# Pressure of a lid

Pressure of a lid!!

## Homework Statement

A lid is put on a box that is 10cm long, 20cm wide and 8cm tall and the box is then evacuated until its inner pressure is 72000 Pa.

1.How much force is required to lift the lid at sea level?

2. How much force is required to lift the lid in Denver on a day when the atmospheric pressure is 67.5 kPa (2/3 the value at sea level)?

P=F/A

A=L x W

## The Attempt at a Solution

I got #1 where I did the following:

A= L x W= 0.1 x 0.2= 0.02m^2

P=F/A
(101325 - 72000)= F/ 0.02
586.5 N= F

I attempted to do #2 as shown below and got both methods wrong:

so first I did: P=F/A
(67500- 72000)= F/0.02

so then I did: P=F/A
67500= F/0.02

I am unsure how to answer this question, so any help is appreciated! It's online homework so there's no given answer either. thanks!

gneill
Mentor

Your first attempt at #2 is along the right lines, you just didn't analyze the result closely enough. Suppose the lid were secured in place with some kind of fastener. If the pressure inside is higher than outside, what would happen when you undid the restraints? If there were no restraints in place when the outside pressure dropped below the inside pressure, what would happen to the excess pressure in the box?

so if you undid the restraints then the pressure inside would decrease? I'm still confused on how to solve the problem.

ehild
Homework Helper

Look at the data: where is the pressure higher, inside or outside? What happens to a lid if the pressure inside the container is higher than the outside pressure? Do you have no kitchen experience with boiling milk in a covered pot?

ehild

no kitchen experience at all! but if the pressure inside the container is higher than the outside, wouldn't the lid pop off? I still don't get how this will help me get the question...

gneill
Mentor

no kitchen experience at all! but if the pressure inside the container is higher than the outside, wouldn't the lid pop off? I still don't get how this will help me get the question...

Think it through. If the lid pops off by itself, how much force is required to remove the lid?

Oh okay I get it. thank-you for all of your help :)!