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

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Discussion Overview

The discussion revolves around constructing a force diagram for a suction cup, specifically in the context of its interaction with a wall. Participants explore the forces acting on the suction cup, including gravitational force, normal force, and friction, while addressing the implications of these forces on the cup's stability and behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the suction cup is not accelerating, suggesting that the gravitational force must be balanced by an equal and opposite force vector.
  • Another participant questions the direction of the suction force, proposing that it might be pointing sideways due to the cup's attachment to the wall.
  • A participant introduces the concept of the normal force, indicating it is perpendicular to the wall and relates it to the suction force.
  • There is a discussion about the relationship between the normal force and gravity, with one participant expressing confusion over the lack of a counteracting force to prevent downward acceleration.
  • Another participant hints at the presence of friction as an additional force, prompting a discussion about how suction creates a large normal force to maintain static friction against gravity.

Areas of Agreement / Disagreement

Participants express differing views on the forces involved, particularly regarding the direction of the suction force and the role of friction. The discussion remains unresolved as participants explore these concepts without reaching a consensus.

Contextual Notes

There are limitations in the discussion regarding assumptions about the forces acting on the suction cup, particularly the definitions of normal force and friction, as well as the conditions under which the suction cup operates.

Mr Davis 97
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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.
 
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Mr Davis 97 said:
suction force be pointing to the side
... and, can you think of any other force at play?
 
Bystander said:
... and, can you think of any other force at play?

Normal force?
 
Mr Davis 97 said:
suction force be pointing to the side
Mr Davis 97 said:
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?"
 
Bystander said:
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...
 
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?
 
Nugatory said:
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)
 

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