# Homework Help: An airtight box, having a lid of area 80 cm2, is partially evacuated.

1. Oct 23, 2011

### 1MileCrash

1. The problem statement, all variables and given/known data

An airtight box, having a lid of area 80 cm2, is partially evacuated. Atmospheric pressure is 1.01 105 Pa. A force of 108 lb is required to pull the lid off the box. The pressure in the box was:

2. Relevant equations

3. The attempt at a solution

I figure that the force that the pressure in the box exerted on the lid is equal to the force exerted on the lid by the atmosphere minus the applied force.

No matter how "creative" I get with the math, I can't come to any of their answers.

Am I on the right track?

2. Oct 23, 2011

### lewando

Re: pressure

I think you are on the right track. Maybe units are causing you grief. Show what you are trying.

3. Oct 23, 2011

### 1MileCrash

Re: pressure

I've tried a boatload of things, most recently:

Box pressure = 1.01x10^5 - 479.52/.008

479.52 being 108lbs in newtons.

4. Oct 23, 2011

### lewando

Re: pressure

I think you need to try, like you said, F(due to P internal) = F(due to P atm) - 479.52 N.

Once you find F(due to P internal), you can find the internal pressure.

5. Oct 23, 2011

### 1MileCrash

Re: pressure

Which would be:

0.008(B) = 0.008(1.01x10^5) - 479.52

This was one of my first processes. I just get the same answer of 41060.

6. Oct 23, 2011

### lewando

Re: pressure

Oh, right--which is what you were doing eariler... That still seems right. What did you get and what is the answer?

7. Oct 23, 2011

### 1MileCrash

Re: pressure

My answer is 41060.

The homework gives me the choices of:

7.50 e4 Pa
6.35 e4 Pa
2.60 e4 Pa
1.76 e5 Pa
1.38 e5 Pa

8. Oct 23, 2011

### lewando

Re: pressure

You got more or less what I got. I don't think your approach is wrong. Anyone else see something we are missing?

9. Oct 24, 2011

### 1MileCrash

Re: pressure

Anyone got any other ideas?