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Homework Help: Friction and Velocity, Simple

  1. Sep 21, 2010 #1
    A car of mass 1000kg is sliding down a hill. The coefficients of friction between the cars tires and the ground are u=0.89 and u[k]=.61. For what inclination angle will the car slide down the hill with a constant velocity.
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    So what I figured was that the car will have constant velocity when there is 0 acceleration. So F-f[friction]=0 ( which I dont know is correct or not). Then I set ma=u[k]mgcosQ, and solved for cosQ to get arccos(a/u[k]g). I just dont think any of what I'm doing is correct. Where have I gone wrong?
     
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  3. Sep 21, 2010 #2

    rock.freak667

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    well firstly, you need to get the component of the weight parallel to the slope, that will be your 'F'. When you get that you can solve it the way you were solving before.
     
  4. Sep 21, 2010 #3
    Well I thought thats what I did, no? Thats what mgcosQ is, the parallel component of the weight..
     
  5. Sep 21, 2010 #4

    rock.freak667

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    Nope, remember that friction =μN = μmgcosθ, which you correctly identified and this is normal to the slope, so there is only one other component left, which is parallel.
     
  6. Sep 21, 2010 #5
    Ok so I have two things here: mgcosQ and mgsinQ. So mgcosQ is perpendicular weight, and mgsinQ is the parallel weight? So then what? ma=uk(mgsinQ-mgcosQ)?
     
  7. Sep 21, 2010 #6

    rock.freak667

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    No you will have ma=mgsinθ - μmgcosθ and you said that for constant velocity, a = 0, so θ is?
     
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