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Homework Help: Cars, Energy, and Work

  1. Nov 12, 2004 #1
    If it takes 50 m to stop a car initially moving at 25 m/s, what distance is required to stop a car moving at 50 m/s under the same condition?

    a. 50 m
    b. 100 m
    c. 200 m
    d. 400 m

    This is probably a really easy question, but I just can't see how to work it out. I tried to set up a ratio between the distance and velocity and that got me 100 m, but b is incorrect. Can anyone explain the correct method to do this problem and tell me why setting up a ratio between those 2 components is a mistake?
     
  2. jcsd
  3. Nov 12, 2004 #2
    Answer is C

    Vf^2=Vi^2+2ad
    0=25^2+2(a)50
    a=-6.25m/s^2

    now you know the deceleration of the car

    Vf^2=Vi^2+2ad
    0=50^2+2(6.25)d
    d=200m
     
  4. Nov 12, 2004 #3
    Thanks. Is this the only way you can solve it?
     
  5. Nov 12, 2004 #4
    This is the way I would solve it. Hope others give some posts
     
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