Work Force Questions: Explaining Why Work Done is Zero

[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.

 

[radou]

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

 

[parwana]

W= Fx

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

 

[rsk]

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

 

[radou]

W= Fx

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

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?

 

[parwana]

work will be zero

 

[parwana]

can anyone help and explain in detail

 

[PhanthomJay]

can anyone help and explain in detail

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.

 

[parwana]

^ that's what I don't get, why doesn't 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?

 

[OlderDan]

^ that's what I don't get, why doesn't 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?

What does static mean?

 

[PhanthomJay]

^ that's what I don't get, why doesn't 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?

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?