# Homework Help: Find speed and direction of a particle after collision

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1. Aug 4, 2017

### jfnn

SOLVED

THANK YOU

Last edited by a moderator: Aug 4, 2017
2. Aug 4, 2017

### CWatters

Yes conservation of momentum is correct approach. I haven't checked your working but you seemed to jump about a bit. The following would be better order..

Then keep going with..
So now you have the x and y components of velocity.

Yes it's the correct way.

Make a drawing showing the components V2x, V2y and the resultant V2. You used Pythagoras so you know looks like a right angled triangle.

Do you know how to work out angles using Sin() , Cos() or Tan() ?

3. Aug 4, 2017

### CWatters

Re...

It would have been better to state at the beginning which directions you will define as positive (eg up and east).

In addition to using trig to find the angle with the horizontal.... You know that one part went directly west. So what does conservation of momentum tell you about the compass direction of the other part? The V2x is positive so we know it's not got a westerly component.

4. Aug 4, 2017

### jfnn

I know how to do the relationships I think. So it would be theta = tan^-1(v2y'/v2x')?

Therefore, I would get theta=tan^-1 (112.5/24) = 78 deg

However, how do I know what the direction is? Is it from the horizontal axis, so would be N or E or is it from another axis? How can I be sure?

Thank you so much!

5. Aug 4, 2017

### jfnn

OH! So since the V2x' is positive, it means that it is going east, and since the y is positive as well (it is north) I can then say it is 78 deg N of E?

6. Aug 4, 2017

### CWatters

Correct. If it only splits into two parts and one goes exactly west the other must have a component that is exactly east. That way momentum in the Z axis is conserved.

No that's not correct. Y was the vertical axis. So your 78 degrees is the angle it makes with the horizontal. I have made a diagram. Everything happens in the XY plane..

7. Aug 4, 2017

### CWatters

PS I still haven't checked your calculations (just the method).

8. Aug 4, 2017

### jfnn

Ah okay, that makes sense now. But the angle can still be 78 degrees N of E as that implies it is north of the horizontal right?

9. Aug 4, 2017

### jfnn

Okay. Let me know if you do check the calculations and see any errors. I double checked it so hoping it is correct.

10. Aug 4, 2017

### CWatters

No "North of east" would mean it had a component in the Z axis. It must go exactly east and upwards.

11. Aug 5, 2017

### jfnn

ahh okay.

Makes great sense now. Thank you!

12. Aug 5, 2017

### CWatters

Just a heads up... Try not to delete the original post in future because it makes it less useful for other people searching the forum for help with similar problems.