# Bernoulli's principle / ref frames

1. Feb 9, 2010

### Kricket

Situation: a car moving quickly down the road with a window open.

Car reference frame: the air outside is moving, thus has lower pressure, thus the air inside the car will be sucked out the window.

World reference frame: the air in the car is moving, thus has lower pressure, thus the ambient air will be sucked into the car.

So, question: if you're in the car, and thus you experience that the outside air has lower pressure, why is the outside air being sucked into the car (due to the outside air's "higher pressure" with respect to the car's air)?

2. Feb 9, 2010

### rcgldr

Bernoulli is not about the speed of the air relative to some frame of reference. Instead it's a close approximation of how air responds to pressure differentials when no work is done and total mechanical energy is constant. A volume of air will accelerate from a higher pressure zone towards a lower pressure zone. During this transition. it's speed increases and it's pressure decreases, and it expands a small amount. Bernoulli equation approximates the relationship between speed, pressure, and expansion of that volume of air (ignoring factors like turbulence). A simplified version of Bernoulli ignores the expansion term of the equation.

At the molecular level, the pressure is the result of the rate of collisions and the impulses due to the average difference in velocities of those collisions. Assuming that the total mechanical energy is constant, then the average velocity of the affected air molecules is constant, but as that volume of air accelerates from a higher pressure zone to a lower pressure zone, the velocities of the affected molecules becomes a bit less random and a bit more directed (towards the lower pressure zone), reducing the rate of collisions and/or the average impulses due to reduced differences in average velocities of those collisions, reducing the static pressure as the net speed of the affected air increases.

Last edited: Feb 9, 2010