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Question about gravity

  1. Sep 12, 2009 #1
    ok I'll be using a home work problem to ask the question. I'm not asking for help on the question I got the correct answer just need to understand this concept better.

    Calculate the acceelration due to gravity on the Moon. The moon's radius is about 1.74 E 6 meters and its mass is 7.35 E 22 kg. For this problem I'll be using m2 as the mass which the moon is orbiting

    Ok then...

    [tex]\Sigma[/tex]F [in radial direction acting on m2] = (m2 a [radial direction] = Fg = r^-2 G m m2)m2^-1

    divide both sides by m2

    a [radial direction] = r^-2 G m

    I rember in like inclined planes and such when two forces equal and opposite each other are present you can just cancel them out...

    so how is there any net force?
    how come like in inclined planes when I find the net force i don't include both masses...

    Net force = (net mass) a

    I dont use net mass in this problem because of why?

    Why do I just use m2 and not m1 + m2

    net force = (m1 + m2) a
  2. jcsd
  3. Sep 12, 2009 #2


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    Homework Helper

    If I understand what you are asking, it depends on whether the two forces are acting on the same mass or on different masses.

    If two equal-and-opposite forces act on the same mass, you may cancel them ... we say that the net force acting on that mass is zero.

    However, when the two forces are acting on different masses, they do not cancel. Each mass experiences a net force, due to the one force acting on it. This is the case with the mass and the moon in your calculation.

    The equation, Fnet = ma, refers to the net (vector sum) of only the forces that act on the mass m. Any other masses, or forces acting on masses other than m, are irrelevant here.
  4. Sep 12, 2009 #3
    Newton's gravitational law: Force = G m_moon m_object / r_moon ^2

    You just pull the mass of the object out of the expression, and put it to the side:
    Force = [ G m_moon / r_moon ^2 ] m_object

    acceleration of object = F / m_object = the stuff in the brackets
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