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Homework Help: Calculating friction forces

  1. Aug 26, 2012 #1
    1. The problem statement, all variables and given/known data
    A 1326kg car lies on a flat surface. To make it move very slowly (a~0), one needs a force of 160N parallel to the surface. To stop it, the force must be reduced to 120N.

    2. Relevant equations
    Calculate the friction forces of the car

    On Tuesday I have an exam regarding this and related topics. I missed some classes due to sickness and now I am a little lost.

    Thanks in advance!
  2. jcsd
  3. Aug 26, 2012 #2
    According Newton's 2nd law,


    Fnet==F1+F2+F3+ ...Fn

    Forces are vectors, so they have magnitude and direction.
    If you take vector force pointing to the right is positive(vector as an arrow) , then vectors pointing to the left as negative.
    As for the friction, the direction is always opposite of the motion of the object.
    The value can be calculated by multiplying the Normal force to the kinetic coefficient μ.
  4. Aug 26, 2012 #3


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    If I have understood correctly, the problem statement is telling you that to overcome the static friction you need to apply 160N. However once it's moving you only need to apply >120N to keep it moving. (eg this is a problem about static and dynamic friction).

    However is the question really asking you to calculate the "friction forces" or the "coefficients of friction" ?

    If the latter, look at the wikipedia entry for coefficient of friction. Hint: It's a ratio.

    Sorry but I might not be able to return to this thread later.
  5. Aug 26, 2012 #4
    Yes, I meant the coefficient.

    And also, the stop the car, you reduce the applied force to 120N

    I don't have the kinetic coefficient
    Last edited: Aug 26, 2012
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