physics

posted by Cassie

A car (mass = 750 kg) is initially moving with velocity 30 m/s. The driver slowly presses on the brakes and bring the car to a stop, traveling 50 m in the process. Calculate the force of friction between the car and the ground that brings it to a stop. Draw a free-body diagram for the car and clearly explain the method behind your solution.

  1. Damon

    F * d = work done by friction = initial Ke = (1/2) m v^2

    F * 50 = (1/2)(750)(900)
    100 F = 750 * 900
    F = 750 * 9

    There are harder ways of course using
    F = change of momentum/ time = 750*30/t
    then
    50 = (1/2)a t^2
    where a = 30/t
    so
    100 = 30 t
    t = 10/3 seconds to stop
    a = 9 m/s^2 (about 1 g)
    F = m a = 750* 9

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