# Question on rotational motion

1. Oct 26, 2009

### semc

A block of mass 2kg and 6kg are connected by a massless string over a pulley in the shape of a solid disk having radius R=0.25m and mass 10kg.These blocks are allowed to move on a fixed wedge of angle 30$$\circ$$ the coefficient of kinetic friction block is 0.36. Find the acceleration of the blocks.

Hmm how do you solve questions that involve pulley? The second part of the question ask to find the tension in the string and the tension is different. Why is that so? Shouldn't the tension be same since the string is connected?

2. Oct 26, 2009

### Staff: Mentor

In addition to your force equations for each mass, you'll need a torque equation for the pulley.

The tension would be the same throughout the string if the pulley were massless, but since the pulley has mass the tension on either side of the pulley will be different. (If the tension was the same, there would be no net torque to accelerate the pulley.)

3. Oct 26, 2009

### semc

So if the blocks keep sliding to a point whereby the acceleration is zero then the tension will be the same? Thanks doc

4. Oct 26, 2009

### Staff: Mentor

I'm not quite sure what you mean. As the blocks slide, the acceleration remains constant. (Until one of the blocks slams into something, of course.)

5. Oct 26, 2009

### semc

I mean will there be a point where the net force acting on the blocks are zero so the tension will be the same?

6. Oct 26, 2009

### Staff: Mentor

No. The net force and the resulting acceleration is determined by the masses and the angle of the incline. By setting up and solving your force equations (from Newton's 2nd law) you'll be able to determine the acceleration. For a given set of conditions, the acceleration is constant.