Consider a box in the back of a pickup truck. If the truck accelerates too rapidly, what happens to the box? Explain why this happens based on physical models.

Answer 1

As long as the truck and box move together at constant velocity, there is no horizontal force on the box.
The box has momentum m v where m is the box mass and v is the box velocity.
m, the mass of the box, does not change in this experiment.
If there is no horizontal force on the box when the truck accelerates, then v does not change either, and the box just keeps on moving at v while the truck steams off toward the horizon.
This assumes that the box does not stick to the truck bed and that there is very good grease between the bottom of the box and the truck bed. In real life there would be some friction force trying to keep the box up with the truck acceleration, but if the truck accelerates fast, as we all know, Newton wins and the box slams against the tailgate.