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How does mg = ma follow from F=ma?

  1. Sep 30, 2007 #1
    Can somebody explain in simpler terms F=ma and why that can be writtent as mg=ma. Which finally can be written as g=a.
  2. jcsd
  3. Sep 30, 2007 #2

    Doc Al

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    Staff: Mentor

    Newton's 2nd law states that the net force on an object must equal ma. In the special case where the only force on an object is gravity, which equals mg, then the net force is just mg. Thus mg = ma, which implies that g = a. (The acceleration of an object in free fall is g downwards.)
  4. Oct 2, 2007 #3
    Doc_al is correct in saying that g = a when the object is in free fall (the only force acting on the object is gravity). However the world is not that simple, there are alot more forces that can act on objects such as support force, air resistance, friction just to name a few. So therefore F=ma means the sum of all forces = ma, usually seen with the symbol sigma (meaning the sum of forces) before F. I hope that clarified things for you memsces.
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