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Homework Help: Adding force vectors

  1. Feb 11, 2005 #1
    I've been given this problem: Two forces act on a block of mass 14.1 kg. The first force has magnitude 28.5 N and is directed along the x axis. The second force has magnitude 6.7 N and is directed at an angle of 38.7 degrees to the x axis. Find (a) the magnitude and (b) the direction (i.e., the angle above the x-axis) of the block's acceleration. The block ONLY has the two indicated forces acting on it. Gravity DOES NOT play a role.

    I keep doing this problem over, including saving the rounding until the end, but I still seem to be off by a little bit. If any of you could help me spot the error, I would appreciate it.

    components of FA:
    FAX = (28.5 N)(cos 38.7) = 22.24 N
    FAY = (28.5 N)(sin 38.7) = 17.82 N

    components of FB:
    FBX = (6.7 N)(cos 38.7) = 5.23 N
    F[size=-2]BY[/size] = (6.7 N)(sin 38.7) = 4.19 N

    F[size=-1]RX[/size] = 22.24 + 5.23 = 27.47 N
    F[size=-1]RY[/size] = 17.82 + 4.19 = 22.01 N

    then I took the square root of the sum of FRX squared plus FRY squared and got 35.2 N.

    I then divided this by 14.1 kg to find the acceleration and ended up with 2.496 m/s^2 but the program says it's incorrect. Close, but wrong. Any takers?
  2. jcsd
  3. Feb 11, 2005 #2

    Doc Al

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    Staff: Mentor

    The 28.5 N force is directed along the x-axis. That means its y component is 0 and its x component equals 28.5 N.
  4. Feb 11, 2005 #3
    Okay, I finally got it! Thanks for the help. :)
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