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Homework Help: Help with Velocities/graph

  1. Oct 14, 2007 #1
    1. The problem statement, all variables and given/known data

    im given a graph. the graph itself isnt so important though

    it is a curve, with 3 point vectors going from the origin to 3 points on the path of motoion of the particle.

    they are at t=2.6, 2, 2.4

    a and b) im asked to draw the average velocity vectors of the particle over t=2 and t=2.4
    and also over t=1.5 and t=2

    c) write an expression for the velocity vecotr v(2)

    d) draw an approximation to the vector v(2) and estimate the speed of the particle and v(2)

    2. Relevant equations

    3. The attempt at a solution

    a and b
    is it just...draw a line form the point t=2.4 and t=2
    and for b from t=1.5 to t=2

    and thats the average velocity vector?

    c) do i use the formula
    Code (Text):

    lim     r(2+h)-r(2)
    h-->0       h

    d) NO CLUE!?
  2. jcsd
  3. Oct 14, 2007 #2
    i think for D i guess based on looking at the slope?
    so in my case it looks like a slope of -1
    so the speed would be -1? (or 1)
  4. Oct 22, 2007 #3
    The average velocity is total distance by total time, so a and b are correct. For c, the equation of a line is given by Z=mX+c where m is the slope. In this case, m will be dx/dt or your average velocity.

    Therefore, x=vt+c (here Z=x and X=t) and you can find the value of c by plugging in the position and time of any one point that you are given. If you need the equation in vector form, its even easier, r(x,t)=(t2-t1)i+(x2-x1)j, here Im assuming that time is graphed on the x axis and the distance on the y axis.

    Now, the equation of the line in vector or cartesian form (z=mx+c is the cartesian form) when graphed gives you the approximation of the vector you desire. The speed of the particle is the magnitude of the velocity, aka, velocity without direction. You should be able to use the graph to figure it out...
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