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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

    dx=25
    dy=20-10t

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

    Redbelly98

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    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,

    √25^2+(20-10t)^2
     
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