Physics help, pressure and vacuums?

Question

An airtight box has a removable lid with an area of .013 m^2 and negligible weight. The box is taken up a mountain where the air pressure outside the box is .85 x 10^5 Pa. The inside of the box is evacuated. What is the magnitude of the force required to pull the lid off the box?

Mjy teacher didn't teach me how to do problems like this. I'm not even sure if there is enough information. Please help me. Thanks.

Answer 1

If there is a total vacuum in the box, then the pressure on the lid is equal to the force of the atmosphere per unit area multiplied by the area. The force you need to lift the lid is the total force of the atmosphere on the lid of the box. Once you begin to lift the lid, some air will enter the box and the force needed will decrease rapidly, but that does not affect the initial force you need.

You have the area of the box lid and the pressure per unit area of the atmosphere, so as long as your units are consistent, you only need to multiply one by the other.

Answer 2

To be really accurate I think you'd need to know the resistance of the air being pushed in. ie; is there a lip around the edge that could cause turbulance.

Also, you'd need to know the negative pressure inside the box, as (i think) pure vacuum is impossible.

Seb