I Does kinetic friction propel a person walking forward?

[sophiecentaur]

Harmful for whom?

Harmful in as far as it generates needless questions about what's really going on and adds confusion. I could ask what the actual difference is between the two effects. There is a real problem about understanding Friction and that is the misconception that 'friction slows things down'. You will have come across this when students find it hard that the friction with the air or dragging a trailer, makes the car slow down but the friction that its slipping drive wheels is a force which drives the things forwards.

kinetic friction is a dissipative force, while static friction isn't.

Agreed and that should solve people's problem. But confusion arises in all practical situations. e.g. between the friction in brakes and, for instance, the friction in a clutch. One makes you slow and one makes you go (in this case). Imo, the very fact that this discussion is so common on PF goes to show that the classification attempt is actually a problem. I realise that, once you've 'got it' there is no problem but so many people just don't get it because of what's really a false dichotomy.

 

[Nugatory]

Ok, then how come with the free body diagram of this simplified system (a person slipping on the floor represented as just a foot in the image below), there seems to be only a forward force, the kinetic friction, propelling the foot forward, but the leg should be overall moving backward due to the slip?

You’ve left a force out of that diagram: the force of the skater’s leg pushing the foot backwards, against the dynamic friction. If the foot is moving backwards at a constant speed (a reasonable simplifying assumption for most of the stroke) then its acceleration is zero, by $F=ma$ the net force on the foot is zero, and the magnitude of the leg-foot force is equal to the magnitude of the dynamic friction.

By Newton’s third law, there is an equal and opposite force exerted by the foot on the leg. That force is what propels the skater forward.

 

[A.T.]

Harmful in as far as it generates needless questions ...

Sounds like the complaints of a teacher, who doesn't want to be bothered by questions. Similar to your endless complaints about the concept of work in other threads. However, the comfort of teachers is not the criterion, based on which concepts in physics and engineering are introduced.

 

[sophiecentaur]

Sounds like the complaints of a teacher, who doesn't want to be bothered by questions.

A bit of a cheap shot there. They are 'needless' because they often don't take you in a useful direction. Insisting on using a false dichotomy is not very useful and can often generate further confusion. So many of those questions end up with further confusion and they have arisen because an approach which doesn't include those two terms gets a result with our them. A force which can be called 'friction' is all you need., in just the same way that a 'force' doesn't need to be named. This actual thread has the same problem with choosing the direction / choice of name of the force which is being dealt with. (Until the free body diagram sorts you out.)

 

[Herman Trivilino]

The motor + battery + mecahnical foot I clumped together as the whole system.

If that's the whole system then why are you involving the floor? The floor is external to your "whole system". I think you are confused about internal and external forces. You can draw an imaginary boundary around any object or collection of objects, and everything within that boundary is internal. Everything else is the environment and is external to the system. You are always free to draw that boundary wherever you choose. It's not something that's dictated by the circumstances, it's something you are free to do in ay way you choose. Of course, some choices will be more useful than others.

 

[sophiecentaur]

Wrong, static friction can take any value between 0 and that limiting force.

I see what you mean but the maximum force just before slipping will be static friction. It's a force that can't be exceeding and the force when slipping will be less. Perhaps I'm really arguing against the term 'dynamic friction'; a better term would be 'slipping friction' as it can mean either the force on, say a wheel when it's accelerating or slowing the car down.
But when a free body diagram is used, the situation would be clear(er).

 

[sophiecentaur]

The floor is external to your "whole system". I think you are confused about internal and external forces.

Trying to describe how a 'chain' of forces transfer work from one place to another risks getting one's self in a twist. The force on the piston manifests itself as a force pushing the car along. The same force[Edit - another different on the top of the cylinder will also be responsible, as will the forces keeping the engine and transmission value] fixed to the body etc. etc.. The only way to deal with this is to assume that the mechanisms take care of themselves (perhaps ignoring the finite efficiency all along the chain) and deal with the road contact situation. Having to cope with the fact that torques along the system will all be different is an even better reason to keep it simple.

 

[A.T.]

Perhaps I'm really arguing against the term 'dynamic friction'; a better term would be 'slipping friction'

If it was up to me, I would rename:

dynamic / kinetic friction -> slip friction or slide friction

static friction -> tractive friction or sticky friction

The terms "dynamic/kinetic vs. static" or often used for frame dependent quantities (like kinetic energy, static force), but the type of friction is frame invariant, as it's based on whether there is relative motion between the material in contact. The current terms also often lead to the wrong reasoning like: "since static forces cannot do work, neither can static friction".

 

[PeroK]

Kinetic friction in slipping does act, but it’s not strong or stable enough to propel you — it just resists sliding a bit.

A force is a force and Newton's third and second laws apply.

To walk effectively, you need static friction.

That's true.

 

[sophiecentaur]

That's true.

Hardly "true". Unless , by 'effectively' he means 'perfectly' with maximum acceleration. In many, if not nearly all cases there is some slip which doesn't involve suddenly ceasing to accelerate forwards at all. This is a great example of the false dichotomy between so called static and dynamic friction that people want to worry about.
Whenever a car accelerates, the drive wheels actually have a finite amount of slip but it's small. This is totally unlike a rack and pinion railway locomotive in which the drive wheels are locked to the track until a tooth breaks off!
Also, wheels fixed to the same axle will have to slip when the wheel goes round a bend. Making the distinction is really nonsensical.

 

[A.T.]

Making the distinction is really nonsensical.

Your rants are nonsensical. Why the distinction is useful has been explained to you several times.

Whenever a car accelerates, the drive wheels actually have a finite amount of slip but it's small.

That pure static friction is often just an idealization isn't a problem, all of physics is.

This is totally unlike a rack and pinion

It's just a different scale.

 

[sophiecentaur]

. Why the distinction is useful has been explained to you several times.

You appear to understand the Physics behind all this and that's good. But you don't need to be explaining the Physics behind it to me (or any others of us in the know). However, you need to appreciate the common confusion about the importance of the use of those two terms. If it were as straightforward as all that then why are there so many "which is which" posts from confused students? There are many people who want to categorise the 'two forces' (as they seem to se it) as being totally distinct. Those two terms only serve to polarise their view of what they're told.

That pure static friction is often just an idealization isn't a problem, all of physics is.

Of course it is but you are approaching this from the standpoint of someone who doesn't have a problem. A simple term like "slip" could get over the problem and avoid so many questions. You will notice that those questions in PF posts don't come from people like you; that's because you have got it straight and they haven't.