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**1. The problem statement, all variables and given/known data**

Suppose an oxygen molecule traveling at the speed of 484 m/s bounces back and forth between opposite sides of a cubical vessel 0.1 m on a side. What is the average force the molecule exerts on one of the walls of the container?

**2. Relevant equations**

F=p/t

**3. The attempt at a solution**

t=0.2 m / 484 m/s = 4.13 *10^-4 seconds

In a collision v changes direction so delta p = 2*mv

(mv was calculated in a previous question so it is known)

The average force can then be calculated as:

F = 2mv / t

My question here is, how come when calculation the time, they use 0.2 m and not 0.1 as given in the assignment? Is this because the molecule moves back and forth 0.1 m during the collision?

Second question: why do they multiply a factor 2 on the impulse? Where does this come from?