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Direction of static friction

  1. Jan 22, 2008 #1
    1. The problem statement, all variables and given/known data
    1) If I have a ball rolling upwards on an incline, why is the static friction pointing upwards as well (my book does not explain this)?

    2) Also, when I deal with a static equilibrium problem, why is it sometimes that the normal force is not perpendicular to the surface? I thought the normal force was always normal?

    Thanks in advance.
  2. jcsd
  3. Jan 22, 2008 #2


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    Think about the direction of the object's motion at the point of contact with the surface. The frictional force opposes the direction of the motion. With a rolling object, the overall direction is up the ramp, but at the point of contact with the ramp the wheel is turning toward the bottom of the ramp. Does that make sense?

    For your second question do you have an example? The normal force, as the name implies, is always normal to the point of contact.

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