# Friction and its direction

1. Dec 2, 2015

### Lisciu

Hi Guys.

I catch myself with confused a little bit of friction and physics.

It's really simple to understand if it's a 1 box. And then you know that friction is opposed the motion. But it's little complicated for me in those cases :

Case 1 :
I assume that we will push the box 1 to right. Then the friction from the ground is to the left. And for the box 2 because of the third law of Newton we will have equal and opposite reaction and the box 1 will exert a force on box 2. So direction on box 2 should be to the left? Is it okay that this force is friction force? Or it's just transfered force from box 1 and then we will have the friction force acting to the right to hold this force?

Case 2 :

The same question probably here as in case 1. The friction for box two will be to the left, and the box 1 will get equal and opposite reaction to the right because of the pushing with force F. But what about the friction? Is it the 3rd Law force? Where is the direction for force of the ground ?

Case 3 and 4 :

I have problem with wheels that I don't know when should I look for motion as rotating and then decided the friction direction as opposed for example for Clockwise movment and when I should consider the as a opposed for translation movement.

All this case we consider as a Static Problems. ΣF=0.

Ps. Can you recommend good book to refresh this fundamentals? I consider the :
Collins Advanced Science by Dobson. But I almost sure that it's not so detailed in things like that.

2. Dec 2, 2015

### ElijahRockers

I am not sure, but my intuition tells me that in case one, friction will be responsible for box 2 moving to the right. Given a choice, box 2 would rather stay where it started, because of Newton's first law. The friction between box 1 and box 2 will cause box 2 to move with box 1.

Think of trying to pull a table cloth out very quickly from under a plate. If the cloth does not grip the plate well (i.e. there is no friction between them) the plate will be okay. But if there's enough friction, the plate will follow the table cloth. More friction = More following the item underneath.

At least this is my guess.

3. Dec 2, 2015

### andrewkirk

It will help you to not get confused if you avoid saying things like this. For every interaction there are two equal and opposing frictional forces, and we need to make clear which is which to avoid confusion.

For example in Case 1, box 1 exerts a rightwards frictional force on Box 2, which causes Box 2 to move along with it. Box 2 exerts a frictional force to the left on Box 1, which causes Box 1 to move leftwards more slowly than it otherwise would.

4. Dec 3, 2015

### Lisciu

Okay.

I will say what I understand. From your advice from example 1. To the box is attached a force to rightwards. So the surface exert a equal and opposite frictional force on box 1. Because we applied force on the box 1 and there is contact between box 1 and 2 we have an equal and opposite reaction and then on box 2 we have frictional force to the right. And question is why you saying something totally diffrent. Because Box 1 is moving to the right side so it can't be opposite reaction between them to the right....like you mention. Because then question again the equal and opposite reaction to what for Box 2?

Are you check on the direction of motion?

And the questions remain the same which pair of 3rd Law of Newton I should left..Confusing is that there is action and reaction. But when we solved problem we don't draw all forces on FBD because they will canceled each other...

5. Dec 3, 2015

### A.T.

3rd Law pairs don't cancel each other, because they act on different bodies. If you are confused, then draw all forces in your FBDs and stay consistent about what the separate bodies are.

6. Dec 3, 2015

### Lisciu

Okay should it looks like that?

I'm just not sure right know about the direction of force N2 Should be downward?

7. Dec 3, 2015

### A.T.

Do both boxes have the same weight w?
If not, you need w1 and w2.

You need both: N12 and N21, just like with friction. Newtons 3rd applies to all contact force components.

8. Dec 3, 2015

### Lisciu

I'm sorry. I assumed that they have the same mass.

Is this okay : On Box 2 should be N12.

9. Dec 3, 2015

### A.T.

The external force F is missing.

10. Dec 3, 2015

### Lisciu

Okay you mean the force acting on Box1. But what about the Box 2. If we take the 2nd Newton Law we get T=0 on ∑Fx so there will be no friction at all. Something is not okay then...

And regarding case 3 and 4. When we trying to take the direction of friction for wheel that have been pushed by force F we take in account it's rotation movment? or translation movment? Because if the wheel going to the right side it's rotate clockwise so the friction agains rotation is to the right but against to translation is to the left...

11. Dec 3, 2015

### A.T.

Why should there be friction if box 2 doesn't accelerate?

