1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Friction for walking

  1. Jun 26, 2013 #1
    Why is friction necessary for walking? For moving/walking ahead we just need that when we push the ground the ground pushes us back. Can't we push the ground if the ground is frictionless?
  2. jcsd
  3. Jun 26, 2013 #2


    User Avatar
    Gold Member

    Try wrapping your shoes in rags soaked in oil and then see if you can walk on glass.
  4. Jun 26, 2013 #3
    I know I wouldn't be able to but why?
  5. Jun 26, 2013 #4
    Make an FBD of a person walking
  6. Jun 26, 2013 #5
    Ok. Got it.
  7. Jun 26, 2013 #6
    But when I draw the FBD, the force which with which the feet pushes on the ground is inclined at an angle of θ with the vertical. Let the force which we exert be F. There Fcosθ=N(Normal Reaction by the ground) and Fsinθ=f(frictional force)
    So the horizontal forces balance then how can we move forward?
  8. Jun 26, 2013 #7
    In response to your original question: without friction, you can jump up and down all you want (courtesy of the normal force) But not move horizontally.

    'Action' and 'reaction' forces do not 'cancel'. You push on the earth, the earth pushes on you. result is that you move, relative to the earth, that is.
  9. Jun 26, 2013 #8
    Why don't action reaction forces cancel? Normal reaction cancels weight. So friction should equal the horizontal component of force applied by the man. Shouldn't it?
  10. Jun 26, 2013 #9
    Can you show the FBD you draw to us?
  11. Jun 26, 2013 #10


    User Avatar
    Science Advisor

    The normal force is not an action reaction pair with gravity! This seems to be a really common misconception; the reaction force of the Earth's gravitational pull on you is simply your pull back on the Earth. Action reaction forces act on two different interacting objects, not on the same object.
  12. Jun 26, 2013 #11
    And WannabeNewton, yes that is correct. We generally assume that because of the FBDs we draw. In fact I don't find any difference between what you said? Not a reaction force to gravity but a reaction force for the force by which the earth is pulling us, i.e the gravitaional force? Aren't gravity and gravitation one and the same thing?

    BTW, this is the working I came up with for the use of friction as necessary for walking is this:

    When we walk on a rough surface it is the roughness of the surface that makes us go forward. Like think of a rough surface as having many imperfections in it. When we walk we push the earth. Zooming in, we see that parts of our shoes/boots interlock with the imperfection. We now apply force to these imperfections which in turn exert a force on us, so we move forward.

    Now had the surface been smooth there would have been nothing that would have been felt by the brunt of the horizontal component of the force we applied. This is because there are no imperfections in the way that meet out horizontal force applied. Is that correct?

    So considering this the way as we walk, why don't we keep accelerating then? Because there would always be a force of friction acting on us and as force causes acceleration so we should be always accelerating as long as we walking with he same force being exerted by our foot on the ground?

    Attached Files:

    Last edited: Jun 26, 2013
  13. Jun 26, 2013 #12


    User Avatar
    Science Advisor

    No we never assume that. The normal force is not a reaction force due ro gravity.
  14. Jun 26, 2013 #13
    Means the same thing? The normal force is not between earth and us I guess. It is between us and the floor we are standing on. The floor experiences a push downwards because of the force exerted by us due to the earth. The floor comes in between. Actually it is the centre of the Earth that is attracting us due to the gravitational pull and not the whole earth. So the floor comes in between. As a result the floor also experiences the same force of attraction that we are experiencing due to gravity. So it thinks it is us who are applying a force on it. So it exerts a force on us. which is equal and opposite? It was written that Normal reaction is between two surfaces in contact.
  15. Jun 26, 2013 #14
    normal force is not reaction to gravity, it counteracts gravity. Ex: gravity of earth acts DOWN on YOU. Normal force acts UP on YOU. They act on the SAME object and cancel. another example: You pull EARTH. earth pulls YOU. Forces act on DIFFERENT objects. Result? YOU smash into EARTH.
  16. Jun 27, 2013 #15
    This FBD is not correct. FBD only shows Force acting on the object. Not the forces exerted by the object. Only the forces acting on the object(that is external force) effects the motion of the object. Re-draw the FBD showing only the forces on the object. Remember Newtons second Law, it says about external forces only.
  17. Jun 27, 2013 #16
    Yes that is correct. But where does the normal force arise from? Is it not an action reaction pair of our push and the floor's upward push? Then Why don't we fly up!Because the gravitational pull balances the the floor's push?
  18. Jun 27, 2013 #17
    True that. Normal Force is not the reaction Force. The reaction Force on earth is 'mg'. Whether the object is falling(where there is no Normal) or is placed at surface(where Normal is present). Normal Force is Contact Force.
  19. Jun 27, 2013 #18
    So we just act as an isolated body on which gravitational force and normal force act. Right?
  20. Jun 27, 2013 #19
    Why don't we accelerate because of frictional force then?
  21. Jun 27, 2013 #20
    We do. A container in trck ses friction to accelerate!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted