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One-Dimentional Kinematics Question

  1. Aug 14, 2009 #1
    1. The problem statement, all variables and given/known data
    "Two cars drive on a straight highway. At time t = 0, car 1 passes mile marker 0 traveling due east with a speed of 20.0 m/s. At the same time, car 2 is 1.0 km east of mile marker 0 traveling at 30.0 m/s due west. Car 1 is speeding up with an acceleration of magnitude 2.5 m/s^2, and car 2 is slowing down with an acceleration of magnitude 3.2 m/s^2. Write x-versus-t equations of motion for both cars"


    2. Relevant equations

    The equation I used was X = Xi + ViT + 1/2 AT^2

    Sorry, I'm bad at typing these equations without subscripts or superscripts.

    3. The attempt at a solution

    I got the equation right for Car 1. However, when I went to solve for Car two, it did not match with the answer key. I plugged in all of the relevant information into the equation:

    X = 1000m + (-30 m/s)t + 1/2(3.2 m/s^2)t^2 = 1000m + (-30 m/s)t + (1.6 m/s^2)t^2

    Since Car 2 is going towards the 0 marker, the velocity should be zero. However, in order for there to be deceleration, accerleration and velocity should have opposite signs. Since velocity is negative, I made acceleration positive.

    However, this doesn't match the textbook answer, which is X = 1000m - (-30 m/s)t - (1.6 m/s)t^2

    Am I doing something wrong?

    Thanks in advance for taking the time to answer my question!
     
  2. jcsd
  3. Aug 14, 2009 #2

    kuruman

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    Homework Helper
    Gold Member

    How about starting with two separate equations (as the problem suggests), one for each car. Fill in the blanks

    x1 =

    x2 =

    Then it should be clearer what you're doing.
     
  4. Aug 14, 2009 #3
    Hm...the textbook answer looks wrong; inserting t=1, we would get X = 1028.4m > 1000m, which is clearly not possible given that the car is heading towards the 0 mark.
     
  5. Aug 14, 2009 #4
    Basically, the first one is simple enough to do without any trouble, and it's the second car that I'm having trouble with. To clear things up:

    What I did for Car 1:

    x1 = xi + vit + 1/2 at2
    x1 = 0 m + (20 m/s)t + 1/2(2.5 m/s2)t2
    x1= (20 m/s)t + (1.25 m/s2)t2

    This answer matched with the textbook answer, so I didn't have any trouble for the first one. I simply plugged in the givens into the equation.

    What I did for Car 2:

    x2 = 1000m + (-30 m/s)t + 1/2(3.2 m/s2)t2
    x2= 1000m + (-30 m/s)t + (1.6 m/s2)t2

    My reasoning: Since Car 2 is going towards the 0 marker, the velocity should be zero. However, in order for there to be deceleration, accerleration and velocity should have opposite signs. Since velocity is negative, I made acceleration positive.

    textbook answer is X2 = 1000m - (-30 m/s)t - (1.6 m/s)t2

    Am I doing things incorrectly or is this a textbook error?

    Edit: Just saw the above post. Thanks for the response!
     
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