Question

A box rests on top of a flat bed truck. The box has a mass of m = 17.0 kg. The coefficient of static friction between the box and truck is μs = 0.81 and the coefficient of kinetic friction between the box and truck is μk = 0.66.
The truck accelerates from rest to vf = 15.0 m/s in t = 14.0 s (which is slow enough that the box will not slide). What is the acceleration of the box?

Answer 1

If it does not slide, its acceleration is the same as that of the truck.

Answer 2

If you think about the forces acting on the box the forces acting are friction and wind resistance. Normally friction acts against the direction of movement, which it really is doing in this case. It seems as if it is moving against the direction of movement but if you think about in this case the wind resistance would cause the box to move to the left so friction is against the direction of motion and friction force is the force exerted by the truck on the box.

Answer 3

I do not agree with azolotor.

It is not necessary to involve air resistance to justify in which direction friction is acting.
All this would be the same in a vacuum.

Remember this important fact: Friction does not act against motion, it acts against RELATIVE motion, real or impending.
In this case, friction is a motive force, and it is the force that sets the box moving AND provides its kinetic energy.
Because of inertia of the box (and not because wind resistance), any slipping would occur towards the rear of the lorry (direction of impending relative motion). Hence friction is toward the front of the lorry.

Answer 4

I agree with you Vincent. I was just trying to present another way to think about it.