# Homework Help: Moment of inertia and angular speed

1. May 2, 2010

### pat666

1. The problem statement, all variables and given/known data
Consider a particle of mass = 19.0 revolving around an axis with angular speed . The perpendicular distance from the particle to the axis is = 0.500 .

2. Relevant equations

I=mr^2
K=0.5Iw^2

3. The attempt at a solution
I=4.75
K=1796 which is 0.5*4.75*27.5^2

i know that w is right, this is one of those stupid mastering physics questions.

2. May 2, 2010

### pgardn

What exactly is the question?

19 what and 0.5 what?

3. May 2, 2010

### pat666

19kg and 0.5m sorry the units were in a picture and didnt copy properly.

4. May 2, 2010

### pgardn

So what do they want to know about the 19 kg mass?

Its moment of inertia?

5. May 2, 2010

### pat666

sorry i should really have checked this before posting. i want to know the kinetic energy of the rotating particle

6. May 2, 2010

7. May 2, 2010

### pgardn

Ok then...
Have they given you any info about the rate of rotation? omega? what you are calling w...

ANd I see the poster above me is assuming its is 27.5 rad/s based on what you wrote in the solution?

8. May 2, 2010

### pat666

i had to figure that out in an earlier question and the answer is definitely 27.5rad/s. but the KE is not what i calculated.

9. May 2, 2010

### pgardn

Well if you know the answer you got is wrong, then I or omega or both are wrong. Do you have the entire question?

10. May 2, 2010

### pat666

That is the entire question, i calculated I from the given info and omega is definitely right???????????????????????????????????????????

11. May 2, 2010

### pgardn

So if we are assuming the 19 kg mass is just a point mass, and it making a ciruclar rotation about an axis that is 0.5 meters in radius, then the moment of inertia looks correct to me. So how did you arrive at the angular speed? I dont know if omega or the angular speed is correct...

Do you have the entire question?

If you know the answer you got for the kinetic energy is wrong, then again, either I, or omega, or both are wrong. Because your math to figure out the kinetic energy looks correct to me using your numbers... Just trying to narrrow down what could have gone wrong.

12. May 2, 2010

### pat666

ok this is my fault w is actually 55rad/s v is 27.5m/s sorry

13. May 2, 2010

### pgardn

No problemo.

As long as you figure out what you did incorrectly then all is good :)

Mistakes can be very instructive. And sometimes its just a small thing like forgetting to multply by 2. It might make you feel a whole lot better to know its just a math error and not a conceptual error.