# Free body diagram with circular motion

1. Feb 27, 2010

### crybllrd

1. The problem statement, all variables and given/known data
In a circular rotating room (think Gravitron), there are two blocks stacked against the wall and do not drop. Block A is touching block B, and block B is touching the wall.

2. Relevant equations

3. The attempt at a solution
I know how to do this problem, but I am not sure about where to draw the friction vectors. My intuition tells me straight up, but it seems like my instructor did something different.

2. Feb 27, 2010

### PhanthomJay

the friction force opposes the direction of relative motion or pending relative motion between the 2 objects.

3. Feb 27, 2010

### ƒ(x)

4. Feb 27, 2010

### PhanthomJay

What's the direction of the acceleration? What force or forces provides the acceleration??

5. Feb 27, 2010

### crybllrd

Constant speed.
OK I guess I was right, I must have gotten it confused with another lecture.
Thanks a lot!

6. Feb 27, 2010

### PhanthomJay

Well, you got the direction of the friction force correctly, but not the question about the acceleration. The acceleration of objects moving in a circle is inward toward the center of the circle (centripetal aceleration). Since there is a centripetal acceleration, there must be a centripetal force acting inward for it to accelerate inward (Newton2). That centripetal force is provided by the normal force between the books or book and wall. If there is no normal force, there can be no friction (Friction force less than or equal to u_s(N))!