Question

Consider a box sitting in the back of a pickup. The pickup accelerates to the right, and because the bed of the pickup is sticky, the box does not slide around the truck when this happens.What direction is the force acting on the box due to the truck?

Answer 1

Explanation:

In the frame of the truck, the box is not moving. The force on the box by the truck also acts in the direction of motion of the truck. But it does not move because of the frictional force acting on the box due to the sticky contact surface. Thus, the net force on the box in trucks reference frame is zero.

From outside, the net force acting on the box is in direction of acceleration of the truck i.e. in the right direction.

Answer 2

The pickup accelerates towards right. The box is sticky to the pickup, thus its acceleration is the same, towards right. Its inertia (force) is oposing the acceleration, thus it is towards left. For the box not to move, it is necessary that the truck acts on it with a force towards right. (The two forces of the truck on box and the box inertia (force) equilibrate themselves).