For ten points, name this force that pulls objects toward large masses like the Earth.
This force is related to an accelerating observer by the Equivalence Principle of general relativity. Spacecraft can speed up by using this force as a "slingshot." Galileo used two balls of different (*) masses to demonstrate that the pull of this force is constant, causing acceleration of 9.8 meters per second squared. For ten points, name this force that pulls objects toward large masses like the Earth.
This force is related to an accelerating observer by the Equivalence Principle of general relativity.
gravity (accept gravitational force)