# Homework Help: Determining the angular speed of the object? I must have done something terribly wron

1. Nov 18, 2012

### riseofphoenix

Use conservation of energy to determine the angular speed of the spool shown in the figure below after the 3.00 kg bucket has fallen 4.40 m, starting from rest. The light string attached to the bucket is wrapped around the spool and does not slip as it unwinds.

I initially thought this would be a simple plug and chug problem but it turns out I was wrong...

This is where I kind of got stuck and so I kind of freestyled... -.-

initial = Iωfinal
(3)(0.6002)ω = (3)(4.402
1.8ω = 58.08ω

-.-
Help?

2. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

Wait...aren't I supposed to do something like...

PEinitial + KEinitial = PEfinal + KEfinal

3. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

Yup, that's the approach to take.

Start by identifying the source of energy for the system. How much energy is made available from that source? Where does it end up?

4. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

The spool is the source of energy for the system?

It would be the Inertia equation no? Since it's rotating according to the image..

I = mr2

5. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

A spool will spontaneously generate energy? I think you've solved the free energy problem!

No, the energy must come from some potential energy source. What's the source of potential energy that drives this system?

6. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

The bucket?

7. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

What about the bucket will make energy available?

8. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

The mass of the bucket and the water that's in it will pull the rope down which will make the spool turn.

9. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

Okay, so what form of potential energy is being exploited here? How much will be made available to the system, being converted to kinetic energy?

10. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

I have no idea :(
My teacher gave us 45 practice problems to do for the test and this one is only #14......

I tried doing...

mghfinal + (1/2)mv2final = mghfinal + (1/2)mv2final

(3)(9.8)(0) + (1/2)(3)(0)2 = (3)(-9.8)(4.40) + (1/2)(3)vf2

0 = -129.36 + 1.5v2
129.36/1.5 = v2
86.24 = v2
9.28 = v

"INCORRECT"

11. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

There is one source of potential energy available here. Your "mgh" terms describe it. What form of potential energy is associated with "mgh"? What's the magnitude of the energy made available?

There are two forms of kinetic energy associated with the system when its in motion, what are they?

12. Nov 18, 2012

### riseofphoenix

Re: Determining the angular speed of the object? I must have done something terribly

translational???

13. Nov 18, 2012

### Staff: Mentor

Re: Determining the angular speed of the object? I must have done something terribly

That's one, but be more specific. What's the other?