I Does kinetic friction propel a person walking forward?

[annamal]

TL;DR Summary

I know that static friction is what helps a person walk forward. I am wondering if the walking person still gets propelled forward if their foot slips and kinetic friction takes place

If we draw the free body diagram of a person walking with the feet slipping, we have kinetic friction applied to the feet. Is this kinetic friction going to propel the person forward? For example if the person tries to walk on ice, the foot just slips and the person doesn’t get propelled forward by the kinetic friction.

 

[Baluncore]

Is this kinetic friction energy going to propel the person forward?

Yes, but inefficiently.
Just like spinning the tires on a car, if there is any friction, there will be a propulsive force.

 

[berkeman]

Or to put it in more quantitative graphical terms...

 

[Chestermiller]

Can you walk forward on ice? Sure, but not very well.

 

[A.T.]

I am wondering if the walking person still gets propelled forward if their foot slips and kinetic friction takes place.

Sure. Newton's 2nd Law doesn't really care what type of force is applied to the body.

For example if the person tries to walk on ice, the foot just slips and the person doesn’t get propelled forward by the kinetic friction.

Unless it's perfect ice and kinetic friction is zero (doesn't happen in the real world), there can be propulsion. However, trying to walk normally is not necessarily the best way to use that low friction for propulsion.

 

[sophiecentaur]

Sure. Newton's 2nd Law doesn't really care what type of force is applied to the body.

Unless it's perfect ice and kinetic friction is zero (doesn't happen in the real world), there can be propulsion. However, trying to walk normally is not necessarily the best way to use that low friction for propulsion.

As in driving car. The best performance on a slippery surface will be when the wheels exert just enough force to avoid wheel-spin which will be just below the limiting friction.

 

[Baluncore]

As in driving car. The best performance on a slippery surface will be when the wheels exert just enough force to avoid wheel-spin which will be just below the limiting friction.

However, with an agricultural tractor, ploughing a field, optimum performance is when there is about 5% wheel slip.

 

[berkeman]

However, with an agricultural tractor, ploughing a field, optimum performance is when there is about 5% wheel slip.

Interesting. Why is that?

 

[A.T.]

Interesting. Why is that?

I guess, because on soft ground (mud, sand, gravel) it's impossible to have no slip at all, if you want any relevant propulsive force. You get most traction force, when you deform and move the soft ground a bit.

 

[A.T.]

As in driving car. The best performance on a slippery surface will be when the wheels exert just enough force to avoid wheel-spin which will be just below the limiting friction.

Yes, but OP specifically asks about the case with slip.

 

[Baluncore]

Interesting. Why is that?

A long time ago, I designed a cruise control that maintained 5% slip, but I believe I under-quoted on what is now considered ideal slip.

It comes down to driving up out of the wheel rut, while continuously sinking back in, as opposed to pumping soil out of the way using the strakes on the tyre. With too much wheel slip, you are digging two deep trenches in the soil.

"The ideal amount of wheel slip is between 8 and 12 percent for a 4WD tractor and 10 and 15 percent for a 2WD tractor."
https://www.ntstiresupply.com/ptk-shared/optimal-wheel-slippage

 

[annamal]

Yes, but inefficiently.
Just like spinning the tires on a car, if there is any friction, there will be a propulsive force.

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?

 

[A.T.]

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?

Yes, kinetic friction is opposite to the relative motion of the slipping surfaces in contact. The foot can move backwards, while the center of mass of the body accelerates forward.

 

[Baluncore]

While one foot is sliding slowly backwards, the other is stepping a longer distance forward.

The work you are doing is the horizontal kinetic friction force, multiplied by the distance your foot slips backwards, generating heat. An equal force will accelerate your body.

Once your body has accelerated to the required walking speed, stop applying a force sufficient to slip.

 

[A.T.]

The work you are doing is the horizontal kinetic friction force, multiplied by the distance your foot slips backwards, generating heat.

Yes, that's the dissipated energy. The propulsive energy is that same force (magnitude), times the distance the center of mass of the body moves forward.

 

[annamal]

Yes, kinetic friction is opposite to the relative motion of the slipping surfaces in contact. The foot can move backwards, while the center of mass of the body accelerates forward.

What if the system is just for example a mechanical/electrical foot that moves and slips on the floor. As the foot pushes down and back on the ground, shouldn't the foot be slipping backwards, but according to the free body diagram, the foot should just be propelling forward due to the kinetic friction.

 

[Herman Trivilino]

TL;DR Summary: I know that static friction is what helps a person walk forward. I am wondering if the walking person still gets propelled forward if their foot slips and kinetic friction takes place

For example if the person tries to walk on ice, the foot just slips and the person doesn’t get propelled forward by the kinetic friction.

