What is the Direction of Friction Force Between Moving and Stationary Surfaces?

[simplescience]

Homework Statement

One surface moves over the other stationary surface.

a. Is friction felt by both the surfaces?
b. Is the friction force same in magnitude?
c. What are the direction of friction forces felt by them?

Homework Equations

The Attempt at a Solution

Since both the surface are in relative motion and touching each other, the friction force should be produced. But i could not imagine how the stationary surface feel the friction.

This is my first attempt at physics forum. If someone can provide the clear example that let's me imagine, it will be greatly helpful.

 

[Doc Al]

Take a shot at answering your own questions! Hint: Does friction help or hinder motion, in general?

 

[pbuk]

Why imagine when you can do an experiment: hold one hand still and use your other hand to move a book over it.

 

[mooncrater]

consider a case:
you're tied with a rope which is tied with the roof , and you're hanging , there is a platform (with small wheels) beneath you and you can walk on that platform . Now what will happen when you walk on that ( definitely you won't go forward ) ? now consider your first and third parts.
For the second part observe and make cases .

 

[simplescience]

Thank you very much for this book and still hand example. I tried it. Now it is clear that if the hand is still and book sliding on the right side, produces two friction forces (i.e. two gripping forces), one that is felt by the book which tries to pull the book to the left and the other that is felt by the still hand which tries to pull the hand to the right. This frictional forces are of the same magnitude but in opposite directions.

Please confirm and correct me if I am wrong.

 

[simplescience]

Why imagine when you can do an experiment: hold one hand still and use your other hand to move a book over it.

Take a shot at answering your own questions! Hint: Does friction help or hinder motion, in general?

It can do both. It is the force of reaction, if I am not wrong. It can help you push forward as well as it can drag you from moving forward when sliding.

 

[Nathanael]

This frictional forces are of the same magnitude but in opposite directions.

Please confirm and correct me if I am wrong.

Yes that is correct. (Friction obeys Newton's third law.)

 

[simplescience]

Yes that is correct. (Friction obeys Newton's third law.)

Thus friction can help push you or drag you. Right?

 

[pbuk]

Thank you very much for this book and still hand example. I tried it. Now it is clear that if the hand is still and book sliding on the right side, produces two friction forces (i.e. two gripping forces), one that is felt by the book which tries to pull the book to the left and the other that is felt by the still hand which tries to pull the hand to the right. This frictional forces are of the same magnitude but in opposite directions.

Please confirm and correct me if I am wrong.

All correct!

 

[jbriggs444]

Thus friction can help push you or drag you. Right?

Newton's third law just says that the force of friction from A on B is equal and opposite to the force of friction from B on A. It says nothing about which direction this is.

As it turns out, dynamic friction acts to oppose the relative motion that exists between A and B.

Static friction acts to oppose the relative motion that would exist between A and B if the static friction were eliminated or reduced.

If you push your foot backwards against the ground, your foot would slip backwards were it not for the friction that exists. So the friction pushes your foot forward. Static friction in action.

If you spin your tires forward, trying to get unstuck from the snow, the treads on your tires are moving backwards. What little friction there is pushes the treads forward. Dynamic friction in action.

If you slide down a rope by not gripping tightly enough with your hands, your hands slide downward on the rope. So the friction pushes your hands upward. Dynamic friction in action.

 

[Doc Al]

Thus friction can help push you or drag you. Right?

Yes. The key is that friction acts to oppose (or prevent) slipping between surfaces.

 

[simplescience]

There is lot of confusion about the ultimate cause of friction over the web. Some say it is the inter-molecular forces, others say it is caused by the roughness. What is it precisely? What actually grips the two surfaces?

 

[HallsofIvy]

Those are the same. "Roughness", looked at from the level of molecules is "inter-molecular forces".

 

[rude man]

I am fortunate to have had, as my engineering tutor, Dr. Frederick Abernathy of Harvard, who showed that static friction always acts in a direction so as to oppose motion.

 

[simplescience]

Those are the same. "Roughness", looked at from the level of molecules is "inter-molecular forces".

I feel the roughness has to do with bumpy ride, whereas friction has to do with the stickiness.

 

[haruspex]

Those are the same. "Roughness", looked at from the level of molecules is "inter-molecular forces".

Roughness implies that at some sufficiently detailed level molecules are being pushed into the body of the object of which they are part in order to get around bumps in the other body. While that does indeed involve intermolecular forces, one can imagine other such forces - maybe attraction across the interface.
That said, it is more obvious how the roughness forces would be proportional to the normal force.