# I Friction Force: Why do we walk?

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1. Aug 12, 2018

### babaliaris

I created this example to see if i can explain how humans walk but I can't. If I'm correct, when we push the ground to move forward we apply a force with the opposite direction (backward). Let's name this force Fp (for push). Because of that force, a friction force is being applied to us with the opposite direction of Fp (forward). While |Fp| <= |Fsmax| , we don't move at all. At some point |Fp| > |Fsmax| so we should slide (move) backwards and then Fs will be Fk (kinematic Friction).

Then why in the real word we move forward? Am I missing something here? Please consider that I'm new in Physics.

Last edited: Aug 12, 2018
2. Aug 12, 2018

### FactChecker

That sounds wrong. The static friction is between the bottom of your shoe and the ground. So this case is normal walking without your feet slipping. Why do you say you wouldn't move at all?

3. Aug 12, 2018

### e-pie

4. Aug 12, 2018

### Bandersnatch

No, that's when we move. What doesn't move is the foot.
We move by pivoting our torso on our legs. The leg has to be anchored on the ground - i.e., not sliding, i.e., Fp <= Fsmax. The thing that moves is the torso.
Put a person on slippery ice, and the torso does not move because the leg is not anchored.

5. Aug 12, 2018

### babaliaris

That explain's it perfectly! Thank you!

6. Aug 12, 2018

### Delta2

Just put in a different way, what is the total force on our body when we walk (assuming we don't touch anything with our hands)? Since our weight is cancelled by the normal force, all that is left is the force of static friction between our foot and the floor(Fp is the force from our foot to the floor, this force is acting upon the floor, not upon us).
The force of static friction is what put us in motion, contrary to the usual case, where we are used to have static friction to try to oppose the motion of an object.

7. Aug 12, 2018

### babaliaris

Also i noticed that when i walk and pivoting my torso forward's I move faster than when it's straight. This is because I changed the direction of my weight and now $$Fg_{x}$$ becomes greater than 0?

Last edited: Aug 12, 2018
8. Aug 12, 2018

### babaliaris

So in other words, because of the static friction you have the ability to anchor your foot on the ground. By anchoring your foot, you can pivot your body and change the direction of your weight and now its your weight that creates the force that accelerates you and you move. If you pivot forward you accelerate forward, if you pivot backwards you accelerate backwards. This force is $$Fg_{x}$$ where normally if you stand straight is 0. OMG I LOVE PHYSICS!!!!!

Last edited: Aug 12, 2018
9. Aug 12, 2018

### Delta2

I don't think that you can change the direction of your weight, you can only change the centre of mass of your body. I think what you do is that you increase the normal force by pressing the ground harder, so that increases the static friction from the ground to body, so greater force from the friction means greater acceleration, cause as I said, the force of static friction is the only force that remains on our body and is what put us in motion.

10. Aug 12, 2018

### babaliaris

Post #7 #8 are wrong you are right... You can't change the direction of weight lol what i was thinking, once again I'm overthinking

11. Aug 12, 2018

### Delta2

Not completely wrong, you might not change the direction of weight, but after thinking it abit more carefully myself, I think what you change is the direction of $F_p$. Do you agree?

12. Aug 12, 2018

### babaliaris

Yes!!!

13. Aug 12, 2018

### Staff: Mentor

As far as I can see, nobody addressed [correctly] the real error here:
A free body diagram includes all of the forces acting on a body. Fp is a force you apply to the ground. It should not be on your diagram. Now what is left is one horizontal force, pushing you forward.

14. Aug 12, 2018

### Delta2

I think I say it inside the parentheses in post #6.

15. Aug 12, 2018

### babaliaris

Lol you are extremely right!!! Fp is being applied to the ground not to your foot!!! And according to Newton's third law, the ground must also apply a force to you and this is the friction force, this is why you move forward!!!

16. Aug 12, 2018

### babaliaris

LoL didn't see that

17. Aug 12, 2018

### Delta2

e hehe lol

18. Aug 12, 2018

### Klystron

If the OP is able to visit San Francisco, California; I recommend a trip to Frank Oppenheimer's Exploratorium https://www.exploratorium.edu/ .

Along with many fascinating exhibits including a rendition of Foucault's Pendulum demonstrating the Earth's rotation, you may find a small interactive model of the human torso, legs and feet. A visitor can study the sequence of nerves and muscles that produce bipedal walking by pressing a sequence of switches simulating the electro-chemical impulses from the brain. Anecdotally it appears that adult visitors tend to over and under control the sequence while young humans quickly master the non-intuitive series of electrical impulses required for smooth gait and (simulated) forward motion. A wireless "docent" explains the physics of bipedal motion.

If the OP still searches for models to simplify walking physics, consider using thrust to model the motive force coupled with torque to simplify the moment vectors through the human body. A more advanced walking model might include swinging arms and even head angle as center of gravity (COG) varies with gait and stance.

As suggested in an earlier post, abandoning the requirement for friction between the foot and ground to produce momentum, say on smooth ice, provides deeper understanding of how altering body COG and manipulating torque provides both forward and angular momentum. Examine limb movements of ice skaters, for example. Very low drag (friction) and controlled stance yields amazing motion. Going beyond simple COG to recognize moment arms, understand how a spinning skater alters their angular momentum by extending and contracting their limbs.

Isaac Asimov and other science writers have described bipedal walking as "controlled falling". There are studies available online where actors wear suits equipped with reflective devices ("dots") while performing various human motions such as walking, running, dancing, etc. within range of cameras and motion sensors.

19. Aug 12, 2018

### babaliaris

I live in Greece...

20. Aug 12, 2018

### jbriggs444

Does that affect your floors like this?