Depending on how much linear force and torque you apply to accelerate the wheel, the static friction can go either way. You have to write down the equations (linear and rotational) and solve for it.

12. Dec 3, 2015

### Lisciu

Okay.

So there is 2 things that bother me.

First one is the friction product of 3rd Newton Law? Consider that I push the ground with my leg to move mny body right. I exert force on the ground by push my foot to left. The ground exert force to push my leg. Is this pushing force from ground isn't friction? Or is is the friction and reaction force acting on the same time? I trying to grasp the concept now. Why we saying that without friction will be no movement at all if actually we have 3rd law. And I recall situation when we have been on ice and can't walk... So where is go reaction force in ice?

13. Dec 3, 2015

### A.T.

Friction obeys 3rd law. Both forces in the pair are friction and act at the same time.

14. Dec 4, 2015

### haruspex

Lisciu,

The rule is: friction acts to oppose relative motion of the surfaces in contact.

Case 1:
You do not say whether box 1 is accelerating, or what the current relative velocities are.
If the boxes are currently moving at different velocities then the frictional force between them will be so as to oppose that, and it will be the kinetic value. The other forces are irrelevant. So assume that at this instant the two boxes have the same velocity.
If F equals the frictional force from the ground then box 2 is moving at constant speed. Box 1 is also moving that that speed, so there is no frictional force between the boxes.
If F exceeds the frictional force from the ground then box 2 is accelerating to the right. That will tend to make box 2 move right relative to box 1, so friction opposes that by acting to the left on box 1 and to the right on box 2.
If F is now less than the frictional force from the ground (but the boxes were somehow set moving together) then box 2 is accelerating to the left. That will tend to make box 2 move left relative to box 1, so friction opposes that by acting to the right on box 1 and to the left on box 2.

Case 2:
Again, assume current velocities are the same.
Because of F, the movement of box 1 either is, is tending to be, to the right. Thus the frictional force from the ground is to the left. Likewise, box 2 will be tending to move to the right relative to box 1, so the friction between them acts to the right on box 1 and to the left on box 2.

Case 3:
This is an interesting one. It is not obvious which way the friction from the ground will act. But you do not need to figure that out in advance. Just suppose it acts some way, to the left say, and work through the algebra. If you guessed wrong you will get a negative answer.
A commonly asked question is: at what height does the horizontal force need to be applied such that there is no frictional force from the ground.

15. Dec 5, 2015

### just dani ok

he did mention that total force is 0 hence the system is not accelerating. According to the picture, only in case 3 we have moving object.

16. Dec 5, 2015

### andrewkirk

I have often wondered this myself. I have managed to ice skate, clumsily, but nevertheless achieving deliberate forward motion. I think what happens is that one creates friction by turning the skate off which is pushing at an angle to the direction of motion, thereby enabling it to cut into the ice a little and generate a push. The push will be at right angles to the angle of the skate, and hence at an angle to the general direction of motion, which is why the micro-motion of a skater is a zig-zag.

17. Dec 5, 2015

### andrewkirk

No. The 3rd law says that all forces must occur in equal and opposite pairs. It does not predict the arising of any forces but just says that, when a force does arise - be it from electostatic, magnetic, nuclear or gravity - it will arise as a pair of equal and opposite forces.

What produces the pair of frictional forces is interactions between the molecules on the two surfaces. I'd guess the interactions are principally electromagnetic.

18. Dec 6, 2015

### A.T.

Yes, same for cross country skiing: Also, similar: Propelling a skate board without touching the ground:

Also see snakeboard and wavebaord.

19. Dec 6, 2015

### just dani ok

back to op. In all those cases the total force is zero, which means no acceleration. the only object that can cancel the external force in the picture is the floor. hence in all those cases the floor applies friction force to the object above it to the left, as much as F.
in case 1 the force on object 1 is already canceled by the friction from the floor, hence object 2 doesnt feel anything.
In case 2 the force acting on object 2 is not directly canceled by the floor. Imagine if the surface between object 1 and 2 is perfectly smooth. friction of the floor can not possibly stop object 2 from moving.
so in case 2 object 2 acting friction on object 1 to the right, while object 1 reacting by friction on object 2 to the left equally. It is similar to a string tension that keeps two objects together when external forces act on those objects unevenly.