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Homework Help: Horsepower moving a mass

  1. Mar 4, 2009 #1
    1. The problem statement, all variables and given/known data
    What minimum horsepower must a motor have to be able to drag a 270 kg box along a level floor at a speed of 1.20 m/s if the coefficient of friction is 0.50?

    2. Relevant equations

    3. The attempt at a solution
    I ended up having W=Fr*d=mu*m*g*d
    (.5)*(270)*(1.2)*(1.2)=194.4 J/s=.26 hp

    That is way too low.

    Can anyone give me any guidance?
  2. jcsd
  3. Mar 4, 2009 #2


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    Homework Helper

    It looks like you just used 1.2 instead of 9.81 in your calc, but it would be better to use P = F*v.
    If you don't know this formula, it is from P = W/t = F*d/t = F*v.
  4. Mar 4, 2009 #3
    Is the way I tried it wrong? I ended up with hp=2.13 and that still seems pretty low
  5. Mar 4, 2009 #4


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    Homework Helper

    That answer looks good!
    Why does it seem low? Picture a pair of horses pulling a sled with a big guy on it at 5 km/hr?
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