Why are some things heavy?

[Liam]

The gravitational force on an object is proportional to its mass. That's why heavier stuff feels heavier. But more mass also means you need more force to accelerate it. Those 2 effects perfectly cancel out, which is why everything experiences the same acceleration due to gravity (ignoring air resistance).

Mathematically, gravitational force = mg, and F=ma, so mg=ma, the m's cancel out and you end up with a=g~=9.8m/s^2