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Homework Help: Parametric equations, find speed and direction

  1. Aug 6, 2012 #1
    1. The problem statement, all variables and given/known data

    An object moves so it's coordinates at the time t is given by the relationships

    x = 25t
    y = 20t-5t^2

    What is the object's speed and direction at 3 sec?

    t = 3 sec

    2. Relevant equations

    v = √(dy/dt)^2 / (dx/dt)^2

    Pythagoras theorem

    3. The attempt at a solution


    I'm not sure how I should use this by combining the two formulas above.
  2. jcsd
  3. Aug 6, 2012 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    What you are calling dx and dy are really
    [tex]\frac{dx}{dt} \text{ and } \frac{dy}{dt}[/tex]
    which is the same as vx and vy. So you have found the two components of the velocity vector v -- or at least you'll have them once you plug the time value into your expression. So as a start, figure out what the values of vx and vy are, using the expressions you got.
  4. Aug 7, 2012 #3
    Ah, but that is the part I get stuck on,

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