# Homework Help: Help with F=mv^2/r

1. Sep 1, 2008

### phynewbie

Hello!

I have a problem with centrifugal force calculus. An arm of 20 cm long with a 100 gr weight at the end is spining with 1000 rpm.

F = mv^2/r
F = 100 x 1,000^2 / 20 = 100 x 1,000,000 / 20 = 100,000,000 / 20 = 5,000,000 Kgf?

2. Sep 1, 2008

### Hootenanny

Staff Emeritus
Re: F=mv^2/r

Welcome to PF,

You need to be careful with your quantities here. It is generally best to use S.I. units (i.e. meters, kilograms, seconds etc.) for such problems. You should also note that 1000 rpm is a frequency as opposed to a speed (which is what v is in you're equation).

3. Sep 1, 2008

### Staff: Mentor

Re: F=mv^2/r

To use that formula, you need to figure out the speed (v) of the weight. 1000 rpm (revolutions per minute) is not the speed. Hint: What's the circumference of the path, which equals one revolution?

4. Sep 1, 2008

### phynewbie

Re: F=mv^2/r

I see, cm to meters, rpm to radians/second? now i think i am closer

m = 100 gr = 0.1 Kg
1000 RPM = 104.72 radian/second
r= 20 cm = 0.2 m

F= 0.1x104.72/0.2=0.1x10,966.2784/0.2=1,096.62784/0.2=5,483.1392 Kgf or Newton?

Am i correct now?

5. Sep 1, 2008

### Staff: Mentor

Re: F=mv^2/r

You correctly found the angular speed ($\omega$, measured in radians/sec), but you still haven't found the tangential speed (v, measured in m/s).

6. Sep 1, 2008

### phynewbie

Re: F=mv^2/r

now im lost, how do i convert radians/sex to m/s?

7. Sep 1, 2008

### Redbelly98

Staff Emeritus
Re: F=mv^2/r

From the hint in post #3.

What distance does the weight travel during 1 revolution?
How long does it take the weight to travel through 1 revolution?

v = distance / time