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

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Discussion Overview

The discussion revolves around the physics of a glass of water resting on a table while a piece of paper or cloth is pulled quickly from underneath it. Participants explore the concepts of friction, acceleration, and the dynamics involved in this scenario, referencing both practical observations and theoretical explanations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes that the glass does move slightly when the cloth is pulled quickly, referencing a magician's trick where the cloth is removed without disturbing the items on the table.
  • Another participant discusses the role of static and kinetic friction, suggesting that overcoming static friction is necessary for the glass to begin moving.
  • There is a mention of how the speed of pulling the cloth affects the distance the glass moves, with a suggestion to calculate the necessary speed to limit the glass's movement to a small distance.
  • One participant questions whether speed affects the force of friction, leading to a clarification that while speed influences the time of contact, it does not change the coefficient of friction.
  • A later reply explains that the friction from the cloth provides acceleration to the glass, and the duration of contact determines the glass's speed before it interacts with the table surface.

Areas of Agreement / Disagreement

Participants generally agree on the mechanics of friction and acceleration but explore different aspects of the interaction, leading to some uncertainty about the implications of speed on frictional forces. The discussion remains open without a definitive consensus on all points raised.

Contextual Notes

Limitations include assumptions about the coefficients of friction and the specific conditions under which the glass is pulled. The discussion does not resolve the mathematical details of the forces involved.

AFG34
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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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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...)
 
so speed affects the force of friction right (but it doesn't change the coefficient)??
 
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.
 
ok makes sense, thanks
 

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