# Homework Help: How does static friction do work?

1. Mar 16, 2014

### c_programmer

1. The problem statement, all variables and given/known data
Static friction can never do work on an object.
a. True
b. False

2. Relevant equations
The answer is False, indicating that static friction can do work.

3. The attempt at a solution
W=fd, when d=0 w=0 regardless of the force.

How does this work? Or is the online homework wrong.

2. Mar 16, 2014

### c_programmer

I think I might have an answer. Imagine a box on top of another, there is a large static friction coefficient between the two boxes and none on the floor. When the bottom box is moved the static friction will cause the box to move, thus doing work. Is this correct?

3. Mar 17, 2014

### consciousness

Yes, I thought of the same counter example. However in the reference frame of the contact surface, static friction can't do work.

4. Mar 17, 2014

### Staff: Mentor

Work always depends on the reference frame.

Chet

5. Mar 17, 2014

### VictorH

Why does work depend of the reference frame? If we do work isn't it the change of kinetic energy equal for all observers? The velocity vectors may change, of course, but the difference shouldn't for two inertial frames. Is it different for non-inertial frames?

6. Mar 17, 2014

### Staff: Mentor

Yes. You hit the nail on the head. Consider the case of a box on a conveyor belt, and the speed of the belt is increasing with time. So the frictional force is causing the box to accelerate, and the box is gaining kinetic energy. Now consider this same situation from the perspective of an accelerating frame of reference that is moving with the box. From this frame of reference, the frictional force is not doing any work, and the box is not accelerating. The frictional force is being balanced by the apparent horizontal body force acting on the box (associated with the acceleration).

Chet