# B Friction Force

1. Feb 27, 2017

### terryds

Is there any case example where the friction force DOESNOT oppose the motion, but rather PROVOKES the motion of some object?

2. Feb 27, 2017

### Staff: Mentor

Friction opposes slipping between surfaces, not necessarily the motion of an object.

3. Feb 27, 2017

### terryds

Is there any example case where the friction force direction is the same as the motion of the object?

4. Feb 27, 2017

### Staff: Mentor

Sure. You can set up one yourself. Stack two books on top of each other on a table. Push the bottom book along. Friction moves the top book and acts in the same direction as its motion.

5. Feb 27, 2017

### terryds

Ah, you're right!

But, is it correct if I say that all the friction force that can do this is only the static frictional force??

Is there any kinetic frictional force that provokes this situation?

6. Feb 27, 2017

### Staff: Mentor

No.
Sure. Just push the bottom book hard enough so that the top book begins to slip. The kinetic friction acting on the top book acts in its direction of motion.

7. Feb 27, 2017

### terryds

Thanks!

But, may I ask you something more?
If a person walks, does it mean that the friction force provokes the forward motion?
Is the friction force the same as reaction forc (Newton 3rd Law)?

8. Feb 27, 2017

### Staff: Mentor

Sure. Without friction, you won't be able to start moving (or regulate your steps). Your feet would just keep slipping.

Not sure what you mean. Friction is an interaction between two surfaces: The surfaces exert equal and opposite friction forces against each other, per Newton's 3rd law.

9. Feb 27, 2017

### terryds

There are two thoughts of mine about it:

1. The friction force is just to prevent you from slipping, but not push your foot forward. The force that push the foot forward is the reaction force because I have pushed my foot onto the floor first.
2. The friction force prevents from slipping and also generate the forward motion of the foot

Which one is right?

10. Feb 27, 2017

### Staff: Mentor

I'd say that the second is more accurate. Realize that the force the floor exerts on you has two components: a normal component and a 'parallel' component. That parallel component is due to friction. Of course, without the normal force there would be no friction.

Imagine you are placed on a frictionless surface. The normal force is still there, but without friction you are stuck.

11. Feb 27, 2017

### terryds

In this case, is it kinetic or static friction that exerts on my foot?
I think it must be kinetic friction right since my foot is going from 0 m/s to some velocity, right?'

But, what about standing on a very slipper surface that makes everyone cannot stand on it? I remember that maximum static force is μ*N , but what is the normal force? Is it equal to the weight of my foot only or my whole body?
In this case, what force exceeds the μ*N? (I can't think of any force since I stand still (no acceleration))

12. Feb 27, 2017

### Staff: Mentor

No. Unless your foot slips, the friction is static.

The normal force will support the entire weight of your body. I suppose you can perhaps stand there, but the slightest movement and you'll begin to slip.

Not sure what you mean. In any case, for a 'perfectly slippery' surface, μ = 0.

13. Feb 27, 2017

### terryds

So, if I stand still (without any movement at all) on the floor with friction, and suddenly, the floor changed somehow into frictionless one, I won't slip and fall, right?

14. Feb 28, 2017

### Staff: Mentor

I wouldn't count on that. We typically rely on friction to keep our legs in place. But assuming that you could stand in such a way as to exert no horizontal force on the ground, you'd still be standing with friction removed.