# I Static friction's role in walking

#### alkaspeltzar

Summary
Confused on friction, does it move the walker or prevent the foot from sliding
Okay, I have been reading when we walk, it is the static friction that propels us forward. See below:

"When we start running we exert a force on the ground. That's why ground also gives us a reaction force equal and opposite to the force exerted by us. The horizontal component of that reaction force is the frictional force. Now if we exert more force on the ground, it will also return us more...so we'll be accelerated,.."

But here is where I get confused. Doesn't friction stop the foot from sliding, so wouldn't the force on the foot and the friction for cancel out, hence the foot stay stationary. Then if so, how is it they say the friction force propels us forward?

I see two forces on the foot. One is the force from our body, other is friction. Those would cancel right? But the above quote makes it sound like friction is a net force on the body moving it forward. What am I looking at wrong. Thank you

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#### A.T.

wouldn't the force on the foot and the friction for cancel out
The horizontal force on the foot is friction.

I see two forces on the foot. One is the force from our body,
The foot is part of the body.

#### BvU

Homework Helper
Hi,

When you stand up straight and still, and you want to get going, what do you do ?
You lean forward
Friction does as you say
friction stops the foot from sliding, so wouldn't the force on the foot and the friction for cancel out, hence the foot stay stationary.
It would! If you don't lift your other foot, and move it forward to stop yourself from going flat out, you get nowhere. And if you don't pull in your first foot, you'll make a full stop.
The experiment is easy: Try it out consciously !

#### alkaspeltzar

The horizontal force on the foot is friction.

The foot is part of the body.
can you explain alittle more? I see two forces on the foot, friction pushing back and the force from our leg on the foot. So how is friction pushing us forward?

#### alkaspeltzar

Hi,

When you stand up straight and still, and you want to get going, what do you do ?
You lean forward
Friction does as you say
It would! If you don't lift your other foot, and move it forward to stop yourself from going flat out, you get nowhere. And if you don't pull in your first foot, you'll make a full stop.
The experiment is easy: Try it out consciously !
okay I agree with this. But then why do other explanations say it is the friction that propels us forward? walking has been described as controlled falling, which agrees with what you and I have said.

Thanks

#### A.T.

force from our leg on the foot.
The text you quoted treats the whole human as one body. It doesn't analyze internal forces.

#### A.T.

But then why do other explanations say it is the friction that propels us forward?
What other external force do you see that could accelerate us forward on level ground?

#### alkaspeltzar

What other external force do you see that could accelerate us forward on level ground?
Okay so if you look at the whole body(leg, foot etc all together), the only external force is the friction. That makes sense.

But would it be wrong in understanding then that the internal forces(of the body) cause that friction force to be translated thru the body and thus moving the torso, because in the end, the foot stay planted. The body as a whole only moves.

#### A.T.

Okay so if you look at the whole body(leg, foot etc all together), the only external force is the friction. That makes sense.

But would it be wrong in understanding then that the internal forces(of the body) cause that friction force to be translated thru the body and thus moving the torso, because in the end, the foot stay planted. The body as a whole only moves.
The momentum from external forces is always transferred through the body they act on. And you can always split a body into parts for a more complex analysis.

#### russ_watters

Mentor
can you explain alittle more? I see two forces on the foot, friction pushing back and the force from our leg on the foot. So how is friction pushing us forward?
All forces come in pairs like that. You need to ignore the reaction force. Look into what a free body diagram is: it show the forces on an object only.

#### alkaspeltzar

The momentum from external forces is always transferred through the body they act on. And you can always split a body into parts for a more complex analysis.
so essentially yes then?

#### alkaspeltzar

All forces come in pairs like that. You need to ignore the reaction force. Look into what a free body diagram is: it show the forces on an object only.
I know what a free body diagram is. I was actually looking at one over this situation which is how it got me thinking. I see the static friction force pushing back on the body as a whole. But then I started wondering why the foot stays planted when in the end, it is our torso that moves etc etc.

Similar example would be swimming. We push on a wall, wall pushes back. In the free body diagram, it is the wall that pushes out into the water. However, the feet don't move. Just got me think why.

If you have an good explanation of that, that would most likely clear my confusion. Thank you

#### jbriggs444

Homework Helper
But then I started wondering why the foot stays planted when in the end, it is our torso that moves etc etc.
If you want to know how the foot moves, create a free body diagram for the foot. It is subject to a frictional force from the ground in the forward direction, a normal force from the ground in the upward direction, a small downward force from gravity and a large diagonal force from the ankle with both a vertical and horizontal component. You might consider enhancing this diagram to reflect the existence of a joint in compression plus an Achilles tendon in tension. That gives the possibility of applying torque through the ankle joint. Do not forget that the ankle joint can support shear forces as well as simple compression

If you want to know how the shin moves, you can build a free body diagram for the shin. You'll have the force from the foot through the ankle joint and from the Achilles tendon through the calf. You'll also have the force from the knee joint with both joints and tendons in play. And you'll have some down force from gravity.

If you want to track how the thing is powered, pay attention to the muscles. The two ends of a muscle move relative to one another. The total work done summed over their external attachment points can be either positive or negative.

#### alkaspeltzar

Can someone answer my questions about the swimmer? I think that is part of my confusion.

Take the swimmer that pushes away from a wall. The wall is the reaction force. As the person pushes away, their body moves but the feet stay planted. But isn't the reaction on the feet/body? Why does this happen in this way?

#### russ_watters

Mentor
Can someone answer my questions about the swimmer? I think that is part of my confusion.

