# Homework Help: Determining the work done on the puck using conservation of angular momentum? Help!

1. Nov 19, 2012

### riseofphoenix

Determining the work done on the puck using conservation of angular momentum?? Help!

This is what I did...

1) Given

mpuck = 0.300 kg
rinitial = 0.4 m
vinitial = 0.6 m/s

mpuck = 0.300 kg
rfinal = 0.15 m
vfinal = ____ m/s

ƩW = KEfinal - KEinitial

2) KEinitial = (1/2)mv2
KEinitial = (1/2)(0.300)(0.62)
KEinitial = 0.054 J

3) KEinitial = (1/2)mv2
KEinitial = (1/2)(0.300)v2
KEinitial = 0.15v2

4) Find v - Angular momentum is conserved due to a lack of friction. The puck goes from 40 cm to 15 cm, so it has a different angular momentum.

Linitial = Lfinal
initial = Iωfinal
[STRIKE](0.300)[/STRIKE](0.4)(0.6)2 = [STRIKE](0.300)[/STRIKE](0.15)v2
(0.4)(0.6)2 = (0.15)v2
0.144/0.15 = v2
0.96 = v2
0.979 = v

5) Plug v back into Net work equation

ƩW = KEfinal - KEinitial
ƩW = (0.979) - (0.054)
ƩW = 0.925 J

Which is wrong...
:(
Help!

2. Nov 19, 2012

### TSny

Re: Determining the work done on the puck using conservation of angular momentum?? He

You want those squares?

3. Nov 19, 2012

### riseofphoenix

Re: Determining the work done on the puck using conservation of angular momentum?? He

Oh...

But even without them I still get the wrong answer.

(0.4)(0.6) = (0.15)vfinal

1.6 = vfinal

4) KEfinal = 0.15(1.62)
KEfinal = 0.15(2.56)
KEfinal = 0.384

So,

W = (0.384) - (0.054) = 0.33

Still wrong...

4. Nov 19, 2012

### riseofphoenix

Re: Determining the work done on the puck using conservation of angular momentum?? He

brb in an hour!!

5. Nov 19, 2012

### TSny

Re: Determining the work done on the puck using conservation of angular momentum?? He

Is 0.15 m correct for the final radius? Another reading of the question might help.

6. Nov 19, 2012

### riseofphoenix

Re: Determining the work done on the puck using conservation of angular momentum?? He

Final radius would be... .4 m - .15 = .25!

Ohhhhh thank you!

Last edited: Nov 19, 2012