That was his student, Johannes Kepler.
Answer:
what is the equation for the earth gravitational force
That depends on several things that we don't know.
A few of them are:
-- the size of the sample
-- what level of radioactivity was measured at the beginning
-- what's the unit of the 12.5
-- the half-life of the substance involved
If both the net force and the mass are doubled, the acceleration will be unchanged.
