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Homework Help: Instantaneous power problem

  1. Oct 13, 2005 #1
    i need a little help on this problem and i'm not sure if i have the equation down right.

    a 1990kg car accelerates uniformly fromrest to a speed of 17.1 m/s in 4.21 sec.
    now the question states what is the instantaneous power delivered by the net force to the car at 2s. answer in units of w.

    so this is what i did.
    i know that the equation is P=dw/dt which is F*v
    to find F i did F=ma but i need to find out what a is
    vf=vi+at

    vf=vi+at
    17.1m/s = 0 m/s + a(2s)
    a = 8.55 m/s

    F = 1990kg (8.55m/s) = 17014.5

    then i plug all that into P= F*v
    P = 17014.5 * 17.1 m/s = 290947.95

    but thats wrong (online submit). my instructor told the class that v in that equation is the final velocity. am i doing something wrong here. i already found the average power in for the 4.21s and i already found work done.
     
  2. jcsd
  3. Oct 13, 2005 #2

    Pyrrhus

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    You need to find the speed at 2s, then multiply it by the net force.
     
  4. Oct 13, 2005 #3
    how would i find speed? isnt speed change of x over change of time?
     
  5. Oct 13, 2005 #4

    Pyrrhus

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    You know the acceleration is uniform and it stars from rest, Kinematics? :smile:
     
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