# Work force question

1. Oct 26, 2006

### parwana

In most circumstances, the normal force acting on an object and the force of static friction do no work on the object. However, the reason that the work is zero is different for the two cases. In each case, explain why the work done by the force is zero.

2. Oct 26, 2006

Look at the definition of work. What does the work equal if the force and displacement are perpendicular?

3. Oct 26, 2006

### parwana

W= Fx

Thats work. But there also can be an angle between them

4. Oct 26, 2006

### rsk

Yes but what is the relation? What happens when that angle is 90 degrees?

5. Oct 26, 2006

Yes, that's work when the angle between the vectors F (the force) and x (the displacement) equals zero. But, when the angle is non zero, as in your case, the work is given with W = F x cos(angle). Now, as rfk asked, what happens when the angle equals 90 degrees?

6. Oct 26, 2006

### parwana

work will be zero

7. Oct 26, 2006

### parwana

can anyone help and explain in detail

8. Oct 26, 2006

### PhanthomJay

You have already I think determined from the help of the responders that the work is zero when the displacement is 90 degrees to the force. Generally the normal force is 90 degrees to the displacemnt vector, so it does no work. Like when you push a block along a level table, the normal force between the block and table acts straight up on the block, and the block moves perpendicularly to it (to the right). so the normal force does no work (W=Ndcos90 = 0). But you still have another question to answer. Why does the static friction force generally not do any work? It's for a different reason.

9. Oct 26, 2006

### parwana

^ thats what I dont get, why doesnt static friction not do any work. Usually friction is in the opposite direction as the movement right. Now how can that make it not work?

10. Oct 26, 2006

### OlderDan

What does static mean?

11. Oct 26, 2006

### PhanthomJay

When an object is moving, it is subject to kinetic friction, not static friction. Kinetic friction does lots of work, since the kinetic friction force is in the same direction of the displacement.. But what can you say about static friction force and displacemnt?