Vacuum required to generate specific amount of force

[Chris@ETI]

Homework Statement

I have a circular vacuum cup. The cup has a diameter of 4.5 inches. I need to calculate the amount of pressure, in inches of mercury (Hg), that the vacuum created by the cup would have to be in order to generate 120 pounds of force on a flat object.

Homework Equations

None given.

The Attempt at a Solution

It's been years since I've had to do any math or physics, but this is what I think is correct.

First, P = F/A, where P = pressure, F = force, and A=area
P = 120 pounds / ((2.25)^2 * pi) = 7.55 psi

Second, convert psi to Hg:
1 psi = 2.036 Hg, so 7.55*2.036 = 15.36 Hg



So, what I'm hoping for here is verification that I did this correctly or someone to set me straight on this.

 

[mgb_phys]

Sounds right, you have to subtract the answer from atmospheric pressure ( about 15psi) to get the absolute pressure inside the cup.
You have calculated the pressure difference.

 

[Chris@ETI]

Thank you for you assistance on this.

I also apologize for my delayed reply, I was on vacation.