# Rotational Motion HW Help

1. Nov 26, 2007

### mmbruns

A skater holds her arms outstretched as she spins at 120rpm. What is the speed of her hands if they are 130cm apart?

2. Nov 26, 2007

### Staff: Mentor

3. Nov 26, 2007

### mmbruns

Work.

From what I understand in class

from my calculations

120rpm = 2rps
and
d=.130m, so C= .4082m

And I multiplied

2*.4082 to find the total distance in one revolution

=.8164m in one sec

Now I'm stuck

4. Nov 26, 2007

### mmbruns

I Got It!

5. Nov 26, 2007

### mmbruns

Nope didn't get it....I tried doing the equation v=w(pi)f wrong! Gerrrrr

6. Nov 26, 2007

### Staff: Mentor

Well one has the answer - =.8164m in one sec => 0.8164 m/s!

rpm is revolutions per minute and rps is revolutions per second.

Now in one revolution, a point on the circumference travels 2$\pi$ radians, which is a measure of angle.

So 2 rps = 2 * 2$\pi$ rad/s = 12.57 rad/s, which is $\omega$, the angular velocity.

Now apply the formula - Vt=Angular velocity * radius.

7. Nov 26, 2007

### mmbruns

The answer I get from that is 0.81705

8. Nov 26, 2007

### Staff: Mentor

Yes. There is round off accuracy involved in both calculations.

9. Nov 26, 2007

### mmbruns

I understand that, but that is not the answer...I tried it already.

10. Nov 26, 2007

### mmbruns

For Future reference, the equation for this problem is V=[2(pi)r]/t

t is found per second
i.e. 60/120

11. Nov 1, 2009

### KatlynEdwards

Hmm the answer of 0.82 doesn't seem to be correct according to Mastering Physics. I've followed the steps provided, and I still can't seem to figure out where we're going wrong. Any help would be appreciated.