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Homework Help: Kinematics Question

  1. Aug 8, 2007 #1
    1. The problem statement, all variables and given/known data
    " A car starts from rest and accelerates @ 4.0m/s^2 [E]. A truck with a constant velocity of 90.0km/h [E] overtakes and passes the car.
    a) how far beyond the starting point is the car after 10.0 s?
    How far beyond the starting point is the truck after 10.0s?
    c) The car passes the truck @ a distance of 312.5m beyond the starting point. How fast is the car travelling at this instant?
    d) How long does the car take to pass the truck?
    e) Both drivers suddenly see a barrier 100.0m away and hit their breaks at the same time. If they take 3.0s to stop and decelerate uniformly, will they stop in time?

    2. Relevant equations

    3. The attempt at a solution

    a) I found the displacement to be 400m [E]
    b) I converted the 90km/h to 25.0m/s [E] and found the displacement to be 250m [E]
    c) here's where I got stuck. I used the uaual velocity formula to solve for the change in time, but I got 12.5s as t. This doesnt make sence since in part a) I found the displacement to be 400m [E] in 10 seconds.
    d) For this, I wanted to state that since the car encounters the truck at 312.5m [E], and the truck is travelling 25.0m/s [E]. it took the car 12.5 seconds to get there (calculated in pt c) but I think my logic and calculations in the previous step are wrong.
    e) for the truck, I used the v2^2 = v1^2 + (2a)(displacement) to solve for displacement. I got 37.7m but since the last one was wrong..well its all connected and I'm frustrated.

    Any help would be greatly appreciated
    Last edited: Aug 8, 2007
  2. jcsd
  3. Aug 8, 2007 #2
    for a)

    you would get d=0.5*4*100=200m

    You forgot to multiply it by 0.5.
  4. Aug 8, 2007 #3
    ooh thanks v. much
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