# Homework Help: Vector diagram - particle collision

1. May 31, 2012

### jsmith613

1. The problem statement, all variables and given/known data

see attached image

2. Relevant equations

3. The attempt at a solution

So I know as it is a VECTOR diagram it must either be B or C

So I checked that 1, 2, 3, 4 all pointed in the correct directions and they do in both diagrams.
I chose B (but at random) and the answer was C.

thanks

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• ###### VectorQuestion.png
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2. May 31, 2012

### Infinitum

Hi jsmith!!

Use the conservation of momentum, this tells you the initial total momentum should be equal to the final total momentum. In which of the options, do you see this??

3. May 31, 2012

### dimension10

First of all, is this supposed to be a Classical Mechanics question or a Quantum mechanics question? Because if it is Classical mechanics, then the question is impossible. But in Quantum mechanics, it is possible.

4. May 31, 2012

### jsmith613

??? classical mechanism

why is it impossibel?

5. May 31, 2012

### dimension10

I'm assuming the mass is constant?

6. May 31, 2012

### jsmith613

momentum is consereved in B??

7. May 31, 2012

### dimension10

How is momentum conserved in B

8. May 31, 2012

### jsmith613

resultant momentum of both is in the same direciton, no?

#### Attached Files:

• ###### VectorQuestion.png
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16.2 KB
Views:
126
9. May 31, 2012

### dimension10

In classical mechanics, you have $m_1v_{1i}+m_2v_{2i}=m_1v_{1f}+m_2v_{2f}$ but that does not hold in the question, so I think it is a Quantum mechanics question...

10. May 31, 2012

### Infinitum

Uh nope, it isn't. And you can't be sure about the direction in B as the exact magnitude and direction of original aren't given.

Also, direction isn't the only necessary factor. The magnitude remains same too. Observe C carefully.

11. May 31, 2012

### jsmith613

I just reaslied that i have to apply head-to-tail rule
if I apply this it ALWAYS works for ALL questions that have bothered me in the past
:)

12. May 31, 2012

### Infinitum

I believe classical mechanics does hold in the question....

13. May 31, 2012

### Infinitum

Yes!! That's it!

14. May 31, 2012

### dimension10

Then I don't think I understood the question properly. What I understand of it is:

$$v_{1i}=1$$
$$v_{2i}=2$$
$$v_{1f}=3$$
$$v_{2f}=4$$
$$m_1,m_2=\mbox{Constant}$$

15. May 31, 2012

### Infinitum

You didn't read the question properly

1, 2, 3, 4 are the momenta before and after collision, not the velocities.

16. May 31, 2012

### dimension10

Oh! I didn't see that!