Why are our feet fixed when walking? Why does the static frictional force acting on our feet not push our feet forward ?
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$\begingroup$ Here is a similar question, but for a car driving in a circle. physics.stackexchange.com/q/495120/37364 This is a little more complex. A key fact is the bottom of rolling tire has speed $0$, the same as the road. Friction from the road is important for steering the car in the same way that friction is important for walking. $\endgroup$– mmesser314Commented Mar 20, 2020 at 15:27
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$\begingroup$ @mmesser314 I don't think the importance of friction is being questioned here $\endgroup$– BioPhysicistCommented Mar 20, 2020 at 16:08
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$\begingroup$ @AaronStevens - I agree. The questions seems to be about what forces act on a foot while walking. You answered it correctly. I just thought the reference might help clarify some concepts if there was still some confusion. Unfortunately, a rolling tire is harder to understand, so it might not help. $\endgroup$– mmesser314Commented Mar 20, 2020 at 16:15
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$\begingroup$ @ mmesser - this question confused me .. $\endgroup$– Bilgehan YılmazCommented Mar 20, 2020 at 16:58
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4 Answers
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If the foot is not accelerating, then that means it has a net force of $0$ acting on it. Since the static friction force acts forward, this means that there is another force acting on the foot that has a component pointing backwards. The only place this force can come from is the leg, and this makes sense. By Newton's third law, if the leg is pushing backwards on the foot$^*$, then the foot is pushing forwards on the leg, which is what we want to have happen if we are trying to walk forward.
Of course this is a huge over simplification of walking, which involves multiple steps (pun always intended) and many interconnected body parts, forces, etc. But I think just the simplistic view I present above is useful in the initial understanding of why the net horizontal force acting on the foot is $0$ for the part of the step where the foot is on the ground.
$^*$Of course vertical components of these forces are important as well, but of course the vertical components of the forces acting on the foot cancel as well (at least in the part of the step where the foot is not accelerating vertically). I don't think analysis of these vertical components is important for this discussion focusing on horizontal movement due to friction though.
answered Mar 20, 2020 at 15:08
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$\begingroup$ @ Aaron Stevens Our foot The forces coming from our legs keep it in balance. and then our leg pushes us forward as per newton 3rd law. did i get it right ? $\endgroup$ Commented Mar 20, 2020 at 15:24
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$\begingroup$ @BilgehanYılmaz Yes, that is the idea I am trying to convey in my answer. Although keep in mind it is a pretty "crude" view, as you can break the foot and leg down into bones, muscles, ligaments, etc. I am just offering a more "zoomed out" view $\endgroup$ Commented Mar 20, 2020 at 15:41
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$\begingroup$ @ Aaron Stevens so can we draw muscle strength and free body diagram of motion here ? For example, while pushing the wall, our hands are immobile for a while while our Body is moving backwards. $\endgroup$ Commented Mar 20, 2020 at 17:07
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$\begingroup$ @BilgehanYılmaz I am not sure what you are asking, but you can apply the same reasoning to that example. The wall pushes on your hand in one direction, your arm pushes on your hand in the other direction. So your hand doesn't accelerate. But by N3L your hand pushes on your arm in the direction that is away from the wall, which is what you want to have happen. $\endgroup$ Commented Mar 20, 2020 at 17:23
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$\begingroup$ yeah Aaron, I already thought of what you said before. However, I cannot show this in the free body diagram. $\endgroup$ Commented Mar 20, 2020 at 19:11
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Because the static friction is just enough to balance the force applied by us. However there is a maximum value to it. Also if its going to make our feet move forward, then kinetic friction will be generated, the friction won't be static in that case.
answered Mar 20, 2020 at 13:31
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$\begingroup$ What is the force balancing the force Fs in the drawing above? $\endgroup$ Commented Mar 20, 2020 at 13:33
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$\begingroup$ That is the force applied by us in the horizontal direction $\endgroup$ Commented Mar 20, 2020 at 13:37
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$\begingroup$ How did you balance the fs force ? , I'm dontunderstand you bro.. $\endgroup$ Commented Mar 20, 2020 at 13:50
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$\begingroup$ This answer, while correct, does not answer the question. $\endgroup$ Commented Mar 20, 2020 at 14:44
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Friction is a force that opposes the relative motion of bodies in contact.when we are stand on our feet ,there is no force that make relative motion So there is no Friction.
If some how there is a force that act on feet somehow the static friction just balance it,it can't be greater than that force. 1:For bodies not in relative motion (static friction), $$0 \leq f\leq \mu N$$.
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$\begingroup$ What is the force balancing Fs as I draw? $\endgroup$ Commented Mar 20, 2020 at 13:28
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$\begingroup$ @BilgehanYılmaz The force from the leg is balancing out $f_s$. $\endgroup$– SteevenCommented Mar 20, 2020 at 13:37
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$\begingroup$ @ Steeven How can I get to you, Steven? is the leg force pushing us forward while balancing the foot ? can you help me steeven? I'm a civil engineer, I'm writing a book, sometimes I want to get half of you for my questions... $\endgroup$ Commented Mar 20, 2020 at 13:48
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$\begingroup$ @BilgehanYılmaz: In your diagram there are two forces label $f_s$, one is orange and the other is blue. They cancel one another. Similarly, there are two forces labeled $f_a$, one is the person's weight and the other is the normal force from the floor. $\endgroup$– JamesCommented Mar 20, 2020 at 14:21
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Friction cannot accelerate an object, only slow it down. This is due to the second law of thermodynamics. The frictional force from the floor and the force exerted on the foot by the leg balance out and the foot does not acclerate while in contact with the floor.
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1$\begingroup$ Friction can accelerate a body. How does a car accelerate on road then? $\endgroup$ Commented Mar 20, 2020 at 13:26
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$\begingroup$ would you do this on the drawing? $\endgroup$ Commented Mar 20, 2020 at 13:27
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$\begingroup$ Friction can cause acceleration and could in principle increase the velocity of an object. If you put something on a treadmill the friction between it any the track will increase its velocity. $\endgroup$– CharlieCommented Mar 20, 2020 at 13:31
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$\begingroup$ Friction opposes relative movement between two surfaces; which can definitely be used to accelerate an object. $\endgroup$– JMacCommented Apr 21, 2020 at 17:17