Could someone please clarify this problem for me... Assume we have a box sitting on the back of a truck. The box can be modeled as a particle, and there is friction between the box and the surface of the truck. Also, lets assume the truck is accelerating slow enough that the block does not slide - IE it remains stationary relative to an observer fixed in the frame of the truck. If we draw the FBD, we have two forces in the vertical direction. The weight and the normal which are obviously equal in magnitude and opposite in direction. The only horizontal force we have acting on the block is static friction. In this example we can use the simple definition of work, being the product of force and displacement. However, I don't believe its necessary correct to say that the static friction is doing work on the box, because technically the displacement should be zero, as the static friction causes no displacement of the block relative to the frame in which it is being applied. So is it more correct to say the truck (I guess the engine or the torque on the wheel) does the work, or does the static friction do the work. We had a big debate about this a few months back at one of the TA meetings. I just wondered what your input was.... One analogy made was something along the lines of how magnetic fields do no work on charged particles, yet they cause the particles to accelerate. Thanks to anyone who responds with some sort of plausible argument!