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Homework Help: Physics problem on force

  1. Jan 12, 2009 #1
    1.Three astronauts, propelled by jet backpacks, push and guide a 85 kg asteroid toward a processing dock, exerting the forces shown in Fig. 5-31.
    http://img82.imageshack.us/img82/6572/0539aw6.gif [Broken]
    (a) What is the asteroid's acceleration in unit-vector notation?
    __m/s2 i + __m/s2 j
    (b) What is the asteroid's acceleration as a magnitude and direction?
    __° (counterclockwise from the +x axis is positive)

    2. Relevant equations
    W= F * d

    3. a) 55/85 m/s2 i + (32 cos 30-40 cos 60)/85 m/s2 j
    = 0.65 m/s2 i + 0.091 m/s2 j

    b) (.65^2 +.091^2) ^.5 is the net acceleration.
    for direction i used tan x=.091/.65
    answer= .14

    i need help
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Jan 12, 2009 #2
    The question does not seem to be asking for Work or Energy, so the equation W = F * d does not seem correct.

    You do have the right idea in terms of calculating acceleration from the force / mass (a = F / m), but you didn't quite get the procedure correct.

    Using unit-vector notation, the i unit vector will be the sum of the forces in the x-direction, and the j unit vector will be the sum of the forces in the y-direction.

    Hopefully this will put you on the right track.
  4. Jan 12, 2009 #3
    still confused on which equation to use
  5. Jan 12, 2009 #4
    Looking at your first post, you used F = ma (or a = F/m). You found one component of the i unit vector as 55 N / 85 kg. This was correct.

    However, the point where you went wrong was in using
    (32 cos 30-40 cos 60)/85 as the j unit vector. The cosine of the angle times the hypotenuse will give you the x-component of each force, which would be part of the i unit vector.
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