# Homework Help: Another Circular motion question

1. Apr 6, 2005

### laker_gurl3

thanks so much for any help..work and formula used would be appreciated...

A record of diameter 30cm roates on a turntable at 33.3r/min.

a.) How fast is the outside edge of the record moving?

b.) how many times as fast would it move if the frequency were raised to 78 r/min.?

2. Apr 6, 2005

### Gale

a) if the whole record moves at 33.3 revs per min, then a point on the outside edge does as well, which means that point has to go around the circumfrence (C) of the record within the minute, your speed= C/min.

b) same idea as part a, change the frequecny, and then compare.

3. Apr 6, 2005

### laker_gurl3

so i did this out..for A.) i got 3138m/s
for B.) i got 7351m/s, therefore it's 2.34 times as fast...is that correct?

4. Apr 7, 2005

### Gale

ok ya, i mean it had to go around the circumfrence 33.3 times per minute, but thats what you did, so good. Those are the numbers i got, except your units are wrong, its cm/min not m/s.

5. Apr 7, 2005

### dextercioby

Gale,i get double for the first number...

$$v=\omega R \ [m \ s^{-1}]$$

R=0.3m

$$\omega=2\pi \nu=\left(2\pi \ \mbox{rad}\right) \left(\frac{33.3}{60} \ Hz \right) \simeq \frac{200\pi}{180} \mbox{rad} \ s^{-1}$$

Ergo

$$v\simeq \frac{\pi}{3} m \ s^{-1} = \frac{6000\pi}{3} cm \ (min)^{-1}$$

which is double that the # you referred to in post #4.

Daniel.

EDIT:As Gale pointed out,the radius is only 0.15m.So that explains the incorrect result i got.

Last edited: Apr 7, 2005
6. Apr 7, 2005

### Gale

you got double because you let R=.3 whereas .3 is the diameter.

7. Apr 7, 2005

### dextercioby

****. :rofl: :tongue2:

Daniel.