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Could someone help w/ a problem

  1. Jun 14, 2007 #1
    Hey I thought i had this figured out but when i submitted it it was wrong.

    A car with a mass of 1150 kg moves at 16 m/s. What braking force is needed to bring the car to a halt in 10s?

    I set it up as
    KE=1/2mv^2
    =1/2(1150)(16)^2
    =147200J

    then v/2= 16/2=8

    force= 147200/8
    =18400 N

    Could someone tell me what I did wrong?
    Thanks
     
  2. jcsd
  3. Jun 14, 2007 #2

    Doc Al

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    You found the energy.

    And then the average speed.

    Huh? Why would you divide KE by average speed?

    Maybe you are thinking of:
    Power = Force * Speed?

    If so, what is the average power required?
     
  4. Jun 14, 2007 #3

    nrqed

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    :confused::confused: What equations are you using?? I don't follow your reasoning of taking half the speed and so on...
     
  5. Jun 14, 2007 #4
    Umm I'm not sure. Maybe I have it set up all wrong. I just need the breaking force in Newtons.
     
  6. Jun 14, 2007 #5

    Doc Al

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    We know what you need, we're trying to understand what you did. :wink:

    If you weren't thinking of power, you might wish to consider how much work is required to stop the car.

    Or you can use kinematics: What's the car's acceleration?
     
  7. Jun 14, 2007 #6
    It doesn't give the car's acceleration
     
  8. Jun 14, 2007 #7

    malawi_glenn

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    F = m*a

    find acceleration
     
  9. Jun 14, 2007 #8

    Doc Al

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    But you have speed and time--everything you need to figure out the acceleration using kinematics.
     
  10. Jun 14, 2007 #9
    if the car moves at 16 m/s the the acceleration would be constant right?
     
  11. Jun 14, 2007 #10

    malawi_glenn

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    but the car shall stop in 10s.. hence an acceleration is done.
     
  12. Jun 14, 2007 #11

    Doc Al

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    You may assume that the acceleration is constant.
     
  13. Jun 14, 2007 #12
    Ok I wrote out everything the problem has in it so how do i set it up to solve it?
     
  14. Jun 14, 2007 #13

    Doc Al

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    Did you find the car's acceleration?
     
  15. Jun 14, 2007 #14
    No could u tell me what it is or how to get it.
     
  16. Jun 14, 2007 #15

    malawi_glenn

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    average a = delta(V)/delta(t) , i.e. how much the velocity changes during a certain interval of time.
     
  17. Jun 14, 2007 #16
    so the acceleration would be -1.6
     
  18. Jun 14, 2007 #17

    malawi_glenn

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    m/s^2 , always keep the units.

    so the magnitude of the force is then?
     
  19. Jun 14, 2007 #18
    so the acceleration is -1.6
     
  20. Jun 14, 2007 #19
    wait what?
     
  21. Jun 14, 2007 #20

    malawi_glenn

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    what are you asking about?...

    you can delete your double post by pressing the "edit" button then choose "delete post".

    The question was what force is needed to stop the car, right?..
     
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