Why doesn't static friction move the other object?

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Discussion Overview

The discussion revolves around the behavior of static friction when a block is pushed on a table, particularly in a scenario where the table is on a frictionless surface. Participants explore the implications of Newton's third law and the conditions under which the block and table interact, questioning why the table does not move when static friction is involved.

Discussion Character

  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that static friction acts leftwards on the block and should act rightwards on the table, leading to a net force on the table.
  • Others question the premise, suggesting that if the table is on a frictionless surface, it must move when a force is applied.
  • A participant proposes that if the table is pushed to the verge of slipping, the block may also move, raising questions about the conditions of motion.
  • Some participants suggest drawing free body diagrams for both the block and the table to clarify the forces at play.
  • There is a discussion about the analogy of a horse on a tablecloth and the type of friction involved in that scenario.
  • A participant draws parallels to real-world scenarios, such as a box in a truck bed or a waitress carrying a tray, to illustrate the concept of maintaining motion without slipping.

Areas of Agreement / Disagreement

Participants express differing views on whether the table moves under the influence of static friction, with some asserting it does and others questioning this conclusion. The discussion remains unresolved, with multiple competing perspectives on the behavior of the system.

Contextual Notes

There are assumptions about the frictionless nature of the table's surface and the conditions under which the block may or may not move. The discussion also touches on the nuances of static friction and the implications of Newton's laws, which are not fully resolved.

phantomvommand
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Consider a block sitting on top of a table.

When we push the block rightwards, static friction opposes its motion by acting leftwards on the block.

By Newton's 3rd Law, static friction should act rightwards on the table.

There is now a net rightwards force on the table. Why doesn't the table move?

Assume the table is on a frictionless surface.
 
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phantomvommand said:
Why doesn't the table move?

Who says it doesn't? Have you TRIED this experiment?
 
phinds said:
Who says it doesn't? Have you TRIED this experiment?
Sorry, I stated my question wrongly.

If we pushed the table (on the frictionless surface) such that it is on the verge of slipping, does the block on the table move?
 
phantomvommand said:
Why doesn't the table move?

Assume the table is on a frictionless surface.
If the table is on a frictionless surface then it does move.
 
Dale said:
If the table is on a frictionless surface then it does move.
Sorry, I stated my question wrongly.

If we pushed the table (on the frictionless surface) such that it is on the verge of slipping, does the block on the table move?
 
You should draw two free body diagrams. One for the block and one for the table. That should make it pretty clear, and will make it easy to solve quantitatively.
 
Dale said:
You should draw two free body diagrams. One for the block and one for the table. That should make it pretty clear, and will make it easy to solve quantitatively.
Wld I be right to say that the object does move?

Screenshot 2022-08-15 at 1.28.41 AM.png

Screenshot 2022-08-15 at 1.28.56 AM.png


I ask this because this exam question seems to suggest that the Horse (analogous to the object) can somehow remain on the verge of slipping.
 
We assume that the table/tablecloth interface is frictionless.

[edit: Regarding the horse/tablecoth, what kind of friction are we talking about ? ]
 
hmmm27 said:
We assume that the table/tablecloth interface is frictionless.
Which means that the leftward static friction force acts on the table cloth along the Horse/cloth interface?

By N3L, a rightward static friction acts on the horse?
 
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phantomvommand said:
If we pushed the table (on the frictionless surface) such that it is on the verge of slipping, does the block on the table move?
Are you pushing the table so that the table is on the verge of slipping against the block?

[Based on the toy horse example, the answer is "yes"]

If the floor is frictionless, this is actually an identical scenario to pushing the block so that the block is on the verge of slipping against the table. You have two objects with a fixed normal force between them and a non-zero coefficient of friction for their mating surfaces. One object is subject to an external force. No other [horizontal] forces are acting.

Similarly, one could place a box of tools in the bed of a pick-up truck and gently accelerate from a red light. Can one maintain a steady acceleration such that the box is always on the verge of slipping toward the rear of the bed?

Can a waitress holding a tray of beer steins keep the tray perfectly horizontal as she moves smoothly across the room to the table of thirsty businessmen, accelerating and then decellerating as she goes?

If you put a coin on top of a textbook, can you keep the book level while moving it back and forth so that the coin stays in place relative to the book? [I tried it 10 seconds ago. Yes, I can]
 
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