Why Doesn't the Glass Move When You Pull the Paper/Cloth?

In summary, when pulling a piece of cloth quickly from under a glass of water resting on a table, the glass will move slightly due to the friction between the cloth and the glass, but will ultimately stop due to the friction between the glass and the table surface. The speed at which the cloth is pulled affects the time over which the friction will act, but not the coefficient of friction or the magnitude of the force. This is a classic magician's trick, and it is important to practice with non-breakable objects before attempting it in front of an audience.
  • #1
AFG34
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0
If a glass of water is resting on a table and there is a piece of paper or cloth or whatever under the glass, and you pull the paper/cloth really fast, the glass does not move. Can someone please explain the physics of this to me. THNX
 
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  • #2
AFG34 said:
If a glass of water is resting on a table and there is a piece of paper or cloth or whatever under the glass, and you pull the paper/cloth really fast, the glass does not move. Can someone please explain the physics of this to me. THNX

Well, the glass does move a little bit, less the faster you can yank out the cloth. This is a classic magician's trick, in which a tablecloth is snapped out from under a fully set table without causing anything to fall over the edge. (Everything will jump a bit, however...)

Think about the friction between the glass and the cloth and then the glass and the table surface. You want to pull quickly to overcome static friction. You will still have kinetic friction between the glass and cloth, so the glass will begin to accelerate in the direction the cloth is being yanked. Before much time passes, though, the cloth will have been completely pulled away, leaving the glass now in contact with the underlying table surface. The kinetic friction in that case will generally be greater, but in any event, the friction will immediately act to decelerate the glass to a stop.

If you pick some reasonable coefficients of kinetic friction between the surfaces, you can work on how fast you have to pull the cloth away to keep the glass from sliding more than a couple inches (about 5 cm.). This has to be done in a (smallish) fraction of a second. (It's definitely suggested that you practice this with non-breakables before demonstrating this in front of an audience...)
 
  • #3
so speed affects the force of friction right (but it doesn't change the coefficient)??
 
  • #4
AFG34 said:
so speed affects the force of friction right (but it doesn't change the coefficient)??

The rate at which you pull out the cloth affects the time over which the friction will act, not the coefficient of friction or the magnitude of the force. The friction from the cloth will give the glass a certain positive acceleration from rest; the time interval of contact will determine the speed the glass reaches. Kinetic friction from the table surface will then decelerate the moving glass at a certain rate.
 
  • #5
ok makes sense, thanks
 

1. Why doesn't the glass move when I pull the paper/cloth?

When you pull the paper or cloth, you are exerting a force on the glass. However, the glass does not move because it is held in place by the force of friction. The friction between the glass and the surface it is resting on prevents it from moving.

2. Can't I just pull harder to make the glass move?

Pulling harder will increase the force on the glass, but it will also increase the force of friction. In most cases, the force of friction will be greater than the pulling force, making it difficult to move the glass.

3. Why can I move other objects on the same surface, but not the glass?

The amount of friction between two surfaces depends on the materials and the force pressing them together. Smooth surfaces, like glass, have a higher coefficient of friction and require more force to move compared to rougher surfaces.

4. Can I reduce the friction to make the glass move?

Yes, you can reduce the friction by using lubricants or by changing the surface the glass is resting on. For example, placing the glass on a piece of paper or using a cloth with a low coefficient of friction can help reduce the force needed to move the glass.

5. Why is it easier to move the glass when I tilt it?

Tilting the glass changes the direction of the force of gravity, which can help reduce the force of friction. This is because the force of gravity now acts at an angle, making it easier to overcome the force of friction and move the glass.

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