Explaining Inertia: Box B Moving Left When A Moves Right

[x86]

Box B sits on top of box A. IF we apply a force to box A and cause it to move to the right; then box B moves to the left.

Why? Why doesn't box B also move to the right? Can someone explain this? I keep doing such experiments but can't figure it out.

 

[berkeman]

Box B sits on top of box A. IF we apply a force to box A and cause it to move to the right; then box B moves to the left.

Why? Why doesn't box B also move to the right? Can someone explain this? I keep doing such experiments but can't figure it out.

It depends on the coefficient of friction between the boxes, and on how much force is applied. If the friction force is high and the applied force is small, the two boxes move together. If the friction force is small and the force on the lower box is high, then the force of friction between the boxes is not high enough to accelerate the top box at the same rate as the lower box. The top box does not move to the left -- it moves to the right, but more slowly than the lower box.

Does that make sense now?