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Dynamic equilibrium - walking in constant speed

  1. Sep 23, 2008 #1
    1. The problem statement, all variables and given/known data
    A man is walking in a straight line in a constant speed V. All variables are known. Since the speed in constant, the net force is zero. But aside from the static friction which is allowing him to move forward, what other force is present (in the direction of the movement), which is supposed to cancel out the friction so that the net forces is zero?

    2. Relevant equations
    [tex]\sum[/tex]F = 0

    3. The attempt at a solution
    I tried comparing it to walking on a treadmill on a constant speed. In this situation, there is no (or isn't supposed to be) static friction, since you're not really pushing yourself forward, only "jumping" while the belt is going backwards, so that you stay at the same position. But when walking on the ground, I have to use friction in order to move forward, so what is the force that is opposite in direction to the static friction that is acting upon me?

    If this is supposed to be in "Classical Mechanics" forum, sorry..please move the thread.
  2. jcsd
  3. Sep 23, 2008 #2


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    Homework Helper

    Welcome to PF.

    Walking is a bit like a controlled fall. And the static friction is a resisting force. But there are motive forces, such as moving your legs one in front of the other, even though the legs may serve more to balance the center of mass on top to keep you from falling under gravity as you move.
  4. Sep 24, 2008 #3
    First thanks for the reply. I think I understand what you're saying, it a situation like this, the friction force has to be a "response" force, as in a force that is equal in magnitude and opposite in direction to a force that was applied beforehand. But what is that force? If I want to look at the entire body of the walking man, there is friction acting in opposite direction of movement, but what was the original force that cause the friction force, if I'm looking at the man's entire body as a whole?
  5. Dec 31, 2011 #4
    But are not these 'motive forces' simply internal forces - are not only external forces are of concern? I presume that when you are walking at a constant speed there is in fact very little friction force (only enough to be equal and opposite to the drag force).
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