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Homework Help: Simple kinematics problem: avg. acceleration if a car's going 12 m/s and accel

  1. Sep 28, 2009 #1
    A basic kinematics problem that I just can't get for some reason. I'm having trouble visualizing the graph.

    1. The problem statement, all variables and given/known data
    What's the average acceleration if a car's going 12 m/s constant speed, then accelerates for 12 seconds at 1.7 m/s^2 acceleration?
    Variables: none?

    2. Relevant equations
    ? I do not know. At all.

    3. The attempt at a solution
    I had graphed speed over time with the function as a horizontal line until it reached the origin, where it gained a slope of 1.7 because of the acceleration. I don't know if I'm visualizing the increase in speed correctly or if it would become an exponential curve. If this was the result, I don't know how I would then find average speed--total distance over time. Would I need to know calculus to figure this out? So this is where I'm stuck.
    Last edited: Sep 28, 2009
  2. jcsd
  3. Sep 28, 2009 #2


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    Homework Helper

    Missing information - it doesn't say how long it is going at constant speed.
    You could use v = vi + at to find the final velocity.
    The average acceleration would be a = (change in velocity)/time
    where "time" would be the total of the constant speed time and the 12 seconds of acceleration.
  4. Sep 29, 2009 #3
    Whoops! Yeah, I forgot to specify that the 12 s was the entire period... the time when it was going 12 m/s was not included in calculations except as a starting value, i.e. v(0).

    I'm sorry, I did ending up figuring this out. It was just averaging 12 m/s with the speed after 12 s with acceleration, which was 32.4 m/sec - 1.7 m/s^2 times 12 seconds and plus 12 m/s initial speed - to get 22.2 m/sec.

    Oh, and I put average acceleration instead of average speed. Well. I'm sorry, I was not being nice to you. :(
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