Kinetic theory of gases momentum

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The discussion centers on the momentum change of a gas particle during an elastic collision with a wall. The equation for momentum change is clarified as delta px = pf - pi, leading to the expression delta px = -2mvx when considering the reversal of velocity upon impact. The second negative arises from the initial velocity being negative after the collision, indicating a change in direction. Participants confirm that understanding this concept helps clarify the momentum calculations. The explanation emphasizes the relationship between velocity direction and momentum in elastic collisions.
lukasz08
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p=momentum=(mass*volume)

delta px = pf - pi
mvf - mvi
mvx - (-mvx) <<<<---- where does this second negative come from ? what am i missing here?
2mvx


in my book it shows that
delta px = - mvx - (mvx) = -2mvx
 
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When a particle bounces elastically off a wall, its velocity (and momentum) changes direction. Does that help?
 
yes i understand that so the second negative appears after the particle has hit the wall resulting in the 2mvx ?
 
ok i got it mvi = -mvf therefore -mvi - mvf = -2mvx
 

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