Free Body Diagram for a Piece of Gum on the Underside of a Desk

[flintstones]

Homework Statement

Draw a free-body diagram for a piece of gum that is stuck to the underside of a desk.

Relevant Equations

None

So I’m a high school physics teacher and I came up with the question above. However I haven’t given it to my students because I’m not sure what the answer would be.

There is a force holding the gum up, and that force must balance the force of gravity. I’d probably call that an applied force, so the FBD would have an arrow going up labelled Fa and one going down labelled Fg.

However the reason I came up with this question in the first place is I was trying to show students how the normal force works. So I wanted to show them that normal force is drawn from the surface/desk towards the gum, 90 degrees to the desk. But normal force can’t be down, because something must be holding the gum UP. So is there zero normal force?

If you were to throw the gum at the table, THEN there’d definitely be a downward normal force as the gum was stopping. But if the gum is at rest does the normal force go away? Or is it that the applied force is balancing the normal and gravity forces together?

I think I won’t give this one to my students as it seems too hard, but it’s bothering me now. Thoughts on what the FBD should look like?

 

[berkeman]

The gum is held onto the underside of the desk by an adhesion force, which has nothing to do with a gravitational normal force with friction.

 

[hmmm27]

Just label it as "adhesive force" ; if anybody asks, mutter something about Van der Waals forces.

If you were to throw the gum at the table, THEN there’d definitely be a downward normal force as the gum was stopping. But if the gum is at rest does the normal force go away? Or is it that the applied force is balancing the normal and gravity forces together?

The normal force is a manifestation of the Pauli Exclusion principle ; keeps the gum from falling through the table.

A good one is this : the forces are clearly labellable.

 

[flintstones]

Just label it as "adhesive force" ; if anybody asks, mutter something about Van der Waals forces.The normal force is a manifestation of the Pauli Exclusion principle ; keeps the gum from falling through the table.

A good one is this : the forces are clearly labellable.

Thank you - that video will work much better as an example.

Adhesive force makes sense, thanks!

 

[flintstones]

The gum is held onto the underside of the desk by an adhesion force, which has nothing to do with a gravitational normal force with friction.

I see. So the free-body diagram would have the adhesion force going up, and the force of gravity going down, and no other forces?

 

[Chestermiller]

Those would be the only forces acting on the gum. There would of course also be a downward (reaction) force exerted by the gum on the bottom of the desk. But that would not figure in the force balance on the gum.

 

[haruspex]

The gum is held onto the underside of the desk by an adhesion force, which has nothing to do with a gravitational normal force with friction.

Could get some help from air pressure?