Positive work done by FRICTION?

[Erik_at_DTU]

Hey,
I have this task that may seem simple, but really isn't:

When a box moves across a surface the force of friction does a negative work. Can a friction force ever do a positive work?

Possibility 1:
Yes, because else what is it then that moves your car forward when you accelerates?

Possibility 2:
No, because the force of friction is always opposit the direction of motion, and therefore W = F*s*cos(180) = -F*s...

Anybody who can offer help?

 

[mezarashi]

Well it's a bit weird to phrase friction doing positive work. Positive with reference to? If you ask, is there a situation where friction does not oppose motion? Is there a situation where friction helps motion? The answer is a solid YES.

The example you mentioned is perfect. I can even recall other situations where friction can help produce a greater acceleration of an object.

 

[comwiz72]

your car does not slide along the road, the friction causes the tyres to grip the road, doing negative work against the motion of the tyre by stopping them slipping.

this "gripping" means they tyres rotate around the axle, which causes the car to move forward.

 

[Doc Al]

When a box moves across a surface the force of friction does a negative work. Can a friction force ever do a positive work?

In a sense, yes. Imagine a small block resting on top of a larger block. If the bottom block accelerates it will drag the top block with it. Since the force of friction on the small block and it's displacement are in the same direction, the "work" is positive. Of course, the static friction is a passive force, not a source of energy; it just transmits the force exerted on the bottom block. (And the "work" done by the friction on the bottom block is exactly the negative of the work done on the top block.)

Possibility 1:
Yes, because else what is it then that moves your car forward when you accelerates?

Bad example: While the friction force does move the car forward, it does no work on the car. (There is no displacement of the point of application of the friction.)

 

[Erik_at_DTU]

Thanks!

I just have to thank you, suddenly it all became much clearer :)

 

[Erik_at_DTU]

Just one last thing:

Well it's a bit weird to phrase friction doing positive work. Positive with reference to? If you ask, is there a situation where friction does not oppose motion? Is there a situation where friction helps motion? The answer is a solid YES.

The example you mentioned is perfect. I can even recall other situations where friction can help produce a greater acceleration of an object.

To mezarashi: Could you give me those examples?

Because as far as I understand it now, kinetic friction will always do negativ work (relative to the direction of motion), while static friction can do positive work, but is a passive force so it really doesn't count...?

 

[Cyrus]

In a sense, yes. Imagine a small block resting on top of a larger block. If the bottom block accelerates it will drag the top block with it. Since the force of friction on the small block and it's displacement are in the same direction, the "work" is positive. Of course, the static friction is a passive force, not a source of energy; it just transmits the force exerted on the bottom block. (And the "work" done by the friction on the bottom block is exactly the negative of the work done on the top block.)



Bad example: While the friction force does move the car forward, it does no work on the car. (There is no displacement of the point of application of the friction.)

Based on that analysis, how is the block any different from the wheel. Doesnt the point of application of the friction force on the upper block stay at the same point as well? The top block is not sliding with respect to the bottom block.

 

[mezarashi]

To mezarashi: Could you give me those examples?

Because as far as I understand it now, kinetic friction will always do negativ work (relative to the direction of motion), while static friction can do positive work, but is a passive force so it really doesn't count...?

Again I'm not comfortable with the idea of friction doing negative work. I'd like to say that in some cases friction will aid motion rather than oppose it. Usually the case of wheels. Cars are a good example. Another one would be a spool. Pull on a spool of thread horizontally and you will find that the acceleration helped by the static friction. And I agree with you about kinetic friction always opposing motion.

 

[Erik_at_DTU]

This is indeed a difficult question to answer!

I think I will answer the task by saying:

" Friction can't do positive work, because:

(1) In the case of kinetic friction: Kinetic friction always oppose motion so
W = F*d*cos(x>90 degrees) = - F*d

(2) In the case of static friction: Their is no displacement and therefore no work is done.

Note: Friction can aid motion, as is the case when you are driving. "

Is there anybody who disagrees with me? If you disagree, please explain why

 

[Cyrus]

(To Doc Al)

For the car, why can't friction do positive work? If it does not, what other force is left to supply the car with the necessary work to increase its kinetic energy? Remove the road, then the free body on the car is the weight, and the static friction at the tires. So the static friction is the ONLY force that can supply the work. But I think this is analagous to your box example. The friction simply "transfers" the work that was supplied by the gasoline in the engine, that in turn went to the drive shaft, in turn gear box, and then to wheels. The friction is not the SOURCE of the energy, but it DOES do positive work, well transmits positive work. I am probably wrong, but I thought I would point that out.

As for your point of application not moving, if I push on a box with a rope tied around the box, the point of application of the force is always the same, i.e. where the rope is hooked to the box, but it DOES do positive work.

 

[Doc Al]

For the car, why can't friction do positive work?

