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Fg = mg QUESTIONED

  1. Sep 12, 2009 #1
    1. The problem statement, all variables and given/known data

    we can say that the force of gravity is equal to mass times acceleration were the acceleration is equal to gravity sense gravity is an acceleration because of newtons second law force = mass times acceleration

    hence Fg = mass times acceleration
    Fg = mass times gravity

    Fg = mg

    however newtons second law states that the net force acting on an object is equal to it's mass times it's acceleration so what allows us to say that

    Fg = mg
    because certainly not for every single situation the

    net force is going to equal to the force of gravity

    please explain...

    what allows us to say

    Fg = mg

    2. Relevant equations

    net force = mass times acceleration

    3. The attempt at a solution

    Thank You!
     
  2. jcsd
  3. Sep 12, 2009 #2

    Pengwuino

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    Gold Member

    When you say Fg, do you mean [tex]F_g [/tex]? As in the force of gravity? If not, and you mean the force of gravity multiplied by gravitational acceleration Fg = mg, then that is not correct. It's simply F=mg.
     
  4. Sep 12, 2009 #3
    i mean in the force of gravity
     
  5. Sep 12, 2009 #4

    LeonhardEuler

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    Gold Member

    If the net force on the object is not Fg, then the object's acceleration is not g, so there is no contradiction because the second law refers to the object's acceleration. g is just the "acceleration due to gravity" in the sense that it is the component of the acceleration caused by the gravitational force.
     
  6. Sep 12, 2009 #5
    so then I'm not sure how to do this problem

    Calculate the acceleration due to gravity on the Moon. The Moon's radius is about 1.74 E 6 m and its mass is 7.35 E 22 kg.

    I know I just use this equation

    a = r^-2 G m

    were m is the mass of the moon but

    Fg = r^-2 G m M

    were M is the the mass of the object the moon orbits were does it go

    Fg = r^-2 G m M
    ???

    why can we just simply set this equal to Mg??

    Fg = r^-2 G m M = M a

    and that's how we get this

    a = r^-2 G m
     
  7. Sep 12, 2009 #6

    LeonhardEuler

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    Gold Member

    I think I might see why you are confused. g is the acceleration due to gravity on the Earth's surface. It is different on other planets and higher in space above the Earth. g is not the acceleration due to gravity on the Moon.
     
  8. Sep 12, 2009 #7
    Do I do it like this

    net force = M a = r^-2 G m M

    or

    r^-2 G m M = M g

    am I solving for how fast M accelerates towards the moon?
     
  9. Sep 12, 2009 #8

    LeonhardEuler

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    Gold Member

    If you are solving a problem on the Moon's surface, why would you use 'g', the acceleration due to gravity on the surface of the Earth?
     
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