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Homework Help: I don't get the same results in a Pytel dynamics problem

  1. Jun 8, 2017 #1
    • Poster has been reminded to post all schoolwork type questions in the Homework Help forums
    I have Pytel Mechanics Dynamics 2nd edition and revising for my own purposes (not a homework).
    I am trying to solve problem 12.4:

    A bead moves along a straight 1500-mm wire that lies along the x-axis. The position of the bead
    is given by
    x=t2 - 10t mm
    where x is measured from the center of the wire, and t is the time in seconds. Determine
    a) the time when the bead leaves the wire; and b) the distance traveled by the bead from t=0 until
    it leaves the wire

    I get (a) to be 32.83sec which is correct according to even-numbered answers given at the back of the
    book but (b) is given to be 2000mm which is not what I get. I get 800mm

    Can you verify if my answer to (b) is correct or tell me what is your answer of (b)?

    Does anyone has all the answers of the book I mentioned?

  2. jcsd
  3. Jun 8, 2017 #2


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    I get 800 too: t2 - 10t = (t-5)2 - 25 , so it evaluates to -25 mm at t=5 .

    I don't have all the answers :frown:
  4. Jun 8, 2017 #3
    After trying to calculate (b) analytically and failed to get 2000mm
    today I differentiated x to get the velocity which is 2t-10. I plotted the graph
    and calculated the area of the two triangles. that gave me aproximately 800mm
    I used https://www.desmos.com/calculator to do the plot.
  5. Jun 8, 2017 #4


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    It goes 25 mm to the left and then 775 to the right. 800 mm of travel.
    Don't waste much more time on this and move on to the next !
  6. Jun 8, 2017 #5


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    Also note that this book is very sloppy concerning dimensions. The correct formula must read
    $$x(t)=\frac{a}{2} t^2 +v t,$$
    where ##a## is the constant acceleration and ##v## the initial velocity. In other words the constants in front of ##t## must have the correct dimensions to be a valid formula in physics.
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