A crate rests on the rough bed ( μs > 0 ) of a parked truck.

A crate rests on the rough bed ( μs > 0 ) of a parked truck. When the driver in the truck accelerates to the right, what is the direction of the force of friction on the crate?

A) the frictional force is zero
B) left
C) up
D) right
E) down
F) none of the above

hmm.. I tried B. left, but it marked me incorrect

Imagine that the crate is replaced by a ball instead. When the truck accelerates to the right, which way will the ball roll? Well, it would roll to the LEFT [ try it if you can]. This is the direction that the crate would try to move in, i.e to the LEFT. So the frictional force, which ALWAYS OPPOSES the direction of motion, will act to the RIGHT. So D) is correct.