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Homework Help: Did i do this f=ma problem right

  1. Sep 18, 2010 #1
    1. The problem statement, all variables and given/known data

    A body of mass m = 10 kg initially at rest has force F = 20Ni + 30Nj applied to it. Find its velocity at t = 3 s


    3. The attempt at a solution

    m(ai,aj) = (20i,30j

    m*ai= 20N

    m*aj = 30N

    ai = 2*m/s^2

    aj = 3*m/s^2

    to find the velocities of each of the vectors i used dimensional analysis:

    vi = (2*m/s^2) * (3*s) = 6m/s
    vj = (3*m/s^2) * (3*s) = 9m/s

    to find the complete velocity, i now calculate the magnitude of the vectors by the equation:

    v = √( vx^2 + vy^2)

    v=√(6^2 + 9^2)

    v= 10.9 m/s <--- is that right?

  2. jcsd
  3. Sep 18, 2010 #2
    yes that is correct.
  4. Sep 18, 2010 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You have the right idea, except that your final answer is not expressed as a vector. They did ask for velocity, and not for the speed.
  5. Sep 18, 2010 #4


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    Science Advisor

    Yes, velocity is the vector you got initially: [itex]6\vec{i}+ 9\vec{j}[/math] m/s. It is not necessary to find the magnitude- that is the speed.
  6. Sep 18, 2010 #5
    thanks i never knew the difference between speed and velocity, now i do!
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