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Kinetmatics coordinate

  1. Jul 23, 2013 #1
    1. The problem statement, all variables and given/known data
    At t=0, a particle leaves the origin with the velocity of 9.0m/s in the positive y direction and moves in the xy plane with a constant acceleration of ( 2.0i-4.0j) m/s ^2 . At the instant the x coordinate of the particle is 15 m , what is the speed of the particle ?


    2. Relevant equations

    V=u+aT , X= ut+ .5at

    3. The attempt at a solution
    X coordinate: X=ut+.5at
    15= 0+ 2t
    find t and plug it in the y - coorodinate
    y coordinate : V=9-4t
    Is that corrected ?
     
  2. jcsd
  3. Jul 23, 2013 #2

    HallsofIvy

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    This is incorrect. X= ut+ .5at^2

    You seem to be completely misunderstanding the notation. V is the vector velocity. It is NOT the y coordinate. X is the vector position, it has both x and y components. It is NOT the x coordinate.

    If the initial speed is u (which itself a vector with x and y components) then V= u+ at. Here, you are told that "At t=0, a particle leaves the origin with the velocity of 9.0m/s in the positive y direction and moves in the xy plane with a constant acceleration of ( 2.0i-4.0j) m/s ^2" so that u= <0, 9> and a= <2, -4>. V= <0, 9>+ <2, -4>t= <2t, 9- 4t>. Then X= <t^2, 9t- 2t^2>.

    The x coordinate will be 15 when t^2= 15 or t= sqrt(15). Then y= 9t- 2t^2= 9sqrt(15)- 30.
     
  4. Jul 23, 2013 #3
    That means I just consider the x and y component of the acceleration and put them to find the velocity of y component
     
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