# Quick M1 conservation of momentum QWatch

#1
Hiya, just did this and got it wrong

A moving particle P collides with a particle Q, which is akso moving. Particle P hs a mass of 0.25kg, with velocities immediatly before and after the collision respectivly being [3,6] ms^-1 and [-1,4] ms^-1 (these are column vectors, I don't know latex )

The mass of Q is 0.1kg. Immediatly after the collision, Q moves with velocity [5,2]ms^-1. Find the velocity immediatly before the collision.

I just did this as best I could and got [-27.5,-3]

The -3 is right

My method was

0.25 x [3,6] + 0.1u = 0.25 x [-1,4] + 0.1 x [5,2]

0.1u = [-2,1.2] - [0.75,1.5]

u = [-2.75,-0.3] / 0.1

u = [-27.5,-3]

I have missed a few lines out

Can anyone see where I have gone wrong, please?

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9 years ago
#2
How do you know that Particle B goes in the other direction? It could just continue in the same direction. Which would mean 1.4ms/1 is positive.
0
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#3
I'm not sure, I'm just going by the velocities that have been given
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9 years ago
#4
(Original post by Loz17)

0.25 x [3,6] + 0.1u = 0.25 x [-1,4] + 0.1 x [5,2]

[0.75,1.5] + 0.1u = [-2,1.2] - [0.75,1.5]
I can't see where you got the bits in bold from.
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#5
(Original post by ghostwalker)
I can't see where you got the bits in bold from.
Ok

from 0.25 x [3,6] + 0.1u = 0.25 x [-1,4] + 0.1 x [5,2]...

[0.75,1.5] + 0.1u = [-2.5,1] + [0.5,0.2]

>> 0.1u = [-2,1.2] - [0.75,1.5]

That was the line I missed out, and seen an error in my typing in the 1st post,which i didn't do in my working
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9 years ago
#6
(Original post by Loz17)
Ok

from 0.25 x [3,6] + 0.1u = 0.25 x [-1,4] + 0.1 x [5,2]...

[0.75,1.5] + 0.1u = [-2.5,1] + [0.5,0.2]

>> 0.1u = [-2,1.2] - [0.75,1.5]

That was the line I missed out, and seen an error in my typing in the 1st post,which i didn't do in my working
Evaluating the bit in bold gives [-0.25,1]. I think that's where you error lies.
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#7
(Original post by ghostwalker)
Evaluating the bit in bold gives [-0.25,1]. I think that's where you error lies.
Aww so simple, hehe ty, rep for you
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