Free Body Diagram: A small box resting on a big box. Big box is accelerating and small box does not slip.?

Question

This is a question from my Gr 12 Physics textbook...and i don't understand it.

A Small box is resting on a larger box sitting on a horizontal surface. When a horizontal force is applied to the larger box, both boxes accelerate together. The small box does not slip on the larger box.
a) Draw a FBD of the small box during its acceleration.

If you can please explain all the forces that are acting on the box. I know there is the force of Gravity and the Normal force. There is supposed to be other forces but im not sure what they are, what direction they are goin in and why they exist. If anyone can explainit, thatd be awesome.

Answer 1

In addition to the normal force acting up, which is equal and opposite to the effect of gravity = mg acting down, there is a horizontal force being applied at the lower surface of the small box to accelerate it forward with the larger box. The magnitude of this force = ma, where "m" = the mass of the small box, and "a" = the acceleration of both boxes acting together. If "a" ever becomes large enough such that ma exceeds (mu) x (the normal force), then the small box will slip.