You put a stack of coins in a box which begins at rest near the top of a tower without any air in it. You release the box so that it is in free fall. What happens to the stack of coins as observed inside the box?

A) The stack remains at rest (with respect to the box) at the bottom of the box.

B) The stack jumps above the bottom of the box and remains together.

C) The individual coins spread out.

D) Something else happens.

Solution: D. All but one of the coins moves up with respect to the box. There is an upward force on the stack as the normal force doesn't instantaneously disappear upon the box dropping. There is a series of collisions between the coins and the bottom one remains at rest (similar to a Newton's cradle). See this video to watch the coins do so.