physics
posted by physics .
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 forc is going to equal to the force of gravity
please explain...

You are totally wrong in postulating the net force=force of gravity.
Nuts.
Force of gravity applies only to the gravitional field, and for a free falling object, Forcegravity=mass*a
But what about other forces? Any unblanced force = mass*acceleartion, whether or not gravity is involved.
Now what if an object is sitting on a shelf..it is not accelerating.
Fgravity=force upward by shelf. What about weight? The downward force is equal to its mass times the gravitional field strength. What is the gravitional field strength? It is commonly called g, and is equal to 9.8N/kg. That is the force per kilogram when under the influence of gravity. What if the object is dropped? what is its acceleration? It is g, or 9.8m/s^2. Hmmm. Note in units, N/kg=m/s^2
One nitpick of your writing, and is the reason I wrote this. Your statement "the acceleration is equal to gravity sense gravity is an acceleration because of newtons..." is inaccurate.
Gravity is not an acceleration, it is a force.