Action Reaction Forces

  • Thread starter Bashyboy
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  • #1
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Main Question or Discussion Point

This is not a question from a textbook, I am just supposing a situation.

A book is lying on a table with some friction. If I push on the book, the book will push on me with an equal and opposite force; the book will then push on the table with friction, and, in return it, the table will push with an equal and opposite force on the book. How does then book move then? I push with a force on the book, the force is transmitted into the book, and it pushes on the table that has friction, and the table pushes back with an equal force. Doesn't, then, the friction force equal the force that I push with?
 

Answers and Replies

  • #2
rcgldr
Homework Helper
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At some point in time, there was a net force on the book, causing it to accelerate to some velocity, then once a near constant velocity the net force returned to zero. During the acceleration, part of the force opposing the pushing was the books reaction force to acceleration.
 
  • #3
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Doesn't, then, the friction force equal the force that I push with?
No it most certainly does not when the book is moving.

The force with which you push the book along is not related to the force of friction between the book and the table.

The book presses down on the table with a force equal to its weight.
There is indeed always an equal and opposite reaction from the table on the book. This is what stops the book falling to the floor.
The mximum force of friction between the book and the table equals some constant (called the coefficient of friction) times this upward reaction from the table.

Meanwhile I can push with any force I like horizontally on the book.

If the force with which I push is less than that maximum friction stated above the book will not move.

If the force with which I push is greater than that maximum friction stated above the book will move.

Up to the point of starting to move actual force of friction will be just enough to prevent movement and no more.ie less than the maximum ffriction stated above.
 

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