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Work done using time, velocity and mass?

  1. Mar 30, 2016 #1
    1. The problem statement, all variables and given/known data

    The law of conservation of energy applies to the motion of vehicles. Find the work done on a 1200kg vehicle when it slows from 90kmh-1 to 50kmh-1 in 8.0 seconds.
    2. Relevant equations

    W=1/2mv^2 - 1/2mu^2
    3. The attempt at a solution

    W=0.5*1200*50^2 - 0.5*1200*50^2
     
  2. jcsd
  3. Mar 30, 2016 #2
    If you assume a constant deceleration, you can use constant acceleration equations such as x = 1/2(vo + v)*t and v = vo + a*t. Work = Fd, where F is the force and d is the displacement. This can be expanded to W = m*a*d. Using the acceleration equations, you should be able to find all of your unknowns and plug them into the work equation.
     
  4. Mar 30, 2016 #3

    haruspex

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    Is that what you meant to post? What happened to the 90kmph?
    Why would TDizzl care what the rate of deceleration is?
     
  5. Mar 31, 2016 #4
    Yes, my bad
    Its W=0.5*1200*50^2 - 0.5*1200*90^2
     
  6. Mar 31, 2016 #5
    The question is from a paper which does not require the use of constant acceleration equations. It would seem bizarre to use a formula that hasn't been introduced to us yet.
    Also, the solution is written as -2.59*10^5 Joules in the paper.
     
  7. Mar 31, 2016 #6

    PeroK

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    You're not proposing to leave that as your answer, are you?
     
  8. Mar 31, 2016 #7
    No, that was how I substituted the values in as an attempt to answer the question.
     
  9. Mar 31, 2016 #8

    haruspex

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    In what units?
     
  10. Mar 31, 2016 #9
    m=1200kg
    v=50kmh-1
    u=90kmh-1
     
  11. Mar 31, 2016 #10

    haruspex

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    So what is it in Joules?
     
  12. Mar 31, 2016 #11
    Yea its like -3360000J, that's what I got
     
  13. Mar 31, 2016 #12

    haruspex

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    No, the speeds you are given are in kmh-1, not ms-1.
     
  14. Mar 31, 2016 #13
    So how do I get the answer as -2.59×105?
     
  15. Mar 31, 2016 #14

    haruspex

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    How do you convert km/h to m/s?
     
  16. Mar 31, 2016 #15
    Alright -2.59×105 is the answer I got, the problem was that I was doing 502/3.6 instead of (50/3.6)2.
    Thanks for the help, feeling extremely stupid. :mad:
     
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