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Static friction on a car on a circle

  1. Sep 30, 2011 #1
    When a car is moving on a circular track which is sloped away from the centre of the track, is the static friction keep that car on the circle (constant speed) towards the outside of the circle, opposite the acceleration?

    I'm having trouble seeing this because to me, the car would want to fall out of the circle, because of the slope. Which would mean it would need the static friction to be towards the centre.
     
  2. jcsd
  3. Sep 30, 2011 #2

    ehild

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    The static friction can act in both directions, towards the centre and away from it. To move along the circle with speed v, the car needs proper centripetal force, which is the resultant of gravity, normal force and static friction between the road and tyres.

    ehild
     
  4. Sep 30, 2011 #3
    OK but in this case which direction would the static friction be? I just need to know to draw the free body diagram.
     
  5. Sep 30, 2011 #4

    ehild

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    It depends on the speed of the car, the radius of circle and the angle of the slope.
     
  6. Sep 30, 2011 #5
    The speed is 20 km/h, the radius is 113m and the angle of the slope is 8 degres
     
  7. Oct 1, 2011 #6

    ehild

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    Draw the free body diagram. You have two unknown forces: the normal force and the static friction. The normal force is perpendicular to the road, the friction is parallel, but you do not know if it points inward or outward. The resultant has to be horizontal, pointing towards the centre and equal to mv2/R. Write the equations for the force components, horizontal and vertical, assuming a direction for the friction. There are two equations with two unknowns. If you get negative result for the friction it points to the other direction you assumed.

    ehild
     
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