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Work and mass problem

  1. Aug 3, 2016 #1
    1. The problem statement,
    all variables and given/known dataIt takes 190 kJ of work to accelerate a car from 20.8 m/s to 29.3 m/s. What is the car's mass?


    2. Relevant equations
    w=fd
    f=ma
    1/2mvf2-1/2mvo2=maΔx=FΔx

    3. The attempt at a solution
    not sure how to start this problem, because they dont give you time or displacement but you can probably work it out from the velocities im assuming? just not 100% sure on how to
     
  2. jcsd
  3. Aug 3, 2016 #2

    Orodruin

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    Why do you think you need time or displacement?
     
  4. Aug 3, 2016 #3
    Could you solve by going m=f/a ??
     
  5. Aug 3, 2016 #4

    Orodruin

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    No. Stop and think a bit about what your relevant equations are telling you about how the input parameters are related to what you want to know.
     
  6. Aug 3, 2016 #5
    in the third equation, using energy. Can somehow use that to solve for mass?
     
  7. Aug 3, 2016 #6

    Orodruin

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    You tell me. In order to solve for the mass, you need to know all other variables of the equation.
     
  8. Aug 3, 2016 #7
    the only other variables you need to know are the velocities right? which we are given
     
  9. Aug 3, 2016 #8

    Orodruin

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    So what is the solution?
     
  10. Aug 3, 2016 #9
    1/2mv2-1/2mvo2=190000J
    1/2m(20.8m/s)2-1/2m(29.3m/s)2 =190000J
    1/2m432.64-1/2m858.49=190000J
    m⋅216.64-m⋅429.245=190000J
    is this correct so far?? not fully sure how to go any further if it is ?
     
  11. Aug 3, 2016 #10

    Orodruin

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    You are aware that multiplication is distributive, right?
    x(y+z) = xy + xz
     
  12. Aug 3, 2016 #11
    yeah
     
  13. Aug 3, 2016 #12

    haruspex

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    The left hand side will clearly be negative, assuming mass is positive.
     
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