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Velocity Time graph question [i'm not getting anywhere]

  1. Jan 21, 2004 #1
    "As a car passes point A on a straight road, it's speed is 10m/s, the car moves with constant acceleration, a m/s/s, along the road for T seconds until it reaches point B, where it's speed is Vm/s.
    The car travels at this speed for a further 10 seconds, when it reaches the point C. From C it travels for a further T seconds with constant acceleration 3a m/s/s until it reaches a speed of 20 m/s at the point D."

    Sketch the [t,v] graph for the motion, and show that V=12.5

    ok, this is the first t,v graph i've drawn, so it could be painfully wrong, which may be my first problem, here it is is paint.

    anyway, i don't know whether my graph is correcy, and even if it is, i just can't seem to work it out.
    tried some trig, trapezium rule, i just feel i have to many unknowns to work this thing out

    any push in the right direction is appreciated

    alternate link to my graph thingy


    drag and drop into your browser, or copy and paste, to get it to work :)

    Attached Files:

    Last edited: Jan 21, 2004
  2. jcsd
  3. Jan 21, 2004 #2
    dunno if attachement worked?

    realised that t should really be a T, and those T's equal

    Attached Files:

    Last edited: Jan 21, 2004
  4. Jan 21, 2004 #3

    Doc Al

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    Staff: Mentor

    Your graph looks fine. Now write down equations describing how the velocity changes during the accelerated periods. (Use [itex]v_f=v_i+a\Delta t[/itex].) Even though there are three unknowns (a, t, v), if you write down the two equations, you will see that you can easily solve for v.
  5. Jan 21, 2004 #4
    thanks very muchfor that tip, i worked it out now, very simple in the end :(

    but the next part says "the distance from A to D is 675m, find a and t" [from earlier.

    the answers are a=1/8 and t=20. i just can't get to these answers, i've tried writing what i know [s, u, v] and trying to work that out, but it doesn't work. also tried taking away the middle section which is 25m, from 675 to get 650, and then dividing that by a ratio of 3:1, as there is a and a 3a , basicly just aimless ideas trying to get to the answers, but i've gone wrong. one more try was to write out the forumlas for the sqaures and triangles areas and making them equal 675, then solving for t, but that didn't work.

    any ideas for this one?
  6. Jan 21, 2004 #5

    Doc Al

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    Staff: Mentor

    You must be making an error somewhere. The distance equals the area under the graph. Just add up the pieces and solve for T. (The middle section does not equal 25m.)
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