You could perhaps explain it with Newton's law: F=ma
Where:
- F: Force in Newtons
- m: Mass in kilograms
- a: acceleration in m/s
- g: the acceleration due to gravity that we find at the surface of the Earth (about 9.8 m/s)
If you have an object of mass m
_{1} sitting on the ground, it exerts a force of F
_{1} =m
_{1}g Newtons.
This applies if the mass m
_{1} is 1g, 1 kg or 1 ton(ne).
If you now drop this object of mass m
_{1} down a hole in the ground, you could imagine the object being subject to a gravitational force F
_{1}.
Rearranging the equation F=ma, you find that the acceleration of the object is now a=F
_{1}/m
_{1} = g = 9.8m/s
It may seem like a circular argument, but this says that regardless of whether the object is 1g, 1 kg or 1 ton(ne), it will accelerate downwards at about 9.8m/s (...in a vacuum, near the Earth's surface).
In physics, there are several types of mass (the above description referred to "passive gravitational mass" and "inertial mass"), which always seem to be identical when we compare them, but nobody is
totally sure why.
See:
https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitational_mass