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Homework Help: Calculating velocity at a certain time

  1. Apr 3, 2012 #1
    1. The problem statement, all variables and given/known data
    The following describes the position of a body as a function of time: [itex]X(t)=9\cdot cos(1.2t)\cdot e^{-t/10}[/itex].

    What is the velocity of the body at t=17sec ?

    2. The attempt at a solution
    I'm trying to use the formula [itex]v(t)=\frac{dr}{dt}[/itex] in order to get the velocity and I get: [itex]-9\cdot e^{\frac{-17}{10}}\cdot [sin(1.2\cdot 17)\cdot 1.2+\frac{1}{10}\cdot cos(1.2\cdot 17)]=-1.975[/itex]

    I guess negative velocity means the body is travelling in the oposite direction but is seems a little odd. Could this be the right answer?
  2. jcsd
  3. Apr 3, 2012 #2


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    Hello phyyy,

    Welcome to Physics Forums!

    That's what I got (ignoring any rounding differences). :approve: But that assumes that the number 1.2 in the cosine function has units of rad/sec (as opposed to deg/sec).
    That shouldn't seem that odd. Velocity is vector. It has both magnitude and direction.
  4. Apr 4, 2012 #3
    Thanks collinsmark!
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