The force exerted by a single molecule In the chapter of the kinetic theory of gases in a basics undergraduate physics book, it says the force exerted by a single molecule on a wall is calculated as $F = \frac{\Delta p}{\Delta t} = \frac{2mv_x}{2L/{v_x}}$, where $2L/{v_x}$ is the time the molecule takes to travel to the opposite wall and back again (a distance $2L$) at speed $v_x$.
However, I don't really understand why we put in $2L/{v_x}$ for $\Delta t$ in the formula. Shouldn't we put instead "the actual time of the collision/contact" between the molecule and the wall for $\Delta t$? I cannot find any reasonable answer to this in basic physics books. Could anyone explain this in a reasonable way? 

 A: In thermodynamics it makes little sense to speak about the real transient force that a molecule exerts on molecules of the wall. Instead we are more interested in pressure, i.e. the force averaged in space and time.
The average force over the time $\Delta t$ is the same as the change of the momentum divided by the time interval.
$$
f_{avg} = \frac1{\Delta t}\int fdt = \frac1{\Delta t}\int \frac{dp}{dt}dt = \frac{\Delta p}{\Delta t}
$$
So if you consider the force that one molecule exerts on a wall by shuttling back and forth, the plot will look like this:

However, instead of going for  the exact shape of this function or even maximal value, we are interested in the average value. The time between spikes is the time the molecule needs to travel through the box twice $\Delta t=2L/v_x$, the integral of one spike is the momentum molecule gains during collision $\Delta p=2mv_x$. The rest you know.
A: 
Shouldn't we put instead "the actual time of the collision/contact"
  between the molecule and the wall for $\Delta t$?

I believe the kinetic theory of gases assumes the gas molecules consist of hard spheres whose collisions with the walls are perfectly elastic. That being the case, the assumption may be that the amount of time the molecule is in contact with the wall will be negligible compared to the time it takes to go from one wall to the other.  In other words, the time it takes to change direction when colliding with the walls is considered negligible compared to the time it takes to traverse the distance between the walls. 
Hope this helps.
A: You can think of the equation as meaning how hard the molecule hits the wall, multiplied by how frequently it hits it. The time taken to travel to the other side and back is just the gap between successive hits.
