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Direction of friction in rotation

  1. Apr 20, 2008 #1
    could someone please explain the technique of how to know the direction of friction in case of rolling.

    eg. when a ball is rolling down an inclined or up or hits a wall and rebounds etc.
  2. jcsd
  3. Apr 20, 2008 #2


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    In the case of a rolling object, the friction force on a rolling (and not slipping) object is zero unless the object is accelerating (or decelerating). Friction force will oppose the acceleration of a rolling object, resulting in an increase in angular velocity (assuming no slippage). So if an object is being accelerated due to gravity on an inclined plane, the friction force will be along the surface of the plane, in the "upward" direction. This retards the rate of linear acceleration, since the gravitational potential energy is used to increase both the linear and angular kinetic energy of a rolling (but non slipping) object. In the case of a sliding object, some of the energy is converted into heat, also reducing the rate of acceleration.
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