| | g is the strength of gravity
Depends on what you mean by "strength of gravity".
The force of gravity is usually given in the Newtonian approximation as
a force between two objects. Let's call the mass of one object M and
the mass of the other object m, and let's say the distance between
their centers is r. Then the force is
F = G*M*m/r^2.
When one of the objects is the Earth and the other object is
comparatively small and close to the surface of the Earth, then M is
Mearth and r is Rearth, and this becomes
F = G*Mearth*m/Rearth^2
But let's pull out everything that doesn't have to do with the other
object m and put it in parentheses (at the risk of causing you some
F = (G*Mearth/Rearth^2)*m
Running the numbers for G, Mearth, Rearth, we find out that
G*Mearth/Rearth^2 = 9.8 m/s^2, so we can say
F = m * (9.8 m/s^2)
Oh, heck that's still too long. Let's just write this as
F = mg.