Question:

Motion problem...?

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When traveling at 100 km/h a car has to overcome a drag force due to air resistance of 800 N. If the car has a mass of 900kg determine the average force that the motor needs to apply if it is to accelerate at 2.ms^-2.

This confuses me greatly. Its a textbook question and the answer is 2600 N. Any help showing steps or explaining how this works is appreciated :)

thanks.

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  1. m = 900kg

    a = 2m/s²

    D = 800N ........ the drag force

    F = ? ........the force of the motor

    ma = sum of the forces on the car

    ma = F - D

    F = ma + D = 900*2 + 800 = 2600N

    .,.,


  2. Consider direction in which car is accelerated as positive.

    Mass m = 900 kg

    Acceleration a = 2 m/s^2

    Drag force Fd = -800 N (negative because drag force opposes motion)

    Force by motor Fm = ?

    Total force F = ma

    Again, F = Fd + Fm

    Therefore, Fd + Fm = ma

    Or, -800 + Fm = 900 * 2

    Or, Fm - 800 = 1800

    Or, Fm = 1800 + 800 = 2600 N

    Ans: 2600 N
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