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Work to stop

  1. Dec 11, 2003 #1
    This should be easy...the problem asks for the amount of work needed to stop a 1000kg car traveling @ 100km/h.

    I am having a hard time do to the fact acceleration,distance nor time is given.

    here is how I think i need to approach this:

    convert 1000km/h to m/sec

    find acceleration

    use Work=Mas*acceleration*Distance to find work

    Here are the variables i know:

    Vf = 0
    V0 = 27.78 m/s

    how can i find acceleration if no distance or time is given? I would think i need to find acceleration to calculate work needed to stop.

    any hints?:smile:

  2. jcsd
  3. Dec 11, 2003 #2


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    Science Advisor

    convert speed to m/s:
    100km/h * 1000m/km * h/3600s = 27.7777m/s

    you have to remember the standard formula for all of these types of problems is energy in = energy out. expanded, it looks like this:
    Wo + Ko + Po = Wf + Kf + Pf

    W is work, K is kinetic energy, P is potential energy. For this equation, there is no work put in, no initial potential energy, no final potential energy, no final kinetic energy. With all the 0 terms removed, it looks like this:
    Ko = Wf

    solfing for equation:
    (1/2)mv^2 = W
    (1/2)(1000)(27.777^2) = W
    W = 385800J
    W = 3.858 x 10^5 J
  4. Dec 12, 2003 #3


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    Science Advisor

    In order to stop any object, you need to reduce its velocity to 0. That means you need to reduce its kinetic energy to 0. Work is energy so: calculate the kinetic energy. That's the work you need to do to stop the object.
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