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Converting engine power into a force

  1. May 30, 2008 #1
    I am wanting to accurately calculate the top speed of a car (using it's power, mass, drag, and friction). I started by suming up the forces, but I cannot think of a way to translate the engines power into the force pushing the car forward.
  2. jcsd
  3. May 30, 2008 #2
    power is energy/time. energy or work, is force*distance. so instantaneous power is force*distance/time, which is force*velocity.
  4. May 30, 2008 #3
    Prob. not enough detail. How do I convert the torque of power the engine is sending to the wheel into a force. I thought about t=rF => F=t/r with r being the wheel radius, but that doesn't make sense. So, I need a way to translate the torque/ power the engine is sending to the wheel into a force that the wheels are exerting (sp?) against the ground to move the car forward. Maybe I am going about this problem in the wrong way idk.
  5. May 30, 2008 #4


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    Homework Helper

    Power = force times speed.

    In English units to calculate horsepower given force and speed:

    power/hp = force (lbs) x speed (mph) / 375 (conversion factor).

    You can ignore the gearing factor, since it's assumed that you gear the car so peak power occurs at top speed, so the rpm, torques, ... don't need to be known. You'll need to estimate drivetrain and rolling friction losses and reduce the peak power by this amount, usually 15% loss is a good estimate for cars with manual trannys. The next problem is calculating aerodynamic drag, which will be related to cross sectional area times coefficient of drag times speed2.

    Driven wheel force = wheel torque divided by effective radius of tire. Wheel torque = engine torque times overall gear ratio (times effeciency factor, 85% is a reasonable esitmate).
    Last edited: May 30, 2008
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