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Work and friction problem

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data
    When an automobile moves with constant velocity, the power developed is used to overcome the frictional forces exerted by the air and the road. If the engine develops 30 hp, what total frictional force acts on the car at 120 mph?

    2. Relevant equations
    1hp = 746W
    power= work/time

    3. The attempt at a solution

    30hp= 22380w
    120mph= 53.6448m/s
    I'm really not sure where to go from there, you need to end up with force which is Newtons, dont you need a mass in kg to for that?
  2. jcsd
  3. Nov 26, 2007 #2


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    Power = Force*velocity.
  4. Nov 26, 2007 #3
    so can you just multiply 22380w and 53.6448m/s, or do you have to change the watts to another unit?
  5. Nov 26, 2007 #4
    oopps you have to divide them actually right?
  6. Nov 26, 2007 #5


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    Yes. No need to change anything.
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