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Homework Help: Distance and Acceleration for One Car to Catch Another

  1. Oct 15, 2014 #1
    1. The problem statement, all variables and given/known data

    You are traveling at a constant speed vM, and there is a car in front of you traveling with a speed vA. You notice that vM>vA, so you start slowing down with a constant acceleration a when the distance between you and the other car is x. What relationship between a and x determines whether or not you run into the car in front of you?

    2. Relevant equations

    x = v0*t - 1/2*a*t^2

    3. The attempt at a solution

    After a time t, the distance between the cars must be something larger than x if they don't want to crash, so:

    vM*t-1/2*a*t^2 - vA*t > x


    What should be the next step after this?
  2. jcsd
  3. Oct 15, 2014 #2


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    I believe you are complicating things too much. It's simple... A uniform deceleration has to cancel the relative velocity within a given distance...

    And I would choose another equation...
  4. Oct 15, 2014 #3
    we can easily find by using...
  5. Oct 15, 2014 #4


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    Remember: you know two things, that are enough: 1) the difference of velocity with respect to the other car Vm - Va. Let's call it U. 2) you also know the distance x to the other car when you start braking with a uniform negative acceleration a.

    Thus, you should use a equation that gives a as a function of U and x... That is, acceleration as a function of velocity and space. You probably know the equation as giving velocity as a function of space and acceleration.

    Just solve for acceleration...
  6. Oct 15, 2014 #5

    I've done this. But what can be said for a and x just looking at this equation?
    (vM- vA)^2 /(2a) must be less than x for cars not to crash?
  7. Oct 15, 2014 #6


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    The negative acceleration to be calculated by a = U2/2*x is exactly the necessary to avoid contact... More deceleration will equalize the speeds earlier, keeping the distance, and less deceleration would result in a crash...
    Last edited: Oct 15, 2014
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