Positive work done by FRICTION?

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

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 gonna write that then.

Cyrus

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.