Of course he does. There's a bit of friction between the feet and the ice.

 

[A.T.]

What if the system is just for example a mechanical/electrical foot that moves and slips on the floor.

What do you mean by that? What exactly makes the "mechanical/electrical foot" slip on the floor? It's not clear what mechanism you have in mind. And if your "vehicle" has moving parts, then modeling it as one rigid block is eventually not useful for understating what's going on.

As the foot pushes down and back on the ground, shouldn't the foot be slipping backwards, but according to the free body diagram, the foot should just be propelling forward due to the kinetic friction.

Velocity can be backwards, while net force and acceleration is forwards. But it's unclear if that's the case in the scenario you envision, because you have not defined it properly.

 

[PeroK]

Can you walk forward on ice? Sure, but not very well.

It depends what you are wearing on your feet!

Also, rutted ice on a "dry" glacier can be an efficient surface to walk on.

 

[annamal]

What do you mean by that? What exactly makes the "mechanical/electrical foot" slip on the floor? It's not clear what mechanism you have in mind. And if your "vehicle" has moving parts, then modeling it as one rigid block is eventually not useful for understating what's going on.

What I mean is that there is a mechanical foot that moves the same way a real foot moves but is powered electrically. The foot starts at zero velocity and then pushes down on the ground, causing the foot to slide backwards, but if we analyze the free body diagram of the foot, the foot should just be moving forward not backward due to the kinetic friction with the ground and because the force causing the foot to slide backward is an internal force. So why is this foot moving backwards not forwards?

 

[A.T.]

if we analyze the free body diagram of the foot, the foot should just be moving forward not backward due to the kinetic friction with the ground and because the force causing the foot to slide backward is an internal force.

Internal to what? If the force is applied to the foot by the leg, then it is external to the foot. You have to make up your mind, if you are analyzing just the foot, or the entire body.

 

[annamal]

Internal to what? If the force is applied to the foot by the leg, then it is external to the foot. You have to make up your mind, if you are analyzing just the foot, or the entire body.

I simplified a person walking into just an electro mechanical foot. The force is applied to the foot to move it due to a battery powering the foot — so internal force

 

[A.T.]

The force is applied to the foot to move it...

Applied by what?

— so internal force

If it's applied "to the foot", then it is external to the foot, per definition.

 

[annamal]

Applied by what?

The force applied to the foot is powered by a battery and motors.

If it's applied "to the foot", then it is external to the foot, per definition.

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

 

[A.T.]

The force applied to the foot is powered by a battery and motors.

The question wasn't where the energy comes from, but what object applies the force to the foot.

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

What is the motor moving relative to what? So far, your "mechanical foot" is completely ill-defined, and sounds like nonsense. Provide a clear diagram of that thing.

 

[sophiecentaur]

the foot should just be moving forward

"should"???? I don't think any argument you can be presented with could get you thinking straight if you don't dig yourself out of this one. Just bear in mind that Newton's third law about action and reaction forces; they are always both there. Choose the right one and you won't be getting it wrong. (You must be wrong or you'd walk backwards. )

 

[Baluncore]

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

I think before considering one foot on slippery ground, you should consider several feet on slippery ground. But before considering several feet on slippery ground, you should consider spinning a wheel on slippery ground.
The Manx flag, from the Isle of Man, is a relevant concept here. As the wheel spins, the feet slide backwards on the ground, while the vehicle body moves forwards, as an equal and opposite reaction to the kinetic friction.

 

[sophiecentaur]

according to the free body diagram, the foot should just be propelling forward due to the kinetic friction.

The distinction between static and kinetic friction 'categories' is really more harmful than useful; there is only one force in all circumstances. the only difference is that static friction force is the limiting force, just before slipping. Whether you're on ice or a very rough surface with rough shoe soles, exactly the same thing applies - only one word better than the other; the 'forward' force on your body is less on ice.

 

[weirdoguy]

The distinction between static and kinetic friction 'categories' is really more harmful than useful;

Harmful for whom? As a teacher with 17 years of experience, I wholeheartedly disagree. In the end these are only labels, yes, but the road to this 'end' is long and bumpy. Not differentiating (lingustically) between forces that do behave differently would make these bumps bigger.

 

[A.T.]

The distinction between static and kinetic friction 'categories' is really more harmful than useful;

Being useful for solving practical problems, is the main reason why this distinction exists. The distinction is also useful from the energy standpoint, because kinetic friction is a dissipative force, while static friction isn't.

the only difference is that static friction force is the limiting force, just before slipping.

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

 

[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.