# Calculating Centrepetal Force

1. Aug 27, 2011

### joe465

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

Calculate the centrepetal force required to rotate a3kg object in a circle at a radius of 3m at one revolution per second.

2. Relevant equations

MV2/r
2*pie*r

3. The attempt at a solution

First i presume i must convert the revolution per second into metres per second.

Calculate the circumference.

2*pie*r
2*pie*3
18.84955592153876m

Since its one revolution per second then it would mean:

18.84955592153876ms-1

Now for centrepetal force:

mv2/r

3*18.84955592153876 squared/3

1065.9172753176507952118585470128 / 3

Centrepetal force = 355.31N (2dp)

I hope this is right, the circular motion stuff still has never sunk in

Thanks, Joe

2. Aug 27, 2011

### rock.freak667

Your calculation of the tangential velocity v is incorrect, you should use v= rω and then put that into F=mv2/r or F=mω2r for a more direct approach.

3. Aug 27, 2011

### Staff: Mentor

Looks good to me.

4. Aug 27, 2011

### Staff: Mentor

Why do you say that?

5. Aug 27, 2011

### rock.freak667

Nevermind, my bad, I used rpm instead of what it was rps.

6. Aug 27, 2011

### Lobezno

That's quite right. I would have written 355.3 N, but that's just my "three significant figures" training.

MIT Open Courseware has an excellent lecture series on Classical Mechanics, with a great video on circular motion.

7. Aug 27, 2011

### PeterO

There are many texts and references that show it, but if you look at the following wiki reference - the formulas right at the start - you will see that there is another formula for centripetal force that can be used in exactly this situation - when you know the Period of rotation rather than how fast it is travelling. That means you can't make a mistake calculating v, because you never calculate it!

http://en.wikipedia.org/wiki/Centripetal_force