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Homework Help: A confusing word problem. Can any one solve it?

  1. Dec 14, 2011 #1
    1. The problem statement, all variables and given/known data

    A 240gram toy car is driven by an electric motor that has a constant power output. The car can climb a 24° incline at 16m/s and can travel on a horizontal table at 39m/s. The frictional force retarding the motion is kv, where k is a constant and v is its speed. How steep an incline can it climb with a speed of 28m/s?

    3. The attempt at a solution

    I've tried solving this for 2 days now and but not succeeded. It seems quite simple but it is the kv part that I don't know how to use.
    Please could someone explain how to solve it?

    Help is very much appreciated!
    Thank you!
  2. jcsd
  3. Dec 14, 2011 #2


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    Staff: Mentor

    Welcome to the PF.

    The k*v part is just like wind resistance and/or rolling resistance. They are saying that the only retarding force on flat ground is this resistance, which is proportional to velocity.

    Do the 3 FBDs for the 3 cases, and include this resistant force in each. You should end up with a set of equations that you can use to solve for the velocity in the 3rd situation. Please show us your work...
  4. Dec 14, 2011 #3
    Oh thank you :)
    I still seem to be going in circles.

    I made a table with the kinetic and potential energies for each one: (by the way the speeds are supposed to be in cm/s - i typed it wrong)

    1) 0.39m/s --> Kinetic E = m(v^2)/2 = 0.018252 J
    Potential E = mgh = 0 J

    2) Kinetic = 0.003072 J
    Potential = 0.1531 J

    3) Kinetic = 0.009408 J
    Potential = ?? (i don't know theta, that is what i need to find)

    So i guess 2) is best to work with since i have both values so i can find friction:
    so friction is PE - KE = 0.1531 - 0.003072 = 0.150028 J

    I have no idea if this is right...
    and how do i include the "motor has a constant power output" ?

  5. Dec 14, 2011 #4


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    Science Advisor
    Homework Helper
    Gold Member

    You are going about this wrong by trying to use energy concepts and incorrect equations. You need to draw free body diagrams as berkeman has suggested, and apply Newton's first law.
    Power is force times velocity, that is, P = Fv. The power delivered by the motor is the same for all cases. Write down all givens and relevant equations. Please show your work using free body diagrams.
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