- #1
- 40
- 2
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.
jeff12, the static friction is a kind of reaction. That means that it is directed against the force with which the body acts on a surface.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.
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.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.
The dynamic friction does not need in any force. It is really 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.Static or dynamic friction always act in a direction opposite to the impressed force.
The dynamic friction does not need in any impressed force.
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.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.
That`s just Newtons 3rd Law, which also applies to dynamic friction.jeff12, the static friction is a kind of reaction. That means that it is directed against the force with which the body acts on a surface.
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.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.
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 sense 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.Can you give me an example of it not going opposite of the motion of the object?
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.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.
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.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.
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.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.
That very example above (which you gave).Can you give me an example of it not going opposite of the motion of the object?
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.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.
If it did, Energy would be manufactured out of nowhere!Can you give me an example of it not going opposite of the motion of the object?
What? Static friction? An car accelerating without slippage for example.Can you give me an example of it not going opposite of the motion of the object?
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.What? Static friction? An car accelerating without slippage for example.
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.Can you give me an example of it not going opposite of the motion of the object?
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.What? Static friction? An car accelerating without slippage for example.
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 really 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.in the middle of the bed
Can you give me an example of it not going opposite of the motion of the object?
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.If it did, Energy would be manufactured out of nowhere!
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.while in the case of static friction there is no such relative motion
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.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.
In a complicated system like the drive to car wheels, at every point of contact between the parts, N3 applies and you can choose a force in either direction. The effect on the car is to slow it down (of course) and it is fruitless to spend too much time looking for a paradox; there is not one. More than 20 posts on this thread is evidence that this approach just generates confusion.static friction force tends to accelerate the braking wheels rather than slow them down.
If it in context of the TC question, - yes I have. Because this:You could say that you have 'sped up' the wheel by making it rotate - but have you?
is out of context as far as I understand. I have asked TC to describe the professor's example with two bodies more detail, but without an answer. So as I have understood the speech goes about the point of contact moving. Otherwise it can be built arbitrarily complex schemes of arbitrarily complex conjugate bodies and constructive discussion on the verge of it will completely lose the constructivity. I have no to add something and I do not know if it needs because TC keeps silence. Let we wait to him reaction.. Are you referring to acceleration of the CM of the wheel, or
Work is frame dependent, and of course you can have inertial frames, where dynamic friction on an object is in the same direction as the motion of that object, thus doing positive work on that object. This doesn't violate energy conservation, because the equal but opposite friction force on the other object is doing more negative work on the other object.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).
That's true, in principle but my point is that, if the brakes and the tyres get hot, the procedure cannot be looked upon as reversible. This is what happens with anything other than operating above the limit of static friction. The operation of wheels on the ground can only result in loss of energy, which determines the relevant choice of direction. Hot brakes won't supply Kinetic Energy to either the car or the road.Work is frame dependent,
It`s not reversible becuse of entropy, not because of energy conservation.That's true, in principle but my point is that, if the brakes and the tyres get hot, the procedure cannot be looked upon as reversible.
The force of dynamic friction can supply kinetic energy to them.Hot brakes won't supply Kinetic Energy to either the car or the road.
Yes but it doesn't work the other way round, does it? I don't understand why you don't seem to understand.??The force of dynamic friction can supply kinetic energy to them.
I didn't think I said it was. But the energy deficit, due to the effect of friction only applies ' one way round' and the heating comes from whichever side of the contact is supplying the energy. If you choose the instantaneous point of contact on the turning wheel as your reference frame, it strikes me that you are just making your life more difficult for yourself. It is a pretty established principle to choose the (vastly) more massive body as the reference for this sort of problem. There could be an alternative scenario involving a conveyor belt and an unpowered vehicle but afaics, the assumption was of the more familiar situation.It`s not reversible becuse of entropy, not because of energy conservation.
The force of dynamic friction can supply kinetic energy to them.
It does. Dynamic friction can add or remove kinetic energy from an object, by doing positve or negative work on it. It depends on the reference frame.Yes but it doesn't work the other way round, does it?
It's frame dependent.Would you say that it is relevant whether the wheels do work on the road or the road does work on the wheels?
It`s not reversible becuse of entropy, not because of energy conservation.
You originally wrote the reversal would manufacture energy out of nowwhere. It wouldn't. But it would decrease entropy.I didn't think I said it was.
I see what you are getting at now and, of course, I could have stated things better. The reversal I am referring to is not 'that' one. I am talking in terms of Energy flow.You originally wrote the reversal would manufacture energy out of nowwhere. It wouldn't. But it would decrease entropy.
Possibly not controversial. But not unambiguously correct.There is less work done in accelerating the car, when there is slippage, than the engine is actually doing.
I don't think that statement is controversial. is it?
This is all a bit disingenuous, I think. The source of the Power that we are discussing is the fuel in the tank in the car. You can hardly be suggesting that the fuel in the tank is somehow being 'used' by the Earth, in order to accelerate the car (not an inertial frame, because there is acceleration). That is, afaiac, a very relevant point in this discussion. There will, of course, be a finite but very small, increase in the KE of the Earth during the interaction. The source of the energy to achieve this is also in the fuel tank. Do you mean the total power developed by the engine?Possibly not controversial. But not unambiguously correct.
Consider a car accelerating eastward at a red light on the streets of Chicago. It spins its tires briefly and a small cloud of smoke forms. The engine is doing a good amount of work on the drive shaft. But if we adopt an earth-centered inertial frame, the power being applied to the surface of the spinning tires by the pavement during this event is even larger than the power being supplied by the engine. That is because the surfaces of the tires are actually moving eastward at 600 to 700 of [earth-centered-inertial] miles per hour despite the engine's best efforts to get them moving westward at, perhaps, 30 [car-relative] miles per hour.
It would be more correct to say that the work done by the pavement on the surface of the tires plus the work done by the surface of the tires on the pavement adds up to a negative total.