# Direction of Kinetic Friction and Static Friction

1. Oct 27, 2015

### jeff12

Can someone please explain me the direction of static friction? I know kinetic friction is always going against the motion of an object but for static friction it depends.

2. Oct 27, 2015

### IgorIGP

jeff12, the static friction is a kind of reaction. That means that it is directed against the force with wich the body acts on a surface.

3. Oct 27, 2015

### sophiecentaur

Static or dynamic friction always act in a direction opposite to the impressed force. If you apply that rule in a totally rigid way, there is really no problem. However, there can be an intuitive problem which makes things look wrong.
The problem often arises when you are considering wheels, driving a vehicle. When you operate the accelerator, there is a force, backwards, against the road surface and a reaction force will be stopping the wheels from slipping (before you have exceeded the limiting static friction). The contact point with the ground will act as a fulcrum (instantaneously) and the torque at the axle will produce a forward accelerating force on the car (as the wheel turns). If there is slipping, the friction force is less and so the accelerating force on the car will be less. When there is slipping, there is a Force times velocity involved and the tyres can get hot and engine power will actually be wasted in producing the heating effect.

4. Oct 27, 2015

### IgorIGP

The dynamic friction does not need in any force. It is realy directed in opposit to the velosity. And the static friction can never be without the shearing force and it is always directed in opposit to it. However, you are right - dynamic friction forces can or slow down the movement of the body or accelerate it. But they are always slow down the part of the body that is in dynamical contact with friction surface. For example, "they are trying to slow down" the rotation of the drive wheels and thus accelerate the car.
But TC asked about the static friction force direction. If the non compensated tangential to contact surface force is present the static friction force is directed in opposit to it. And it compensate it until the $k_f \cdot N$ is more or equal to that force.

Last edited: Oct 27, 2015
5. Oct 27, 2015

### sophiecentaur

If you are talking in terms of the dynamic friction on a 'coasting' object, it is directed against the motion and, likewise, dynamic friction is acting against the motion of a driving wheel over the road surface (and that can involve forward or backward motion, depending on the actual situation). The same rule applies and there is no contradiction. I can't see the OP's problem with the direction of the static friction force as long as we are talking of the forces rather than the motion. Static friction is the odd one out. in as far as it only works 'against' a force and can't be acting in a dynamic situation (slipping). But is this confusing? I don't see how.

As for the dynamic friction force, you can generalise by saying that it will be a cause of energy loss, whether in a slipping drive or a brake. It produces negative (vectorially) acceleration, in both situations, in as far as the reaction (accelerating) force on the road is against the direction that it is applied by the wheel.

But, to be honest, there are far too many posts which are centred on examining how a classification produces confusion during an arm waving discussion. This is one of them. Once you get down to drawing a proper diagram of any situation involving friction, including all forces and velocities, it will give you an equation of motion which can be solved and give the correct prediction. What more can a chap want?

6. Oct 27, 2015

### Chandra Prayaga

The direction of static friction is decided like this. Imagine that there is no friction, figure out which way the object would slide in such a case. Static friction will be opposite to that direction.

7. Oct 27, 2015

### Aryarising

Actually static friction is a self adjusting force.The maximum static friction that can act between the surfaces in contact is directly proportional to the normal reaction.The formula is f=μsN.If the applied force is less than the maximum static frictional force then the static frictional force that acts between the surfaces is equal and opposite to the force applied.

8. Oct 27, 2015

### jeff12

It is because when my professor he explained it using two blocks, the big one on the bottom and a small on top and he said the static friction was moving in the direction the objects were moving.

9. Oct 27, 2015

### A.T.

That`s just Newtons 3rd Law, which also applies to dynamic friction.

10. Oct 27, 2015

### A.T.

Yes, it depends. You have to look at all other forces on the body to find the static friction required to hold the contact static.

11. Oct 27, 2015

### jeff12

Can you give me an example of it not going opposite of the motion of the object?

12. Oct 28, 2015

### IgorIGP

No one surface moves (relative one to another) when the static friction force acts between them. That force is not "working force" since there is no displacement. Can you give an example where "object motion" is present and it have sence in the static friction context? Or can you explane more detal of your professor example, please? Remember that we are talking about static (non dynamic) friction force.

13. Oct 28, 2015

### IgorIGP

The topic caster spoke about the professor and the two bodies ... Maybe he did not understand the example and then professor of law but we do not know what. But if the meaning is transmitted correctly, the professor also is not to blame. Just when he was a student, another professor showed him this example. The idea of static friction connection with the motion is just of plain bad. But if we do this it must be said that the static friction is directed in oposit to the acceleration of the system and not to its movement (velocity). Application of a non-inertial frame of reference to illustrate the static friction is not justified. This technique is rarely justified.

Last edited: Oct 28, 2015
14. Oct 28, 2015

### Staff: Mentor

Just remember that friction opposes slipping between surfaces. For the static case, if you can figure out which way the surfaces would tend to slip (in the absence of friction), then friction will act to oppose that slipping.

15. Oct 28, 2015

### Staff: Mentor

Let's break that down. Say the big block is being pushed to the right by some force and the small block is going along for the ride. So you know that it's static friction accelerating the small block and the static friction on the small block must act to the right. Thinking in terms of surfaces: without friction, the top block would tend to slip to the left with respect to the bottom block, and the static friction must oppose that and act to the right.

Of course, it's the opposite for the bottom block. The static friction acting on the bottom block acts to the left.

That very example above (which you gave).

In any case, it's best to think of slipping between surfaces to determine the direction of friction and not the "motion" of the object.

16. Oct 28, 2015

### IgorIGP

Absolutely agree. The movement is superfluous. I do not understand why that professor has applied a dynamic scheme and even of two bodies. I think the assumption that in this case there is an effect of Sigmund Freud is not sufficiently substantiated.
I think, anyway, that the one body is enough. jeff12, let we have only the one body which is placed on a rough surface in rest. Suppose that at some point in time a very small force begins to act on the body. Let this force is directed to the left (for definiteness). Let the direction of this force does not change, and a module that force is slowly growing until the body begins the slowly moving (to the left). Let denote the magnitude of this force at the start of the movement as a Fmax. Today we have one answer to the question "why the body did not move when the force has a value from zero to Fmax". This answer is: "the force is equal in magnitude and opposite in direction acted on the body" . This answer is the logical consequence of Newton's Law II. Analyzing the bodies that can affect the body, this can be found only one body that can be the source of action and that body is the surface. The force that applied from the surface on the body is a reaction force. This is because this force is always in need of a driving force It occurs as a response to the emergence of the driving force. And it responds to changes in the driving force to make a balance with. The nature of this force is the engagement of micro-roughness of the surface and of the body. As you can see here, there is no any friction, because there is no movement of surfaces relative to each other. However, the engagement of the micro-roughness plays a decisive role in dynamical friction and this force was called the static friction force. Traditions sometimes generate confusion, but all is even worse without them. I beleive that spich will help you to find the right direction. :)

Last edited: Oct 28, 2015
17. Oct 28, 2015

### sophiecentaur

If it did, Energy would be manufactured out of nowhere!
As is usual with these problems, considering the Energy situation is a much more reliable approach. If the situation is a static one then using the principle of Virtual Work (From way back in my schooldays), you let things move by an infinitessimal amount and check that the Energy is dropping. If it isn't, then you have assigned the wrong sign to the friction direction.
But you have to make sure which object you are considering. (Is it the portion of the wheel in the footprint or the car that counts, for instance?)

18. Oct 28, 2015

### A.T.

What? Static friction? An car accelarating without slippage for example.

19. Oct 28, 2015

### IgorIGP

I think this is bright example, and at the same time it is a bad example. The trouble is that it is true as far as is true idealization: "without slippage". So, the slightest slip leads to the movement of the wheels against the surface of the friction force. And there is the kinetic (sliding) friction appeared...The movement is a harmful thing to demonstrate the static friction. The static friction is not a friction at all.

20. Oct 28, 2015

### jbriggs444

A brick sitting in the middle of the bed on a pick-up truck rounding a corner. The force of static friction on the brick from the truck is inward toward the corner. The force of static friction on the truck from the brick is outward away from the corner. Neither is in the direction of motion of either truck or brick.

Static friction does not oppose motion. It opposes relative motion -- by just enough to prevent it.

21. Oct 28, 2015

### sophiecentaur

Hence my caveat about what is the object to consider here. The thing that the friction is stopping is the slipping of the wheel, not the acceleration of the car. N3 rears its head and seems to confuse a lot of people. With or without slippage, the situation is the same and so is the direction of the friction force.

22. Oct 29, 2015

### IgorIGP

z
Yes! And the pick-up is rounding over the Earth placed on the 3 elephants those with wet feet are standing on a slippery turtle... You are right! If only you know how I understand all that. The complex model with motion and even accelerated motion ( non inertial systems and imaginary forces) is a realy road to hell. Going by that road (TC asks an example with a moving object) I would suggest a braking car. Here, the variant of soft braking without sliding is available and static friction force tends to accelerate the braking wheels rather than slow them down. But what does that prove? That an example offered by Professor wrong? I think, the way to inventing of such examples is an example in itself.

Last edited: Oct 29, 2015
23. Oct 29, 2015

### A.T.

What do you mean by that? You can easily construct inertial frames where the same force (dynamic or static friction) is either opposed to motion of the object or in the same direction.

The relative motion of the contact patches is always opposed by dynamic friction, while in the case of static friction there is no such relative motion.

24. Oct 29, 2015

### IgorIGP

You are absolutely right. But, to be honest, we have not pure reaction in real world. The houses in my street that runs to the sea are moving to the sea with velocity 2sm per year (or two meters per century). What about this... The reaction is always slightly lower than its an active force. This is absolutely fundamental thing. That means that the massive body at a rest on an inclined plane someday will be at the bottom. That means also that the statical friction on driving/braking whils is never purely free of the slip. I am not finding fault. I just want to show that this is no a good way to refute this a wrong model once and for all. This is because this evidence is true up to the idealization and the opponent can always argue that in the real world, his model is correct.

25. Oct 29, 2015

### sophiecentaur

My point was that, if the (dynamic) friction force is in the same direction as the motion, this would involve getting energy out of the system (Work being force dot displacement). If you ever see a situation where this appears to be happening, you have got the direction of the friction force wrong. Isn't that a very basic requirement, related to thermodynamics? I don't think you can invoke 'symmetry' here.