# Force diagram of a suction cup

1. Aug 18, 2015

### 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.

2. Aug 19, 2015

### Bystander

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

3. Aug 19, 2015

### Mr Davis 97

Normal force?

4. Aug 19, 2015

### Bystander

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?"

5. Aug 19, 2015

### Mr Davis 97

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...

6. Aug 19, 2015

### Staff: Mentor

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?

7. Aug 19, 2015

### Mr Davis 97

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)

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