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Homework Help: Friction question

  1. Jun 15, 2008 #1
    1. The problem statement, all variables and given/known data

    I am trying to understand a concept that pops up in my hwk problems about friction, specifically static friction. In an example in my book, it shows that when you turn a car around a corner, you have 3 forces: normal, gravity (these cancel and are in the vertical direction) and static friction (provides the inward centripetal force to allow you to turn in a circle). However, I don't understand how static friction can exist alone as a horizontal force. I thought that static friction results in response to another force. For example, if i try to push a block, static friction force keeps it from moving. However, if the block is just sitting there, there won't be a static friction force. Can someone please explain this to me?

    Also, when we walk, I understand that when I push the ground, the ground pushes back on me and that allows me to walk forward. I don't understand how friction plays into the FBD of this situation. Is friction the force of the 'ground pushing back on me'? How does friction play into newton's third laws?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jun 15, 2008 #2


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    Homework Helper

    This is the problem with teaching about some forces without explaining to students how those forces come about.

    If you think about the surfaces of two objects in contact at the molecular scale, you have irregular-looking arrangements of atoms at the surfaces of each object. In many places, the atoms of each object will be very close to each other, close enough that the valence electrons (the electrons in the outermost orbitals) will not be able to distinguish which atom they "belong to". So molecular bonding takes place at those points of "contact" between the two objects.

    When the objects are just sitting there, you won't notice this taking place. But when you go to move one object past the other, say, by sliding, you are putting energy into breaking those bonds between the "contacting" atoms. So your applied force will be resisted by those bonds, up to a limiting point. At the large scale, you experience this as a resisting "static frictional force" which has a maximum magnitude, generally expressed as (mu_s)ยท(normal force), where mu_s is the proportionality constant we call the "coefficient of static friction". This is why "static friction" can vary from zero (no applied force, so no resistance experienced) up to f_s_max .

    As for your second question, this static friction must act equally on both objects in contact, and in opposite directions on each object (Newton's Third Law).

    Think about what happens when you try to walk on dry sand, wet ice, wet linoleum, etc. (something with very little friction). When you push off with your foot, your shoe sole or skin slides easily along the surface with little resistance, giving you little traction. (This is why fiendish exercise programs make you run on dry sand.) In the limit of frictionlessness, your foot would simply slip along the surface and your body's center-of-mass would go nowhere.

    So the static friction is resisting the motion of your foot relative to the surface you are trying to move on. In a free-body diagram, you would show the static friction, f_s, acting on the ground pointing behind you and acting on your foot pointing ahead of you. So the static friction acts to push you forward, to the extent that it is present. The static friction is also acting to push the ground backwards from you; think about which way loose sand or dirt flies underfoot as you are running on it.
    Last edited: Jun 15, 2008
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