How Do You Construct a Force Diagram for a Suction Cup on a Wall?

[Mr Davis 97]

I am a little confused about how to construct a force diagram for a suction cup, like a plunger, as it is stuck to a wall. First, I know that the cup is not accelerating, so the gravitational force pulling down on the cup has to be met with an equal but opposite force vector pointing up. However, why would this vector be pulling up? It is stuck to the side of the wall, so wouldn't the suction force be pointing to the side. This is my question.

 

[Bystander]

suction force be pointing to the side

... and, can you think of any other force at play?

 

[Mr Davis 97]

... and, can you think of any other force at play?

Normal force?

 

[Bystander]

suction force be pointing to the side

Normal force?

That's another name for "suction force pointing to the side," the word "normal" meaning "perpendicular to the surface (wall)." What's perpendicular to "normal?"

 

[Mr Davis 97]

That's another name for "suction force pointing to the side," the word "normal" meaning "perpendicular to the surface (wall)." What's perpendicular to "normal?"

The only other force seems to be gravity, which acts perpendicular to the normal. But this would mean that the body should accelerate downwards sense there is not counteracting force, and this is why I am confused...

 

[Nugatory]

You've overlooked one more force that is at play here. A hint: What would happen if you tried sliding the plunger sideways? What forces would be involved in that case?

 

[Mr Davis 97]

You've overlooked one more force that is at play here. A hint: What would happen if you tried sliding the plunger sideways? What forces would be involved in that case?

Friction? So what the cup does it use suction to obtain a large normal force in order to have a static friction threshold that always matches the force of gravity? (that is, until the pressure inside the suction equalizes)