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Homework Help: Finding tension and power

  1. Nov 6, 2011 #1
    1. The problem statement, all variables and given/known data

    An Eskimo pulls a 100kg sled with a constant force through the snow
    at a constant speed of 1.5m/s. He applies this force by pulling on a rope at 30◦
    to the horizontal. There is a coefficient of kinetic friction between the sled and the snow of 0.1

    1)Find the tension in the rope
    2)What is the power due to the frictional force (rate at which energy is being dissipated)
    the instant he stops pulling on the sled

    2. Relevant equations
    The Force of Tension = (mass)(acceleration) - (Coefficient of Friction)(-mass · gravity)
    2) Power= Work/Time

    3. The attempt at a solution
    1) I thought it was a simple just plug it in, so i did Fτ=(100kg)(1.5m/s)-(.1)(-100kg*9.8)=248N
    2) I used the 248N to find the amount of work that was done, so Work=248N*1.5m/s*Cos(30)=322.16. However after this I am confused as to how i find time.
  2. jcsd
  3. Nov 6, 2011 #2
    A sled of MASS 100kg has a WEIGHT of 100 x 9.81N =981N
    A coeff of friction of 0.1 means that the force of friction between the sled and the snow is 0.1 x 981N = 98.1N
    So to pull the sled at (ANY) CONSTANT SPEED (velocity) needs a horizontal force of 98.1N
    This force is provided by the horizontal component of the tension in the rope
    So T x Cos30 = 98.1 gives T = 98.1/Cos30 = 113N
    Power = force x velocity
    So with a friction force of 98.1N at a velocity of 1.5m/s the power is 98.1 x 1.5 = 147W
  4. Nov 6, 2011 #3
    Thanks :) i guess my initial equation was off
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