Friction

[thestrokes14]

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From EK Audio Osmosis:

If a book is placed on ice, and you push the book with your hand so that it moves away from you, which direction is friction? I understand that the only possible force here is friction, and must be pointed away from you (because the motion of the book is away from you), but I am confused about how this relates to the definition of friction: opposing relative sliding of two contiguous surfaces.

My initial assumption was that your hand wants to slide forward relative to the surface of the book, and that the frictional force would thus be backwards. But this can't be right, because then the net Force would cause the book to slide backwards. Can someone explain why I was wrong?

 

[milski]

From EK Audio Osmosis:

If a book is placed on ice, and you push the book with your hand so that it moves away from you, which direction is friction? I understand that the only possible force here is friction, and must be pointed away from you (because the motion of the book is away from you), but I am confused about how this relates to the definition of friction: opposing relative sliding of two contiguous surfaces.

My initial assumption was that your hand wants to slide forward relative to the surface of the book, and that the frictional force would thus be backwards. But this can't be right, because then the net Force would cause the book to slide backwards. Can someone explain why I was wrong?

I assume that you're talking about putting your hand on top of the book and pushing it forward that way. The static friction force is always opposite in direction to the force applied. What is confusing you is that this has to applied to the forces action on the same object. Your hand is pushing forward and the friction force from the book on your hand is pushing your hand backwards. From the third law, there is a friction force from your hand on the book with the same size but in opposite direction - forward. This is the force that move the book away from you.

 

[thestrokes14]

I assume that you're talking about putting your hand on top of the book and pushing it forward that way. The static friction force is always opposite in direction to the force applied. What is confusing you is that this has to applied to the forces action on the same object. Your hand is pushing forward and the friction force from the book on your hand is pushing your hand backwards. From the third law, there is a friction force from your hand on the book with the same size but in opposite direction - forward. This is the force that move the book away from you.

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