Relative Pressure?

• August 19th, 2011, 02:50 PM
JEllington
Relative Pressure?
While driving down the street today with the window down, it hit me that while velocity is relative, pressure is absolute.

My question is, how can something measured as an absolute quantity (pressure) depend on a relative quantity without itself also being relative?

For instance, perhaps I'm standing next to a road and I measure the air pressure as X.
Now, if someone were to drive by me, they would see the air as having some velocity V, so would they measure the same air pressure?

If they did measure the same air pressure, there must be something wrong with the equation relating pressure and velocity, as pressure should decrease with velocity.
If they did not measure the same air pressure, pressure must be relative!

Am I missing something?
• August 19th, 2011, 05:09 PM
Bunbury
The air pressure you are measuring is the static pressure and it is the same for the people in the car and for you standing still. Even though the car is moving, the air is not (assuming someone with a long arm is holding the pressure instrument well clear of the car itself). If the wind is blowing then you would still both measure the same static pressure, which would be a little less than the value you measured with no wind. The fact that the car is moving does not change the static pressure, but if the air itself is moving the static pressure will change, but you will both see the same change.

If the car passenger uses a Pitot tube pointed horizontally into the direction the car is traveling, the car passengers will see a higher pressure because the velocity pressure is added to the static pressure. This is the total pressure (or stagnation pressure). For you, standing still, the total pressure and the static pressure are the same when the wind isn't blowing, so you will get a lower reading than the people in the car.
• August 19th, 2011, 05:11 PM
msafwan
Both person will measure the same pressure. I just don't understand why you would think otherwise. Air pressure would only be different if you measure it at different location, eg: front-end of the car and the back-end of the car.

Both person will measure same pressure if they are at the same place and the same time (regardless of their relative velocity).