How do suction cups work? My roommate and I are having trouble agreeing on how suction cups work.
Presumably, the ambient air pressure is the only thing which keeps the suction cup attached to the object.
When lifting an object with a suction cup (or a rigid chamber with air pumped out):


*

*Given that the suction cup isn't physically affixed to the object, it can't apply any force to lift it.
How does pulling actually lift the object?

*What dictates how much weight can be lifted?


*If the object is touching the ground (so there's no air underneath it), can it be lifted? If not, can the suction cup be detached by pulling?


*In practice, why does a rubbery suction cup work better than a rigid object (such as an inverted glass cup)?

*Inside a rigid spherical shell (with gravity towards the center), presumably a suction cup can be used to hang a mass.

Yet, the air pressure is acting nearly uniformly on the sphere. How does the gravitational force on the mass get balanced out so the whole system doesn't move?
 A: I think this picture covers the gist of it.   You need rubber or plastic, a mailable material that flattens when pressed against the object, but that wants to return to it's original shape when released.   
Think of, as part of measuring the weight of an object, you measure the pressure all around it.   If you have more air pressure on the bottom than the top, the object weighs less and the suction cup works by creating a low air pressure pocket on the object it's lifting.  

Source:  http://www.wired.com/2013/08/how-strong-are-the-suction-wheels-in-bad-piggies/
If the object is on the ground, the air pressure is still there unless it's kind of vacuum sealed, like sink-holes or mud can sometimes create a vacuum seal and make it very difficult to lift things - kinda like this picture.  It's not that the boots and mud are that heavy, it's the vacuum seal that makes the boots hard to lift.  If air pressure didn't exist under objects sitting on the floor, everything would be enormously heavy.

A: Suction cups work by having lower air pressure inside of the cup and more on the outside. The strength of a suction cup is dependent on both the difference in air pressure between the outside and inside of the cup and the area the cup covers, if the force applied to lift an object exceeds this amount then the cup will come free.
A: When the suction cup presses down it squeezes all the air out, which creates an area of low pressure and sucks the cup in. When you lift the cup the area inside the cup stretches out, and the area of low pressure turns slightly towards high pressure which lets some air out. The air keeps going until the low pressure turns completely to high pressure and cup detaches from the object. That's my theory/explanation.
