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Homework Help: Calculating distance, acceleration and time

  1. Nov 25, 2016 #1
    1. The problem statement, all variables and given/known data
    To save fuel, some truck drivers try to maintain a constant speed when possible. A truck traveling at 28.0 km/hr approaches a car stopped at the red light. When the truck is 110.7 meters from the car the light turns green and the car immediately begins to accelerate at 3.8 m/s2. How close does the truck come to the car assuming the truck does not slow down?

    2. Relevant equations
    Equations used:

    and r(t)=r0+v0t+0.5a2

    3. The attempt at a solution
    my attempt: http://imgur.com/RHuv2tq
    I got an incorrect answer, what did i get wrong?
    Last edited by a moderator: Nov 25, 2016
  2. jcsd
  3. Nov 25, 2016 #2


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    Did I manage to get my reply across before the good spirit that watches over us took action ?

    I asked you to check Dcar: that's not the expression for uniformly accelerated linear motion.

    [edit] Oh, I see: you mixed them up. Nevertheless, answer's incorrect: you want to check what you do with the signs

    By the way, the numerical values in this exercise are strange (truck speed very low wrt distance)
  4. Nov 26, 2016 #3
    Sorry, I didn't completely understand where the problem is?
  5. Nov 26, 2016 #4
    It looks like you have the car and truck equations reversed. Also, I think your final equation would indicate that the vehicles are headed toward each other.
  6. Nov 26, 2016 #5
    You know, that's what I thought when I worked out the result - that the numbers were kind of strange. I was wondering if the acceleration number might have been wrong; it seems kind of high. But you're right that the truck's speed seems very low also. @agargento, please check that you copied all of the values correctly.
  7. Nov 26, 2016 #6
    Hey, I got the correct result eventually, the numbers are correct. I've built an equation of their distance x(t) = 110.7 + 0.5 * 3.8 * t^2 -(28/3.6) * t and used the time I found to find the minimal distance, and it was correct.
  8. Nov 26, 2016 #7
    Okay good. Glad to hear that. My personal opinion though is that the problem writers should add a little more drama and suspense. :)
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