To obtain force from momentum we use the formula change in momentum/time. The time in the equation refers to the amount of time the force is exerted on the gas molecules. So when considering the kinetic theory of gases we are taught that F ∝ change in momentum/ time so F ∝ 2mv/(2d/v) and hence F ∝ mv2/d. The thing that I don't understand is why the time is 2d/v. 2d/v gives us the time taken for the molecule to travel from one wall to the other. But the "time" in the change in momentum/time equation shouldn't be the time taken for the molecule to travel from one wall to another and instead should be the time the molecule spends in collision with the wall instead. So why is the 2d/v always used?