Part A
A 15 kg box is sitting in the bed of a pickup truck. The truck begins to accelerate at a constant rate of 3.5 m/s 2. Given that the friction between the box and the truck bed is characterized by a coefficient of kinetic friction of 0.25 and a coefficient of static friction equal to 0.55, what is the magnitude of the friction force acting on the box once the truck begins its acceleration?
System: We will treat the box as a point particle subject to external influence from the earth (gravity) and the truck bed (normal force and friction force).
Model: Point Particle Dynamics.
Approach: We begin with a sketch that represents the situation, and then create the appropriate free body diagram.
PICTURE & FBD
It is important to note that friction works to prevent movement of the interface between the box and the truck bed. The truck bed is moving forward, so friction will attempt to pull the box forward as well. If the box moves at the same rate as bed, then the interface is static. For this reason, "static" friction will actually cause motion of the box in this case!