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More force problems

  1. Jul 12, 2007 #1
    [SOLVED] more force problems

    1. The only two forces acting on a body have magnitudes of 20 N and 36 N and directions that differ by 72°. The resulting acceleration has a magnitude of 20 m/s2. What is the mass of the body?

    2. F=ma

    3. My original attempt was to add the two force magnitudes and divide by the acceleration magnitude, but I knew that wasn't right. I tried breaking up the force magnitudes up into x,y components, but that didn't seem to work either. So I'm pretty clueless. I'm having a hard time understanding this chapter so far. Hints to point me in the right direction would be great. Thanks.
  2. jcsd
  3. Jul 12, 2007 #2


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    You can't just add the magnitudes together.

    Try using vector addition and some trig to find the magnitude of the resultant force.
  4. Jan 17, 2008 #3
    Let's determine that 36N is on x-axis. Then find x and y components of 20N, which is 20cos(72) and 20sin(72).
    Add the x components and 36N together because both are on same direction and axis.
    Find magnitude of the force by use pyth. theorem.
    Then apply it to Newton's second law, F=ma. Then divide the acceleration, 20 m/s^2, and you will get the mass of the body.
  5. Jan 17, 2008 #4

    Shooting Star

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    No need to break up into x and y components this time. You need only the magnitude.

    R^2 = P^2 + Q^2 + 2PQcos(theta), where R is the resultant.

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