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In a cube with side length L, consider a gas molecule with x-component of velocity being $C_x$. The question arises while dealing to calculate rate at which molecule transfers momentum to the wall. Let $m$ be the mass of gas molecule. While the change in momentum after the collision with the wall is $2mC_x$, my question remains for the time that is considered to calculate the rate.

According to the source of Cambridge International A levels Physics course book, the time used to calculate rate is time between collisions but according to me the rate of change in momentum should consider the time the molecule is in contact with the wall, though negligibly small. How can the time taken for successive collisions be used?

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When we measure pressure we are measuring it on timescales that are much longer than the collision frequency of the gas molecules. So we aren't that concerned with the details of the force exerted on an individual molecule in an individual collision. We want the average force over multiple collisions.

This average force is simply the total momentum change divided by time. So if for example there are $N$ collisions in one second then the total momentum change per second is just $2Nmv$, and that gives our average force. We don't need to know exactly what goes on during any individual collision to do this calculation.

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