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Homework Help: Find work from braking force.

  1. Mar 8, 2013 #1
    With the brakes fully applied , a 1470 kg automobile decelerates at the rate of 7.5 m/s^2.
    a) what is the magnitude of the breaking force acting on the automobile?
    1470 *7.5=11025N ( I think this is correct)

    b)if the initial speed is 90km/h (25 m/s) what is the stopping distance?
    im thinking 25/7.5=3.33s
    3.33*25=83.25 m

    c)what is the work done by the breaking force at 90km/h.
    part a) *part b)=J

    d) what is the change in kinetic energy of the automobile?

    I believe I know how to do the proceeding parts but I need the previous values.
  2. jcsd
  3. Mar 8, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor


    Careful: The speed is not constant.
  4. Mar 8, 2013 #3
    (b) is incorrect. The distance formula is [itex] d = v_{i}t+\frac{1}{2}at^{2} [/itex]. Note that acceleration is negative in this case, and see if you can go from there.
  5. Mar 8, 2013 #4
    if I use d = vt + (1/2)at^2
    did I use the correct acceleration?
    Last edited: Mar 8, 2013
  6. Mar 8, 2013 #5
    my distance is correct but for part c when I multiplied 41.66 * 11025N= 459301.5 J it is wrong.
  7. Mar 8, 2013 #6
    never mind the work is negative, Thanks for the help!
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