# Bernouli's Equation/Pressure Problem

1. Dec 16, 2008

### alicemunro

1. The problem statement, all variables and given/known data
Consider a truck with a tarpaulin on top of it. When the truck is stationary the tarpaulin lies flat, but it bulges outward when the truck is speeding down the highway. The truck is traveling at 27 m/s. The density of air is 1.29 kg/m^3. by how much does the pressure inside the cargo area beneath the tarpaulin exceed the outside pressure

2. Relevant equations
bernoulis:
P(1) + 1/2pv^2 = P(2) + 1/2pv^2

3. The attempt at a solution
i plugged in the density and but with two unknown pressures and one unknown velocity i dont know how you would find the difference

2. Dec 16, 2008

### alicemunro

and i think the pgy's on either side of the equation cancel out because this is horizontal, so y=0.

3. Dec 16, 2008

### nasu

I would consider the air inside the truck (under the tarpaulin) as stationary.
You can imagine the truck at rest and someone blowing some air on the outside.

4. Dec 16, 2008

### rl.bhat

When the truck is traveling at 27 m/s, the air above the tarpaulin is moving with the same velocity but in the opposite direction. The pressure inside this blowing air decreases. In side the tarpaulin air is at rest.

5. Dec 17, 2008

### alicemunro

so if the air inside the tarpaulin can be considered stationary, you can use P(2) = P(1) + pgh. And the pgh would = 0, so the pressure inside would equal 1 x 10^5 Pa?

6. Dec 17, 2008

### alicemunro

AH oh wait is this how you solve it:

P(1) + 1/2pv^2 = P(2) + 1/2pv^2
P(1) - P(2) = 1/2p(v2^2 - v1^2)
P(1) - P(2) = 1/2(1.29)(27^2 - 0^2)
P(1) - P(2) = 470 Pa

correct??

7. Dec 17, 2008

### rl.bhat

Correct.

8. Dec 17, 2008

### alicemunro

thanks profusely