# Rope being spun with mass

1. Oct 17, 2013

### Panphobia

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

2. Relevant equations

mv^2/r
v^2/r

3. The attempt at a solution
My physics midterm is tomorrow and I was doing review today, so I did this question, then looked at the professor's solutions. For the radius in both equations he used the length of the rope, but shouldn't the radius be Lsin20??? Also the way he positioned his y and x axis was that the Y axis was parallel to tension(EXTRA INFO).

2. Oct 17, 2013

### tiny-tim

Hi Panphobia!

(your professor can't spell "vertical"! )
I think you're misunderstanding "swings in a vertical circle".

It means that the circle is in a vertical plane.

(Lsin20° would be for a horizontal circle)

3. Oct 17, 2013

### Panphobia

Yea that is what I meant, it is pointing down. So is my professor wrong? OHHHH CRAPP didn't know it was a pendulum, sorry man this question was dumb :(

4. Oct 17, 2013

### tiny-tim

no, the ball is moving in the plane of the paper, and so the radius of the circle is L

5. Oct 17, 2013

### Panphobia

Yea I pointed out earlier that it is moving as a pendulum and not in a circle.

6. Oct 17, 2013

### tiny-tim

if the radius of the circle is L, why would you use Lsin20° ?

7. Oct 17, 2013

### Panphobia

I figured that out earlier, I thought original that it the mass was spinning about the vertical axis at angle theta, THEN the radius would be from the end of the mass to the vertical axis. But later I read the question more in depth and found that it was a pendulum, and if it is a pendulum then the radius is the length of the rope.

8. Oct 17, 2013

### tiny-tim

(isn't that what i said? )

so is it all ok now?

9. Oct 17, 2013

### Panphobia

Yes thank you for your help!