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Very Vague Graphing Motion Problem

  1. May 28, 2013 #1
    1. The problem statement, all variables and given/known data

    A stone is thrown upward from the edge of a cliff, reaches its maximum height, and then falls down into the valley below. A motion diagram for this situation is given, beginning the instant the stone leaves the thrower’s hand. Construct the corresponding motion graphs taking the acceleration due to gravity as exactly 10m/s2 . Ignore air resistance. In all three motion graphs, the unit of time is in seconds and the unit of displacement is in meters. In plotting the points, round-off the coordinate values to the nearest integer.

    I need to make three graphs with the vertical displacement, vertical velocity, and vertical acceleration. The graphs are from t=0 to t=6 s.

    I attached a screenshot of the diagram they gave me.



    2. Relevant equations

    V=V0+at

    y=y0+V0t+0.5at2


    3. The attempt at a solution

    I need to find the initial velocity! It doesn't tell me. I'm not sure if I can find the initial velocity from what is given. The image shows that the highest point is at t=2s. But does that mean that v=0 at that point?? There seems to be two points up there.. It's confusing me.

    Can I use V = V0+at to find it?
    I already tried doing that and I got 20 m/s from (at the highest point):

    0=V0+(-10m/s2)(2 s)

    Another thing! Is the gravitational acceleration negative? The question didn't tell me which direction is positive! How am I supposed to know??

    So is V0= 20m/s??
     

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  3. May 28, 2013 #2

    SteamKing

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    In your experience, in which direction does gravity act? Do things go flying into the air for no particular reason?

    The problem statement is very clear. A stone is thrown upward from the edge of a cliff. Can you find that on the attached diagram? The stone reaches its max. height 2 s after it is thrown. What happens at max. height?

    The diagram is drawn the way it is, I think, for clarity, to show that the stone changes direction 2 s after it is thrown from the cliff.

    Stay calm and work through the problem. There are no tricks here.
     
  4. May 29, 2013 #3

    haruspex

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    Looks right.
    It's up to you to choose whether up or down is positive. It doesn't matter as long as you are consistent. Usual is to take up as positive, and that seems to be what you have done.
     
  5. Jun 3, 2013 #4
    Thanks so much. I was just unsure.
     
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