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Dynamic coeffecient of friction Problem

  1. May 30, 2016 #1
    1. The problem statement, all variables and given/known data
    The static co-efficient of friction between a football blocking sled (90 kg) and grass is 0.56. (i) If an80 kg coach stands on top of the blocking sled, what is the maximal static frictional force?(ii) Will the dynamic co-efficient of friction between the sled (with the coach on top) and grass be greater than,less than or equal to 0.56?

    2. Relevant equations
    To solve for (i) i used frictional force= coefficient force x normal force

    3. The attempt at a solution

    for (i) i got frictional force to equal= 933N

    I want to know how to solve for (ii) why is it less than 0.56 thanks
     
  2. jcsd
  3. May 30, 2016 #2

    Simon Bridge

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    The static friction tells you the minimum force needed to get something moving.
    The dynamic friction tells the force to keep something moving at a constant speed.
    If the dynamic friction were bigger than the static, that means the force to keep something moving is bigger than the force to get it moving ... does that make sense?
     
  4. May 30, 2016 #3
    yes it does, but how do I know when the dynamic is greater than the static? Is there a calculation? or is it usually said in the problem


    thanks
     
  5. May 30, 2016 #4

    Simon Bridge

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    How does that make sense to you?
    If the force to keep something moving is bigger than the force to start it moving then how does anything move?
     
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