# Pulley

• Sep 18th 2006, 09:26 AM
ThePerfectHacker
Pulley
When there is a cable going through a frictionless pulley what are the forces on it?

Area the lines of actions are algined with the cables?

Area the two tensions equal from both sides of the pulley?
• Sep 18th 2006, 10:49 AM
topsquark
Quote:

Originally Posted by ThePerfectHacker
When there is a cable going through a frictionless pulley what are the forces on it?

An "ideal" pulley has no mass, does not rotate as the cord/rope/wire/string is being pulled through it, and does not provide friction for the cord/rope/wire/string. There ARE forces on it, however, but they are action-reaction pairs with the forces that are connecting the pulley to whatever surface it is attached to. As such we rarely talk about forces on the pulley since these action-reaction pairs do not affect the string that is moving over the pulley.

Quote:

Originally Posted by ThePerfectHacker
Are the lines of actions are aligned with the cables?

I'm not quite sure what you mean. Taking a guess at it, an ideal pulley does nothing except change the direction of the tension along the cable. And, of course, all tensions are directly in line with the direction of the cable.

Quote:

Originally Posted by ThePerfectHacker
Area the two tensions equal from both sides of the pulley?

(The pulley actually never comes into this.) If we are considering an ideal string, yes. The tension on an ideal string is the same all along its length. If the string has mass, all bets are off on this because the string accelerates along with whatever is on either end, and it takes force to do this.

-Dan