# Homework Help: Rigid body motion problem

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1. Apr 8, 2015

### physaru86

1. A grinding wheel of mass 5 kg and diameter 0.20 m is rotating with an angular speed of 100 rad/s. Calculate its kinetic energy. Through what distance would it have to be dropped in free fall to acquire this kinetic energy? m = 5 kg, r = 0.20/2 m = 0.10 m, g = 10 m/s2, ω = 100 rad/s

2. v = r*ω, kinetic energy = (1/2)*m*v2

3. v = (0.10 m)*(100 rad/s) = 10 m/s, kinetic energy = (1/2)*(5 kg)*(10 m/s)2 = 250 kgm2/s2.................. is this right?

2. Apr 8, 2015

### PWiz

Yes it is. Just calculate the second part, it should be easy for you now.

3. Apr 8, 2015

### physaru86

what formula to use?

4. Apr 8, 2015

### PWiz

Do you know the formula for gravitational potential energy?

5. Apr 8, 2015

### physaru86

U = m*g*h....?

6. Apr 8, 2015

### PWiz

Correct. Now for what value of $h$ does this expression equal 250 joules?

7. Apr 8, 2015

### physaru86

um... is it a question or are you helping me with the solution..?

8. Apr 8, 2015

### PWiz

We don't provide direct answers to homework questions here. And I wouldn't be posting in this thread trying to help you if I didn't know the concept myself :)

9. Apr 8, 2015

### physaru86

Is it related to
Energy conservation during free-fall

10. Apr 8, 2015

### PWiz

Not exactly. It is more to do with energy transformations. The question is simply asking you from which height you'd have to drop this mass for it to have the same kinetic energy when it's about to strike the surface as the value you have calculated (that's what I think the question is asking, since it appears that the question has not been posted verbatim).

11. Apr 8, 2015

### physaru86

m = 5 kg, g = 10 m/s2, m*g*h = 250 joules, h = 250/5*10 m, h = 5 m...... ?

12. Apr 8, 2015

### PWiz

That's right.

13. Apr 8, 2015

### physaru86

So the problem is solved, right.......? thanks

14. Apr 8, 2015

### PWiz

Your welcome

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