# Puck Collision Problem

1. Feb 1, 2008

### davidphysics

Hey. This is my first post... heres my question [Diagram Attached]:

The drawing shows a collision between two pucks on an air-hockey table. Puck A has a mass of 0.028 kg and is moving along the x axis with a velocity of +5.5 m/s. It makes a collision with puck B, which has a mass of 0.068 kg and is initially at rest. After the collision, the two pucks fly apart with the angles shown in the drawing.

Find the final speed of
(a) puck A and

(b) puck B. [both in m/s]

I am having trouble with this and although I know to use conservation of momentum, should I assume its elastic?

If you could post an explanation that would be great, but I would also like a final answer in numbers please.

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2. Feb 1, 2008

### tony873004

No one is interested in doing your homework for you, but rather in helping you understand. So don't be too disappointed with only an explanation.

Have you tried using momentum formulas and a little trig? Put in a little effort so people can see where you're having trouble.

3. Feb 1, 2008

### davidphysics

Ok sorry let me explain what I've gotten to... Basically I've set up both horizontal and vertical equations for both Po=Pf and Ko=Kf. Before I go any further, am I correct to assume that it is elastic and that the kinetic energy is conserved?

To be honest I don't wan't to use up my 4 trys on webassign, which I'm sure your familiar with.

By the way, thanks for the quick response.

4. Feb 1, 2008

### davidphysics

And when I plug in the values for "v" in those formulas I should use for example Vacos65 and Vasin65 for the horiz and vert respectively, right?

For some reason I keep getting the Vert for a and b=0, because both the initial momentum and kin energy=0. So when I solve I get Vert: Va=Vb=0... I feel like I'm doing something very stupid.

5. Feb 1, 2008

### tony873004

Yes, I'm familiar with the 4 attempts :)

Unless energy is being carried away in sound waves or by other means, I would guess your assumption is correct. Your picture isn't approved yet, so I can't be sure.

6. Feb 1, 2008

### davidphysics

O didn't realize that, please read my post that I just posted a second ago, and I'll put the image on imageshack.

POSTED:

And when I plug in the values for "v" in those formulas I should use for example Vacos65 and Vasin65 for the horiz and vert respectively, right?

For some reason I keep getting the Vert for a and b=0, because both the initial momentum and kin energy=0. So when I solve I get Vert: Va=Vb=0... I feel like I'm doing something very stupid.

Last edited: Feb 1, 2008
7. Feb 1, 2008

Bump (sorry)

8. Feb 1, 2008

### tony873004

Keep in mind your total momentum before the collision was completely on the x-axis. There was no momentum on the y-axis. So afterwards, the y-components of the momentums of your two objects should also equal zero since it is conserved.

I'm on my way out the door, so I can't help you further. But others are sure to jump in. Good luck, and welcome to the forum.

9. Feb 1, 2008

### davidphysics

Wow, then that image is very deceiving... thanks for your help -Everyone else JUMP IN! :)

Just thinking of it that makes perfect sense, now I see how these pictures mess with your mind.

Disregard everything above, yea so the Vert momentum up = vert momentum down

10. Feb 1, 2008

### hage567

But the initial momentum is not zero. It is in the vertical direction, but not in the horizontal. The initial kinetic energy is not zero either, since A is moving.

11. Feb 1, 2008

### davidphysics

I mean the initial VERT momentum. But thanks for "The initial kinetic energy is not zero either, since A is moving." Can't believe I screwed that up. I don't have any more time now, but I'll work on that tomorrow and will come back to this forum later.

Thanks! and cya

12. Feb 1, 2008

### usman27

You need to apply conservation of momentum and energy. Initial conditions do not require you to resolve the velocity vector into components. You need to resolve the velocity vector into x and y components for the final condition.

13. Feb 3, 2008

### davidphysics

Hi. I am still having trouble and would like help. Its not working out.

Thanks

14. Feb 3, 2008

### hage567

Show how you set up your equations. We can't find what you're doing wrong if you don't show us your work.

15. Feb 3, 2008

### davidphysics

Nevermind - I figured it out. I was making it much too difficult. All I had to do was Po=Pf for the horiz and vert, and solve that set of equations.