# Confusion about tension and pulleys

1. Mar 29, 2013

### hgfhh123

1. The problem statement, all variables and given/known data

I am a little confused about the concept of tension as it relates to pulleys. I am studying torque right now, and it says in the textbook that the tension in the rope around a pulley produces a torque that causes the pulley to turn. I don't fully understand this. I thought that tension was a pair of opposing forces that acted throughout the rope at each point in the rope. However, I don't understand how the same force would cause the pulley to turn. Since there is no friction between the rope and pulley, I don't understand why the pulley would turn in the first place.

2. Relevant equations

3. The attempt at a solution

2. Mar 29, 2013

### ehild

There is static friction between the rope and pulley. That makes the pulley move together with the rope. The two pieces of the rope exert torques RTa and RTb on the pulley at points A and B. (R is the radius of the pulley). If the pulley is massless zero torque is enough to rotate it with a certain angular acceleration, so the tension is the same in both pieces of the rope. If the pulley has got I moment of inertia, the tensions Ta and Tb must be different to rotate the pulley with angular acceleration α: α=R(Tb-Ta)/I.

ehild

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3. Mar 29, 2013

### bapul4u

Its the frictional force between the string and pulley that cause the pulley to rotate when the string moves..