I Momentum Conservation in Special Relativity: mwu

Feynmanisthegoat
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the vertical component of momentum in the (b) figure before the collision is ∆p = mwu. Can someone please explain why we take the mass in the world component and not the u component
 

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I can't see anything there.
 
Ya man I am sorry this is my first post so I don't know what went wrong but it's figure 16.4 from the Feynman lectures on physics.
 
Feynmanisthegoat said:
Ya man I am sorry this is my first post so I don't know what went wrong but it's figure 16.4 from the Feynman lectures on physics.
You're better to find some way to post it here.
 

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