Good one!

The vacuum creates the normal force. If you pull against the normal force, the vacuum is holding the cup, or cups together. If you pull at 90 degrees to the normal force (like the vacuum cup holding a soap dish in the shower) then its friction that holds it in place. But it is the vacuum that generates the normal force which is necessary for the friction. So both are true, depending on the angle.

If the suction cup was designed such that its surface is essentially flat when it initially contacts the surface, there should be no air under it, When you begin to pull up on the center there should be close to a vacuum condition under it. The only reason that you need to lift the center up is to ensure that there is enough pressure around the outer edges to seal it and prevent air from entering the space under the cup.

I would think that Allison could figure out a way to actually measure the separation force and see how close it comes to 14.2 psi. (I am assuming that Atlanta is at 1000′ altitude).

Yeah, it is not 180. 100 sounds more like it, and the force is proportional to the force you had to apply to the lever that you used to close the suction cup, isn’t it?

After thinking about what you wrote, Paul, I believe the force would be more like 80 to 100 pounds. I think at best, the volume doubles, which cuts the pressure in half. That would make the pressure difference about half of the atmospheric pressure you stated (14.7 pounds per square inch). Right?

John: In some cases, it’s friction holding the two cups together. But as I pulled on them to separate them, it wasn’t friction. It was the force created by the difference in air pressure. And as Paul Szymkiewicz wrote in his comment below, that would be ~180 pounds if the inside pressure were zero. It’s not zero, though, so let’s say it’s ~100 pounds of force.

Not telling!

Paul: Yeah, “pressure cup” or “pressure difference cup” would never catch on. Thanks for doing that little calculation. I should have done that for the article.