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Final speed with mass, initial velocity, force, displacement

  1. Apr 29, 2016 #1
    1. The problem statement, all variables and given/known data
    A 1000kg car is moving at 25ms when a net force of 200N acts to slow the car. This force acts while the car moves through a distance of 300m. Find the new speed.
    2. Relevant equations
    Kinetic Energy equation
    v2=u2+2as

    3. The attempt at a solution
    Attempted the use of Kinetic Energy equation and v2=u2+2as
    However the answer I get is 27.4 when the correct answer is 22.5
     
    Last edited: Apr 29, 2016
  2. jcsd
  3. Apr 29, 2016 #2

    SteamKing

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    Why don't you show us your calculations?
     
  4. Apr 29, 2016 #3

    ehild

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    Are the relevant equations unknown to you? How did you solve the problem then?
    You result is correct with the data you gave. Check if they are the same given in the original text. And do not forget the unit next time. What is the unit of the speed? Is it really ms?
     
  5. Apr 29, 2016 #4
    W=Fd
    =200*300
    =60000
    60000=½*1000*v2-½*1000*252
    =500v2-312500
    372500 = 500v2
    v2=745
    v=27.3
    Yea see for yourself, its a multiple choice question. And I changed the original post so it makes some sense.
     

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  6. Apr 29, 2016 #5

    SteamKing

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    The force of 200 N is acting to slow the car, which is traveling initially at 25 m/s. If the force is acting to slow the car, it makes little sense to say that the car is going to speed up to 27.4 m/s. :oops:

    Care to try again? I suggest you use F = ma here, in addition to v2=u2+2as
     
  7. Apr 29, 2016 #6
    Thanks, I managed to get the answer, all I done was
    27.5-25=2.5
    25-2.5=22.5
     
  8. Apr 29, 2016 #7

    ehild

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    As the car slows down, the acceleration is negative. The force is opposite to velocity and displacement, so the work W is negative. You should have written W=Fd=-200*300= -60000 = 1/2mv2-1/2 m u2.

    You changed the original post by changing the unit of speed from ms-1 to ms. Is it correct?
     
  9. Apr 29, 2016 #8
    The answer is correct as ms-1
     
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