# Tension at Rotating Pulleys

1. Dec 11, 2008

### ximath

Dear All,

Suppose there is a a pulley with mass and a massless cord around it, if pulley is rotating then I have learned that there are different tensions on either side of the pulley. I am having some trouble visualizing it. Moreover, if there are two different tensions on the cords, where exactly they are separated from each other ?

I have searched a little bit and read that thread several times :

There is a statement on that thread which is;

Could you please explain this further ? Why and how it can have different tension on either side, if a string is touching somewhere and where these tensions are seperated ?

2. Dec 11, 2008

### tiny-tim

oooh, that's me!

the reason the tension is usually the same on either side of a point on a string is that Newton's third law requires that the tensions are equal and opposite

but if you have an outside force … in this case, friction … then Newton's third law is irrelevant (to be precise, it just says that the string exerts a force on the pulley ), and the friction acts in the same direction on each side of the string

and so on one side it acts with the tension, and on the other it acts against the tension