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Homework Help: Very beginner physics 1D motion question!

  1. Apr 16, 2008 #1
    [SOLVED] Very beginner physics 1D motion question!

    1. The problem statement, all variables and given/known data
    Two cars move in opposite directions toward each other on a level, straight, one-lane road. When both drivers begin to brake, their front bumpers are 275 meters apart. The smaller car is initially moving at 25.0 m/s and the larger is initially moving at 30.0 m/s. The magnitude of the acceleration of the smaller car is 20.0% greater than the magnitude of the acceleration of the larger. Determine the magnitude of acceleration for each car such that when their front bumpers touch, they have just come to rest

    2. Relevant equations

    3. The attempt at a solution
    1 stands for the larger car, 2 for the smaller car...
    Xi1=0 Xi2=275
    Xf1=unknown Xf2=unknown Xf1=Xf2
    Vi1=30 Vi2=-25
    Vf1=0 Vf2=0
    a1=? a2=? a2=1.2a1


    To solve for a1 I isolated Xf1 in the first equation and then plugged that in where Xf2 is on the second equation, since they are equal. I ended up with:

    a1= ((-25)^2-1.2(30)^2)/(2.4*275)
    a1=-.689 m/s^2
    and therefore
    a2=-.827 m/s^2
    which is the magnitude if you take the absolute value....

    So I am wondering if this is correct becaue I am having difficulty verifying my answers and I'm not sure if this is reasonable..
  2. jcsd
  3. Apr 16, 2008 #2


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    Homework Helper

    Hi mistymoon_38,

    To check this problem you can use the accelerations you found to find the displacement that each car takes during their trip and see if the distances each travels adds up to 275 meters.

    There's a subtle error in one of your expressions. You have set up the problem so that for each car the positive and negative directions are the same. However, the accelerations of the cars are in opposite directions (so one is negative and one is positive). So you cannot write [itex]a_2 = 1.2 a_1[/itex] because this forces both accelerations to have the same sign. If you use [itex]a_2 = -1.2 a_1[/itex] I believe you'll get the proper relationship between the accelerations.
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