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Homework Help: Friction directions

  1. Jun 12, 2005 #1
    Just a quick question that I can't figure out the answer to...

    I know that if I have a car accelerating forwards, the friction force will be forwards to oppose the relative motion between the wheel and road. Now if I put, say, a cylinder on a slope, the friction force will be uphill (I think?)... why is it uphill?
     
  2. jcsd
  3. Jun 12, 2005 #2

    OlderDan

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    Friction always acts in a direction that tends to oppose motion. For a cylinder on a slope gravity tends to pull the cylinder down the slope, so friction acts in the opposite direction IF you start the cylinder from rest. Friction could be directed up the slope under other conditions, such as if you were rolling the cylinder up the slope.
     
  4. Jun 12, 2005 #3
    Simplify your car tire to a cylinder rolling down a slope :)
     
  5. Jun 13, 2005 #4
    I still don't understand what it means to 'act in a direction to oppose motion'. To oppose the motion of what exactly?
     
  6. Jun 13, 2005 #5
    Good point. :smile: So friction acts differently when things start stationary and when they're moving...?
     
  7. Jun 13, 2005 #6
    In a sense, yeah, but this wasnt Dan's point. Dan was saying how if you initially threw the cylinder up the ramp, then as it is rolling up, the friction force will point down the ramp (friction always acts opposite the direction of motion). Once the cylinder stopped rolling and started rolling back down, the friction force will start pointing up the ramp.
     
  8. Jun 13, 2005 #7
    Mmm, ok. Thanks for the replies.
     
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