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I have some related questions about a body moving in uniform horizontal circular motion:

The body moves with a constant angular velocity on a rough horizontal surface. It is attached to a string that is attached to a fixed point on the surface. The body revolves around that fixed point. If the body is continuously in contact with the surface, in which direction will friction act and what type will it be (static or kinetic)?

Now what if it moves with constant angular acceleration? In which direction will friction act?

What if the body is on a rough inclined plane undergoing circular motion?

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  • $\begingroup$ friction is (supposed to be) opposite to the direction of motion (this means is opposite to tangent of the path) and depends on body weight, then it can be analysed to perpendicular and horizontal components (the perpendicular would be associated with the force that keeps the object in orbit) $\endgroup$
    – Nikos M.
    Commented Oct 27, 2014 at 15:04
  • $\begingroup$ what if a string is attached to the body. then tension provides some centripetal force. what about friction?? $\endgroup$
    – Sashurocks
    Commented Oct 27, 2014 at 15:11
  • $\begingroup$ The friction is opposed to the direction of motion - but there are other forces acting on the object to maintain either constant angular velocity, constant acceleration, and/or when it is moving on a plane. I wonder if you need to rephrase your question a bit... $\endgroup$
    – Floris
    Commented Oct 27, 2014 at 15:12
  • $\begingroup$ i rephrased it. is it clear now?? $\endgroup$
    – Sashurocks
    Commented Oct 27, 2014 at 15:15
  • $\begingroup$ what type of friction? the body is moving right? $\endgroup$
    – Nikos M.
    Commented Oct 27, 2014 at 15:15

1 Answer 1

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Since the object is moving and not at rest, it is undergoing kinetic friction. The force of kinetic friction will always be directly opposite the instantaneous direction of motion of the object. This does not depend on the angle of the surface or the value of angular acceleration. The force of kinetic friction will point directly opposite the direction of motion of the object.

Friction can only oppose motion in the contacting object, it does not create motion in that object. If the force of friction points anywhere other than directly opposite the direction of motion, then it has the effect of creating motion along a separate direction.

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  • $\begingroup$ When a car turns on a horizontal road, im told that friction provides necessary centripetal force. in this case friction is perpendicular to motion of the car right? $\endgroup$
    – Sashurocks
    Commented Oct 27, 2014 at 15:25
  • $\begingroup$ @Sashurocks: There are two aspects to tyre friction: rolling resistance in the direction the tyre is rolling and a sideways friction opposing the sideways slipping of the tyre. If the tyre is not slipping sideways, you can regard the second as static friction. It acts opposite the direction the tyre would otherwise slip towards. $\endgroup$ Commented Oct 27, 2014 at 15:40
  • $\begingroup$ @RedGrittyBrick - not quite. If you turn the front wheels of a moving car, they will cause a lateral force; but if the wheel were to slip, it would slip in the forward direction. So "it acts opposite the direction the tyre would otherwise slip towards" is not accurate, I think. $\endgroup$
    – Floris
    Commented Oct 27, 2014 at 16:11
  • $\begingroup$ @Sashurocks the point was that kinetic friction always points opposite the motion. $\endgroup$
    – Jim
    Commented Oct 27, 2014 at 18:52

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