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Question about friction? Dynamics

  1. Feb 16, 2017 #1
    1. The problem statement, all variables and given/known data
    This is the problem
    http://imgur.com/a/1XFlt
    rp7otFI.png
    2. Relevant equations

    This is the given FBD
    http://imgur.com/a/CURqA
    KpA1w1u.png
    3. The attempt at a solution

    Now the car should have a tendency to slip down the ramp thus causing the frictional force to oppose this motion. But the frictional force is pointed down the ramp in this worked out problem. Can someone explain to me why this is?
     
  2. jcsd
  3. Feb 16, 2017 #2
    Why is friction pointed down the ramp? Shouldnt it be pointed up the ramp?
     
  4. Feb 16, 2017 #3
    If there were no ramp (i.e. the ground was flat) and the car was going around a curve, what direction would the friction force have to point?
     
  5. Feb 16, 2017 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Note that they ask for minimum and maximum speeds. What happens when you take the curve too fast? Which direction do you tend to slip?
     
  6. Feb 16, 2017 #5

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    Follow the sequence of the questions asked by the problem.
    First draw the FBD to determine how fast the car must be moving if there were no friction, i.e. sliding around a frictionless curve.
    If the car is moving faster than that, friction down the incline is needed to keep it in a horizontal circle.
    If the car is moving slower than that, friction up the incline is needed to keep it in a horizontal circle.
     
  7. Feb 16, 2017 #6
    Thanks everyone for the great responses! It really helped clarify things for me (:
     
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