# Homework Help: Positive work done by FRICTION?

1. Oct 7, 2005

### 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?

2. Oct 7, 2005

### 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.

3. Oct 7, 2005

### 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.

4. Oct 7, 2005

### Staff: Mentor

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.)

5. Oct 8, 2005

### Erik_at_DTU

Thanks!!

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

6. Oct 8, 2005

### Erik_at_DTU

Just one last thing:

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...?

7. Oct 9, 2005

### Cyrus

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.

Last edited: Oct 9, 2005
8. Oct 9, 2005

### mezarashi

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.

9. Oct 9, 2005

### Erik_at_DTU

This is indeed a difficult question to answer!

" 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

10. Oct 9, 2005

### Cyrus

(To Doc Al)

For the car, why cant 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. Im 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.

11. Oct 9, 2005

### Staff: Mentor

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

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

12. Oct 9, 2005

### Bonulo

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

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

Last edited by a moderator: May 2, 2017
13. Oct 9, 2005

### Erik_at_DTU

Now I'm really confused!

Can friction do positive work?

14. Oct 9, 2005

### 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 [Broken]

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

Last edited by a moderator: May 2, 2017
15. Oct 9, 2005

### Staff: Mentor

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

Last edited by a moderator: May 2, 2017
16. Oct 9, 2005

### Bonulo

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

17. Oct 9, 2005

### Andrew Mason

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

AM

18. Oct 9, 2005

### 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.

19. Oct 9, 2005

### Staff: Mentor

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.

20. Oct 9, 2005

### 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?