Answer: Gravity is the force that keeps planets in orbit around the Sun. Gravity alone holds us to Earth's surface.
Planets have measurable properties, such as size, mass, density, and composition. A planet's size and mass determines its gravitational pull.
A planet's mass and size determines how strong its gravitational pull is.
Models can help us experiment with the motions of objects in space, which are determined by the gravitational pull between them.
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