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I saw this animation (in which a man is running in place) and thought to repeat it and guess what ... I was able to do this.

enter image description here

My legs were pushing the earth back (I know this since my legs were sliding on the rough surface) but I was not being displaced. I would like to know how is this even possible. There is just a single friction force on me in the forward direction (from Newton's third law) but then also I didn't move at all.

Can someone explain the physics behind this ?

If we keep some cloth below us and run over it in the same way , the cloth is thrown back which indicates we are pushing the surface below us.

Note : Those who think it's a useless question just about the animation and nothing real, they should try to do this themselves :)

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    $\begingroup$ It's an animation. What are you asking? Is there zero friction? Is he on a treadmill? is the video tracking him as he moves forward? $\endgroup$ Nov 16 '20 at 18:22
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    $\begingroup$ Now this feels somewhat like moonwalk. $\endgroup$
    – Anonymous
    Nov 16 '20 at 19:06
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    $\begingroup$ @annav I do not see this as "running in place, but that may be how i interpret the leg motion - I don't think it can be done in-place $\endgroup$ Nov 16 '20 at 19:53
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    $\begingroup$ I'm not sure if there is something wrong with me but I couldn't pull this same result out by trying the experiment. I also think to prove this phenomena actually exists you would need to do controlled experiments, if you could at that to this question maybe it will gain a lot of upvotes. One suggestion for now is to make the title better $\endgroup$
    – Buraian
    Nov 23 '20 at 12:27
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    $\begingroup$ Yes one can do it, at least I did it,but I'm not sure if I really was pushing the earth back. $\endgroup$
    – Kashmiri
    Nov 23 '20 at 14:37
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My legs were pushing the earth back but I was not being displaced.

How do you know your legs were pushing back against the earth? Did you have a force meter?

What you would actually find, were you to measure the situation properly, is that you did not actually push back against the earth. You only pushed up and down, effectively jogging in place.

The human body is a mighty complicated structure, and our intuition is surprisingly incomplete, given how much time we spend with that body. When you say "my legs were pushing the earth back," what you actually mean is that you felt as if your legs were pushing the earth back. This is proprioception, the word for our feeling for where our body is and what it is doing. It's an astonishingly powerful system, but it has flaws based on how it goes about measuring things. You can indeed fool yourself into thinking you are pushing on the earth. How? Well, you observe that for your current body position, contracting a particular set of muscles should push against the earth. However, our muscles come in pairs. If the antagonistic element of that pair fires at the same time, your muscle will tense as usual, but it will not generate any force on the ground. Instead, it will simply oppose the forces applied by the antagonist.

If you wanted to prove this, one thing you could do is try to repeat the experiment on ice, where the coefficient of friction is much lower. If you find that you can indeed do it on ice (which you should be able to), and feel "the same" as you did on solid ground, then you know the backwards force is insufficient to break the static friction on ice... which is pretty weak. This would quickly demonstrate that there was indeed no backwards force on the earth in the first place, and leave us with biological reasons for the feeling you experience.

Edit: The above answer was provided before the OP edited in the fact that they were letting their feet slip. In the case where the feet slip, the situation is actually more intuitive. When the feet stuck, one had to explain why there was the feeling of motion when there was no motion. In this case, there is motion.

In this situation, you are not actually staying perfectly still. While your foot is sliding backwards, it is indeed applying a force that propels you forward, accelerating your body. To stay still on average, there must be another force applied which decelerates your body on average. This force is almost certainly applied at the start of the footfall, when your foot is in front of you.

This is actually rather easy to see physically. How do you slip your foot backwards? If your full weight was on the ground when you did so, you would have quite a lot of forward acceleration. Your motion would look more like a step forward and a hop back! So it is almost certain that you are unweighting your foot during the slide. This is typically accomplished by a slight jump upwards, and then sliding your foot backwards once your upward acceleration is sufficient. This also means that, during that initial upward movement, right after the footfall, you are putting a very large amount of force onto the front foot (your weight plus enough to accelerate you upwards). This means you can have a very high force of static friction.

So when your foot falls, you use static friction to arrest your forward motion. You push upwards, unweighting the foot, and letting it slide back. This applies a little forward motion. When your other foot falls, it arrests that motion, pushing you back to where you were.

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  • $\begingroup$ This is a good answer but I’m confused by your last paragraph. Can you explain me this ice experiment in more detail? Isn’t the backwards force being insufficient to break the static friction also required on regular running? Or else the person would slip and fall? $\endgroup$
    – user137288
    Nov 23 '20 at 17:23
  • $\begingroup$ The OP also mentions his feet sliding through the surface. Doesn’t that mean he broke the static friction? Overall I don’t understand why trying the same thing on ice would help prove something. Please help me understand. $\endgroup$
    – user137288
    Nov 23 '20 at 17:31
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    $\begingroup$ @RobertoValente The idea of the ice was that it is very easy to break static friction on ice, thus we know the force of friction is very low. This was before the edit you mention, where the OP added the bit about the feet sliding. That changes the story substantially and I will have to amend my answer, as it is basically a completely unrelated physics problem. $\endgroup$
    – Cort Ammon
    Nov 23 '20 at 18:06
  • $\begingroup$ I think I understand what you mean. In the original case, you believed there wasn’t really any backward force made by the feet. If the OP tried it on ice, and there was indeed some backward force, it would exceed static friction and he would slip and fall. But that probably wouldn’t happen, as you thought there wasn’t any backward force. Is this interpretation correct? $\endgroup$
    – user137288
    Nov 23 '20 at 19:01
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    $\begingroup$ @RobertoValente Yep. And while our intuition isn't very good about friction on a floor, we understand that there's very little friction on ice quite well, making it a useful test case for intuition. $\endgroup$
    – Cort Ammon
    Nov 23 '20 at 19:31
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It is an exercise called "running in place"

Running in place doesn’t provide the same benefits as running since you use different muscles and movements, but many of the benefits are similar

From this article on the physics of running,

runner

to run in place you must use your feet in such a way that there is no forward momentum due to friction of your shoes with the ground , which explains the quoted sentence above. It will be more like the frictional forces present in jumping in place.

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  • $\begingroup$ Respectfully disagree, because if "legs push the earth back" (OP's words) then there is no way not to move forward. $\endgroup$ Nov 16 '20 at 19:54
  • $\begingroup$ @CarlWitthoft the op's "feelings" are not "facts". The physics fact is that one does not move forward, this means that the forces in the horizontal must be zero. This means that the muscles used act so that the total direction of momentum is only in the vertical direction. There even exist walking machines, useful in total lockdown ( I unearthed mine) , these compenste for the forward motion with pumps and springs, which is what the muscles do in the "running in place" at a chemical level. $\endgroup$
    – anna v
    Nov 17 '20 at 5:28
  • $\begingroup$ I mean compensate for any small forward motion that gives rise to the "feeling". Do not forget radiation takes away momentum. $\endgroup$
    – anna v
    Nov 17 '20 at 5:40
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    $\begingroup$ if we were not sufficiently agile,yes we might slip, but we use the force of our feet to get an equal and opposite frictional force to propel us forward with a constant velocity more or less. Even if you ran in jumps, the constant velocity would land you to continue getting the next impulse , if agile enough. $\endgroup$
    – anna v
    Nov 23 '20 at 19:55
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    $\begingroup$ @RobertoValente the heat in the muscles and the friction with the floor is directional also , and heat is finally radiation $\endgroup$
    – anna v
    Nov 23 '20 at 20:23

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