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Friction concept

  1. Feb 9, 2006 #1
    Please someone of you help me in this concept.
    My Prof told us that if we need to move with contact with respect to an object, then friction is essential. I cannot think the dynamics behind it. There was a question was like somone jumped from a platform and we had to clculat the back velocity of the platform due to the impulsive force of the man jumped. While explaining this he said thatfriction is essential forthe manto jump. I don't understand the basis behind this.
     
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  3. Feb 9, 2006 #2

    Chi Meson

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    In order to jump forward, we have to be able to push backward on the ground. If the ground is flat, the only way we can apply a force compnant that is horizontal to that surface is through the frictional force. Simply put, we can not walk without friction since to walk we must pushe the ground backwards in order for the ground to push us forward.
     
  4. Feb 10, 2006 #3
    I mean if we exert force forward in a frictionless plane then we must move forward since our motion forward is not restricted. The case is differant when there is friction as we exert force backward to move forward. I also know that external force is reqd to move a body. But human body can do such a thing as I think due to the complexity of structure of huan body.
     
  5. Feb 10, 2006 #4
    An external force is needed if the object cannot do itself: a car on ice will just just spin its wheels without sufficient friction so it must be pushed in order to move. No external force is necessary if the object itself can exert it: a car on pavement can apply the force through its tires so it does not need a push. The force does not have to be external when you have friction.
     
  6. Feb 10, 2006 #5

    Doc Al

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    Just to be clear: The friction force acting on the tires is an external force; the road is pushing on the car.
     
  7. Feb 10, 2006 #6
    Hmmm, the road and the car are pushing against each other since one force does not exist without its equal and opposite force. But since the road is not the one with the engine, isn't it more correct to say that the origin of the force is the car rather than the road?
     
  8. Feb 10, 2006 #7

    Doc Al

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    I know what you mean, but the point to be clear about is that the car will not be able to start moving without an external force. For example, keep the engine but put the car on oil-covered ice: No friction, no acceleration.

    Of course, in order to have the tires push against the road in the first place, you need an engine. But the direct external force is supplied by the road, not the engine.

    PS: That friction force (if the tires don't slip) does no work on the car; the source of the car's increasing kinetic energy is the engine converting chemical energy into mechanical energy.
     
    Last edited: Feb 10, 2006
  9. Feb 10, 2006 #8
    So all accelerating forces can be called "external" to the accelerating body, which makes the adjective redundant, sure.

    Since the OP used the term, I think he is interested in the source of the force. And for completeness, you might comment on his mention of "the complexity of structure of human body" to clarify that this is not so relevant here, we are dealing with a more common principle.
     
  10. Feb 10, 2006 #9

    Doc Al

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    An external force is always required to produce an acceleration of the center of mass of a body. And an external force requires an external object to supply the force: something outside the body. You cannot just "start moving" by performing some gymnastic contortion, if there's no friction (or surface to push off of). (Unless you throw something, in which case the thrown object provides the external force. :wink: )
     
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