Direction of frictional force during walking

  1. Will you please explain where actually the force due to friction acts while a man is walking forward.
    I am really confused with a lot of confusing things in the internet.
    I am unable to give a fixed answer to my students.

    I saw somewhere that friction acts in forward direction, please explain it to me.

    Since this is my first post in here, I don't actually know the rules, please consider.
     
  2. jcsd
  3. A.T.

    A.T. 5,490
    Gold Member

  4. adjacent

    adjacent 1,533
    Gold Member

    Friction always opposes the relative motions between two surfaces in contact.
    So when the foot moves from left to right,friction is from right to left

    What is the website where you got confused?

    Zz
     
  5. Chestermiller

    Staff: Mentor

    When you are walking forward, the bottom of your rear foot exerts a backward frictional force on the ground. The ground exerts an equal and opposite forward force on your rear foot. This force moves you forward.
     
  6. A.T.

    A.T. 5,490
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    He is asking about normal walking, not sliding around. In normal walking there is no relative motion between shoe and ground during most of the stance phase. The horizontal ground reaction force is static friction, not dynamic friction. Its direction is the same as the horizontal acceleration of the body's center of mass.
     
  7. A.T.

    A.T. 5,490
    Gold Member

    Only in the late stance phase. In the early stance phase it is the other way around, and you are braking. See diagram from above link:

    [​IMG]
     
  8. Chestermiller

    Staff: Mentor

    Hi A.T.:

    Very interesting. Am I correct in saying that the integral of the frictional force by the ground over each gait cycle has to be positive? I can't tell from the figure whether this is the case.

    Chet
     
  9. A.T.

    A.T. 5,490
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    If your final horizontal speed equals your initial horizontal speed, the integral is only slightly positive due to air resistance.
     
  10. AlephZero

    AlephZero 7,300
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    If all the friction is static (i.e. no sliding), the friction force does no work. The work done to overcome air resistance comes from your muscles, not from friction. The work is done in changing the angles of the joints in the legs.

    I don't see any obvious reason why the time-average of the friction force should be take any particular value. You could speculate that it the average would be in different directions depending on how you were moving (e.g. sprinting, or goose-stepping)
     
  11. Chestermiller

    Staff: Mentor

    Thanks AT and AlephZero. Excellent answers.

    Chet
     
  12. adjacent

    adjacent 1,533
    Gold Member

    I think we should wait for the OP to return now.
     
  13. A.T.

    A.T. 5,490
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    Work is not everything. Momentum conservation still applies. At constant average speed the horizontal momentum transfer from the ground, must cancel the horizontal momentum transfer to the air.

    Different speeds have different air resistance, and therefore require different horizontal momentum transfer from the ground, in order to maintain a constant average speed.
     
  14. AlephZero

    AlephZero 7,300
    Science Advisor
    Homework Helper

    Thinking a bit more, from the free body diagram for the human, the time-average friction force on the ground + the time average horizontal air resistance force = 0, for constant average walking velocity.

    (And that seems like a different way to state A.T.s momentum transfer argument).
     
  15. A.T.

    A.T. 5,490
    Gold Member

    Human walking can be modeled as an inverted pendulum. Without air resistance and muscle action, the horizontal ground reaction force (friction) would be a perfectly symmetrical back/forward profile and sum up to zero during the stance phase.

    [​IMG]

    In reality there is damping and propulsion by the muscles.
     
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