How much force will be required to remove the lid?

[iseidthat]

Homework Statement

An airtight box has a removable (massless) lid of area 4·10-2 m2. A lid is placed on it while it is on top of a mountain (where Patm=0.81·105 Pa). It is then taken to sea level, where Patm = 1.013·105 Pa. How much force will be required to remove the lid?

Homework Equations

F=PA

The Attempt at a Solution

For the first part:
F=PA=.04 x 81000=3240 N
For the second part:
F=PA= .04 x 101200=4052 N

is there a change in pressure type of equation?

 

[mgb_phys]

You are only interested in the change of pressure. - this gives you the pressure differnce between the inside and outside of the box and so the force to open it