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Homework Help: Direction of friction when a body moves in a circular track?

  1. Dec 14, 2015 #1
    please explain why?
    Take 2 cases,and explain how is the direction of friction different in these 2 cases.

    CASE 1:uniform circular motion
    CASE 2:non uniform circular motion

    my thought was,friction opposes relative motion,so it must be tangential to the particle at each and every point.
    i very well know this is wrong,but i need to know why?please help.
    Last edited: Dec 14, 2015
  2. jcsd
  3. Dec 14, 2015 #2
    When you say that friction opposes relative motion, you are being too specific (although one could actually be more specific by saying that the motion has to be between two objects in contact, in a direction parallel to the contact plane). What you should really say is that friction is a force that acts against any motion parallel to and between the two surfaces, and against any force that tries to create such motion.

    In circular motion, there is of course tangential friction opposing the motion of the particle, but we also need some way of maintaining the circular path. In space, this is done by gravity, but think about steering your car. You turn the front wheels, which then push the ground away from where you are turning. The ground produces a force that acts opposite to this, and your direction changes the way you want it to. This is centripetal force, which you might have heard of if you've studied circular motion. In the case of the turning car and the particle you've described, the centripetal force is friction.

    I'm not sure what you're referring to exactly with the two cases. Perhaps you forgot to finish the question?
  4. Dec 14, 2015 #3
    Thank you for replying sir,do you mean to say there will be 2 components?centripetel and tangential?i know about circular motion but i don't know about friction that acts which acts in a rough circular track,i would be really gratified if you can explain the entire phenomena and how this is different from static and limiting friction,or atleast some website or video lecture which explain this phenomena.
    sir,i forgot to enter a sentence it is edited now.sorry for the inconvenience.
  5. Dec 14, 2015 #4


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    There are several possible interpretations of the OP. Is this kinetic friction from a stationary surface opposing the motion of a block sliding around under control of another force? Static friction from a rotating surface, holding a mass in place on the surface? Static friction on a wheeled vehicle driving in a circular path on a stationary surface? .....
  6. Dec 15, 2015 #5
    thank you very much for replying,sir.i really don't understand the above cases,i would be really gratified if you could explain all the cases and the entire phenomenon of friction acting on a circular track.
    by the way,the above question says about a car moving in a circular track,sir but please explain the entire phenomenon of friction acting on a circular track and please explain how is each case different from each other.or atleast suggest a website or video lecture which explain the phenomenon.
  7. Dec 15, 2015 #6


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    For now, let's just work with the problem as given to you. (It would help if you were to provide the full text in the first place.)
    You have a car following a circular track on level ground. You have two cases to consider: constant speed and varying speed.
    Friction opposes relative motion of surfaces in contact. When a wheel rolls at steady speed in a straight line on level ground, there is no tendency for the rim (tyre, say) and ground to move relative to each other, so no frictional force.
    Consider a car negotiating a curve at steady speed. Without friction, what would happen?
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