# Two blocks stacked centripetal motion

1. Jan 27, 2014

### thatoneguy6531

1. The problem statement, all variables and given/known data
Mass 1(2.0kg) sits on top of mass 2(5 kg), which rests on a frictionless surface. The coefficient of static friction between mass 1 and 2 is 0.30. A string of length 5.0m is tied to mass 2, and both masses are swung around in a horizontal circle. Calculate a) the max speed of the masses and b) the tension in the string.

2. Relevant equations
Fc=mv^2/r

3. The attempt at a solution
I got the correct answer to a) by assuming static friction was the centripetal force but I don't understand why it is. Wouldn't the direction of the static friction be perpendicular to the rope to oppose mass 1 from falling of mass 2 as they're swung? And for b) I don't know what to do because if I just use T as the centripetal force for mass 2 I don't have its mass so I cant calculate it.

2. Jan 27, 2014

### lightgrav

In this scenario, envision that the blocks' speed around the circle increases very gradually,
so that the rope pulls "essentially" toward the center of the circular path; ignore air drag!

a) What ELSE might deflect the top block from straight-line path to circular path?
(nothing else touching it but bottom block, only gravity as field Force)
b) block #2 has 5kg mass. the rope is the only thing pulling the 2 blocks inward.

3. Jan 27, 2014

### thatoneguy6531

Wow. I feel like an idiot now... I didn't realize the masses were given even after typing that out. Thanks though.

4. Jan 27, 2014

### lightgrav

by the way, recall that ropes always pull inward along their length;
a rope pulling bottom block in a straight line would also need friction applied to top block, pointing along the rope.
and in the same strength ratio.

5. Jan 27, 2014

### thatoneguy6531

By pointing along the rope do you mean in the outward direction?

6. Jan 27, 2014

### lightgrav

same direction that the rope pulls:
the rope pulls the bottom block inward, right?
what pulls the top block inward? nothing touching it except bottom block, by friction.

7. Jan 27, 2014

Ok thanks.