Take the swimmer that pushes away from a wall. The wall is the reaction force. As the person pushes away, their body moves but the feet stay planted. But isn't the reaction on the feet/body? Why does this happen in this way?
Again; no reaction forces. There is one force and it is applied to your feet by the wall. Then through a complex series of levers, this force is transmitted to your torso. You can simplify this and draw a free body diagram of your torso, floating in water, with one force applied to it. Then f=ma

It works very much like a simple machine of a spring-launched mass. For the free body diagram, you ignore everything but the mass and the one force being applied. The obvious difference for your body is that the "pring" (your legs/feet) is not massless, so after the spring finishes accelerating the mass, there is a new interaction between them. But that's a separate part of the motion that comes later.

#### alkaspeltzar

Again; no reaction forces. There is one force and it is applied to your feet by the wall. Then through a complex series of levers, this force is transmitted to your torso. You can simplify this and draw a free body diagram of your torso, floating in water, with one force applied to it. Then f=ma

It works very much like a simple machine of a spring-launched mass. For the free body diagram, you ignore everything but the mass and the one force being applied. The obvious difference for your body is that the "pring" (your legs/feet) is not massless, so after the spring finishes accelerating the mass, there is a new interaction between them. But that's a separate part of the motion that comes later.
Thank you. That makes sense now. I guess I was trying to break it down too far and you are right, I need to look at the free body diagram as a model of a rigid body, but know in reality, thru the muscles and such, the force is transmitted.

The same make sense with the leg/foot in the walking example. I get the free body diagram and see frictions role. I was then trying to break it down from the foot to the leg to the torso and in the end, it was confusing me trying to look at it all together.

#### A.T.

I was then trying to break it down from the foot to the leg to the torso and in the end, it was confusing me trying to look at it all together.
Mixing up different ways to cut a scenario into bodies is a common source of confusion, along with mixing up different reference frames. It leads to apparent contradictions.

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#### alkaspeltzar

I do have two questions still that confuse me.

In my physics book, Tipler, it gives an example of walking. It states "to walk forward, you push backwards on the floor and the floor pushes forward on you, with the force of static friction". So my question is really the static friction is the reaction force to us pushing on the floor correct? Newtons third law?

My second question however is what keeps my foot from slipping. Everyone says friction, but the friction here is pushing me forward. My foot is still apply force against the earth, which for most case is un movable. So isn't this similar to pressing against a wall with large mass. hence our foot doesn't slip because it pushing on the earth at a strength equal or less than the max friction force the earth is pushing back on us?

most cases we encounter friction subtracting our input force( like pulling a box with a rope). here it is a reaction force, I think that is confusing me.

#### russ_watters

Mentor
So my question is really the static friction is the reaction force to us pushing on the floor correct? Newtons third law?
This sentence isn't grammatically correct, and that makes it hard to answer. Can you try again? To repeat previous answers, though, yes the floor pushing on your foot and your foot on the floor form a Newton's 3rd Law pair.
My second question however is what keeps my foot from slipping.
Friction.
Everyone says friction, but the friction here is pushing me forward. My foot is still apply force against the earth, which for most case is un movable. So isn't this similar to pressing against a wall with large mass.
I guess, but why muddy the water? Usually we keep the friction force and normal force separate because it's easier to keep track of them. Swapping them for no reason is just confusing.

But if you really want, consider a runner pushing off a vertical starting block; Two normal forces.
hence our foot doesn't slip because it pushing on the earth at a strength equal or less than the max friction force the earth is pushing back on us?
Cumbersome wording, but yes.
most cases we encounter friction subtracting our input force( like pulling a box with a rope). here it is a reaction force, I think that is confusing me.
Sure, but that's only because you are focusing on one friction force while ignoring the other!

#### A.T.

So my question is really the static friction is the reaction force to us pushing on the floor correct? Newtons third law?
Both forces in Newton's 3rd Law are static friction here.
My second question however is what keeps my foot from slipping. Everyone says friction, but the friction here is pushing me forward.
You are again being inconsistent about about defining your bodies. One time the friction acts on the foot, then on you as a whole.

#### alkaspeltzar

Russ and A.T.

You said friction keeps our foot from slipping, but then agreed with this statement:
"hence our foot doesn't slip because it pushing on the earth at a strength equal or less than the max friction force the earth is pushing back on us?"

I am getting confused because as I read the responses, it seems like friction at one point is the reaction force from newtons third law, only acting on the body. But then in other responses, the friction appears to be equaling/subtracting out other forces, keeping the foot in place.

How can it do both?

#### sophiecentaur

Gold Member
It could make life easier if we acknowledge that Friction is a mechanism (interaction beteen two surfaces or substances) that produces a force - rather than a force in itself. If you look upon friction as something that happens to a thin layer between two objects in this case and that layer exerts forces both on your foot and on the ground. (like a piece of string under tension or a strut in a lattice).
One has to avoid applying too much intuition, right at the end of a logical flow of ideas, rather than accepting where that flow is taking you. The word Friction carries a lot of baggage with it; it must slow things down, it must make things hot etc. etc. and that interferes with straightforward understanding of a perfectly good line of reasoning.

#### A.T.

I am getting confused because as I read the responses, it seems like friction at one point is the reaction force from newtons third law, only acting on the body. But then in other responses, the friction appears to be equaling/subtracting out other forces, keeping the foot in place.
There you go again, talking about the body then just the foot. Did you read post #17 and #20.

#### russ_watters

Mentor
How can it do both?
There's two friction forces: the floor applies a friction force to your foot and your foot applies a friction force to the floor. These forces do not cancel out because they are acting on different objects and they do not tell you if your foot moves because there are other forces being applied to your foot.

You said you know what a free body diagram is, but you seem to be repeatedly describing it wrong. Please draw one and post it. It will become more clear when you can actually see the error.

"Static friction's role in walking"

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