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Homework Help: Work And Power Problem

  1. Dec 21, 2004 #1
    Hey this is my last work and power problem haha...this is the hardest of them all.

    the question is

    A tractor pulls a loaded sled, massing 4110 kg, along a level snow packed road at a constant speed of 45 km/h. If the amount of friction between the road and the sled is 585 N find the power required.

    I found the weight of the load in newtons to be 40319.1 Nand subtracted the friction to get a net force of 39734.1 N but i dont know how to find the power from here.

    help please
  2. jcsd
  3. Dec 21, 2004 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    I am not sure why you are doing that. The mass of the sled is irrelevant since you know the actual force of friction.

    Power is: Work (against the friction force) / unit time. Work is: Force (friction) x distance. So power is Force (friction) x distance/unit time = Force (friction) x speed.

  4. Dec 21, 2004 #3
    Make sure you use consistent units as well. If you're going to use Newtons, then you need to use m/s as well instead of km/hr. More generally, for reasons pointed out by Andrew Mason, power is given by:

    [tex]P=\vec F \cdot \vec v = |\vec F| |\vec v| \cos{(\theta)}[/tex]
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