I know I am asking a very basic question but still I wanted to make my understanding clear. Please correct me if I am wrong about what I know so far. Whenever someone starts to walk , he/she pushes the ground backward, since there is no sliding between the shoes and the ground, so static friction is acting which enable us to walk as it acts in the direction in which we want to walk . But my question was , that eventually , when we lift our leg , does our leg starts to slide over the surface , has it overcome the static friction and kinetic friction starts to act or is it just that , we just lift our leg and broke the contact between the leg and the surface. Moreover , I wanted to know , if friction is the cause of our motion , then there must be certain limit above which we cannot accelerate and that is the limit when we start slipping . Is all this of correct?
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$\begingroup$ Welcome to PhysicsSE. Feel free to take a tour of this site to get a good understanding of how this site works. Hope you get an answer soon. $\endgroup$– Pritt BalagopalCommented Sep 12, 2017 at 5:32
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$\begingroup$ see aplusphysics.com/community/index.php?/blogs/entry/…. Although not directly related to your question, the energetics are discussed here: europhysicsnews.org/articles/epn/pdf/2008/05/epn080505.pdf $\endgroup$– ZeroTheHeroCommented Sep 12, 2017 at 5:53
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$\begingroup$ Your picture is not complete. We're also falling forward as we walk, which also places a limit on the maximum speed, check: en.wikipedia.org/wiki/Walking#Biomechanics $\endgroup$– stafusaCommented Sep 12, 2017 at 8:03
2 Answers
Practically speaking, one can think of it as simply breaking the contact between the foot and the surface. If one dives deeper into the milliseconds as the foot is becoming unweighted, the maximum force of friction that can be applied to our foot becomes smaller and smaller, so there's technically going to be some slipping. However, it is very minor.
When Ninjas learn the art of walking on rice paper, they're developing awareness for these tiny forces. They are learning to transfer their weight such that the back foot has very little lateral forces on it as they lift it. This sort of awareness is very useful for them.
For a more close-to-home example, we can answer your second question by considering someone walking on ice. When people slip and fall, quite often it's because their back leg slides out from beneath them. This is proof of the conjecture you had. On ice, the maximum force of friction you can attain is quite low. As people walk, they typically issue one last quick correction with their back foot before letting it rise off the ground. If this movement is too forceful, the back foot can slip.
Yes, all your statements are correct and for each one all of us have real-life experience.
On a very slippery, that is low-friction, surface you can move by pushing and sliding. Indeed, this is how people naturally cross any slippery surface: Keeping both feet on the ground and slowly sliding forward. However, this is not how you walk. Instead, you lift your foot off the ground and move it through the air. This is much more efficient as you experience less friction moving your feet through the air than sliding over the ground.
There also is a maximum acceleration and conversely a maximum deceleration. We all know this from slippery surfaces where you can't just stop, but continue to slide for a few meters.
