How Do You Calculate Centripetal Force for Circular Motion?

[joe465]

Homework Statement

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

Homework Equations

MV2/r
2*pie*r

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

 

[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.

 

[Doc Al]

Looks good to me.

 

[Doc Al]

Your calculation of the tangential velocity v is incorrect,

Why do you say that?

 

[rock.freak667]

Why do you say that?

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

 

[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.

 

[PeterO]

Homework Statement

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

Homework Equations

MV2/r
2*pie*r

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

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