at $45°$ and it is moving.
$(A)$: frame of reference of the wall (the wall is steady)
$(B)$: frame of reference with respect to which the wall is moving towards the right with velocity $V$
The ball collides on the wall in both cases with an angle $\theta_1=45°$ with respect to the normal to the wall. In $(A)$, using conservation of momentum, the ball will be reflected at an angle $\theta_2=\theta_1=45°$.
But what is the value of $\theta_2$ is $(B)$?
My guess is that the conservation of momentum implies the equality of the angles of incidence and reflection only in the frame where the wall is steady, while in the other reference the ball is reflected at an other angle.
But, thinking about it, it seems strange that the ball gets momentum in the direction of the motion of the wall just being reflected on it. So is the situation really like the picture $(B)$, i.e. the angle of reflection is bigger?
Best Answer
The answer "what is the value of $\theta$ in B" is answered by simple vector summation: you go back to the frame of reference where the wall is stationary, determine the magnitude and direction of the velocity after the impact, then add the velocity of the frame of reference back in.
That's really not so strange. If you imagine the situation where a ball is moving slowing at an angle to a wall that is coming rapidly towards it, you know intuitively that the ball will be moving roughly in the direction of the motion of the wall afterwards (think what happens with a tennis racket and a ball that was tossed in the air to be served... the ball flies across the net, doesn't it?)