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How to find applied force when given force of friction (constant velocity)

  1. Dec 17, 2012 #1
    1. The problem statement, all variables and given/known data
    Five dogs, each having a mass of 30kg, pull a 250kg sled horizontally across the snow (coefficient of kinetic friction = 0.14).
    How much force must each dog exert in order to move the sled at a constant velocity?

    2. Relevant equations
    Force of kinetic friction = coefficient of kinetic friction*normal force
    Rewritten, the equation above would be F{k}=μ*F{n}
    Also, normal force = gravitational force = mass*gravity

    3. The attempt at a solution
    F{k}=μ*F{n}
    =0.14*m*g
    =0.14*400kg*9.8N/kg
    =549N
    Therefore each dog would have to pull with 549/5=110N of force in order for the sled to move at constant velocity.

    The problem is, in my workbook it shows that the answer should be 78.4N. What am I doing wrong?
    Thanks :smile:
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Dec 17, 2012 #2

    lewando

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    Gold Member

    400kg? The force due to friction is based on the mass of the sled. Not sled + dogs.
     
  4. Dec 17, 2012 #3
    Tried it that way. Doesn't work out right either.
     
  5. Dec 17, 2012 #4

    lewando

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    Gold Member

    Typo? (250kg*9.8m/s2*0.16)/5 = 78.4N
     
  6. Dec 18, 2012 #5

    CWatters

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    Science Advisor
    Homework Helper

    I agree with lewando.

    If the problem statement is correct then answer should be..

    250kg*9.8m/s2*0.14/5 = 68.7N

    Has to be a typo somewhere.
     
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