# Fluid Pressure With Acceleration

Q:
I've been told that, unlike in the case of liquids, when a container containing gas is accelerated the pressure on its rear end does not increase. Why?
- Annie (age 18)
NY
A:
Here it's convenient to use the equivalence principle- phenomena in a uniformly accelerating object look just like those in a uniform gravitational field. Any type of fluid will squash some toward the rear end of that accelerating container. So the pressure increase per unit length will be proportional to the density of the fluid, just as it is in the earth's gravitational field. Because the effect is proportional to the fluid density, the gravitational effect makes the  pressure increase as much as you go down about a meter in water as it does when you go down a kilometer in air. However, the effect is still there in the gas, as you will certainly notice if you go to anyplace with high altitude and low air pressure.

Mike W.

(published on 04/05/2011)

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