# Direction of friction between disks

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1. Dec 9, 2014

### edsoneicc

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

Disk D turns with a constant clockwise angular
velocity of 30 . Disk E has a weight of 60 lb and is
initially at rest when it is brought into contact with D.
Determine the time required for disk E to attain the same
angular velocity as disk D. The coefficient of kinetic
friction between the two disks is 0.3 . Neglect the
weight of bar BC.

2. Relevant equation
Summation of forces and moments.

3. The attempt at a solution
I am confused with the direction of friction. In my calculations, the friction points to the left because the velocity where the disk meets points to the right; which resulted to a wrong answer. However, the correct free body diagram is shown above.

2. Dec 9, 2014

### Bystander

That is the force that "what" exerts on "what else?" And what is the reaction force to that?

3. Dec 9, 2014

### edsoneicc

So when I isolate disk D, the friction points to the left to counter the clockwise motion, and when I isolate disk E, it now points to the right. Is this right?

4. Dec 9, 2014

Yes.

5. Dec 9, 2014

Thanks!