No displacement = no work. (The tires do not slip along the ground.)

If it does not, what other force is left to supply the car with the necessary work to increase its kinetic energy?

The friction force, while necessary to get the car moving, does not do work or provide energy. The energy comes from the internal energy of the car (gas) which is transformed into KE.

Remove the road, then the free body on the car is the weight, and the static friction at the tires. So the static friction is the ONLY force that can supply the work. But I think this is analagous to your box example. The friction simply "transfers" the work that was supplied by the gasoline in the engine, that in turn went to the drive shaft, in turn gear box, and then to wheels. The friction is not the SOURCE of the energy, but it DOES do positive work, well transmits positive work. I am probably wrong, but I thought I would point that out.

The friction does no work at all, in this case. But you are pretty close in your thinking; the friction is used to "transfer" the internal energy of the rotating wheels to the translational KE of the car itself.

As for your point of application not moving, if I push on a box with a rope tied around the box, the point of application of the force is always the same, i.e. where the rope is hooked to the box, but it DOES do positive work.

Relative to the ground the point of application certainly does move!

 

[Bonulo]

So the professor who did these solutions is wrong?
http://www.artsci.gmcc.ab.ca/courses/enph131oa/sem08_w05_solutions.pdf

I must underline that kinetic friction counters RELATIVE motion, not motion with respect to the planet Earth (or any other inertial frame of reference).

 

[Erik_at_DTU]

Now I'm really confused!

Can friction do positive work?

 

[Bonulo]

I am too! I've only found this one thing, the solution, where kinetic friction is said to be positive. And there is all this talk about the situation being unspecific. It's not specified because it's simply the standard situation; can the kinetic friction (or the static friction) do positive work, if the displacement is relative to the standard, inertial frame of reference, namely - the planet Earth?

Or, in other words - is this guy right?:

http://www.artsci.gmcc.ab.ca/courses/enph131oa/sem08_w05_solutions.pdf

The question should be quite simple to answer (i.e. - for the skilled helpers here).

 

[Doc Al]

So the professor who did these solutions is wrong?
http://www.artsci.gmcc.ab.ca/courses/enph131oa/sem08_w05_solutions.pdf

I don't see anything wrong with the answers to #1.

 

[Bonulo]

So kinetic friction CAN do positive work, when displacement is measured relative to an inertial FR, e.g. the planet Earth?

 

[Andrew Mason]

I don't see anything wrong with the answers to #1.

I don't agree with it, but it is somewhat of a semantic point. See the other thread on this

AM

 

[Bonulo]

I believe it's quite problematic that we're given basic mechanics homework papers containing assignments whose quite far-reaching solutions apparently change radically by semantics.

Our professor generally doesn't elaborate. I guess he either 1. assumes that we'll assume what he's assuming or 2. that he wants us to be on slipping ground, and build the foundation first of all things at the beginning of each solution process. The latter is quite problematic.

You still haven't answered strictly based on the .pdf-solution; only given an example with static friction, whose analogy you might have asserted, but didn't explain.

 

[Doc Al]

So kinetic friction CAN do positive work, when displacement is measured relative to an inertial FR, e.g. the planet Earth?

Sure, in the following sense: Does the kinetic friction exert a force on an object? Is the displacement of the object in the same direction as the friction? Then friction does positive work on the object.

 

[Bonulo]

Ok, that's one guy convinced. But only one. Everyone else seems to say the opposite. I'm inclined to follow your argumentation here, since it equals my initial thinking, when I thought about the conveyor belt situation, where a system moving relatively to the Earth contains elements moving relatively to each other.

But what level of mechanics are you at?

 

[Physics Monkey]

It seems to me that the .pdf solution is fine. The box starts at rest in the lab frame, it is dropped onto the conveyor, the conveyor exerts a force called friction on the box trying to reduce the relative motion, the box speeds up in the lab frame, friction did positive work by increasing the kinetic energy of the box. I think Doc Al is right on this, and I'm a physicist so mechanics is my bread and butter .

 

[Physics Monkey]

There isn't anything magical about this, all it says is that the work done by a force depends on the frame of reference. In the lab frame the block gains kinetic energy meaning positive work is done. In the conveyer belt frame the block loses kinetic energy meaning negative work is done.

 

[Bonulo]

My point exactly. I'm going to write that then.

 

[Cyrus]

Relative to the ground the point of application certainly does move!

No displacement = no work. (The tires do not slip along the ground.)

Hmmm, how are you concluding that there is no displacement of the friction force? Relative to the ground, the friction force acts tangentially at all points where the tire is in contact with the road. This means as the car moves forwards, and the tire rolls, the friction force follows a straight line, equal to the distance the car travels. So I still do not see how its not allowed to transmit positive work to the car? (notice i was careful in not saying DOES positive work, but TRANSMITS positive work). The friction force will have a displacement, and thus a work that is transfered.