Does kinetic friction propel a person walking forward?

[annamal]

TL;DR

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.