Gravity and orbits

Objective: Explain gravity and inertia, distinguish their meanings, and apply them to a described case.

Gravity: Gravity is an attractive interaction between masses that depends on mass and separation. Inertia: Inertia is an object's tendency to maintain its motion unless a net force acts. Gravity supplies the acceleration that bends an orbiting body's path. Without that acceleration, inertia would carry the body along a straighter trajectory rather than holding it above Earth. Orbital motion does not require an engine continually pushing in the direction of travel. Distance and the masses involved influence motion. Mass remains an intrinsic property while weight changes with the local gravitational field; apparent weightlessness does not mean gravity is absent. Application: A planet's mass attracts an orbiting moon. This most directly illustrates gravity. A spacecraft would continue straight without a deflecting force. This most directly illustrates inertia.

Objective: Explain orbit and elliptical orbit, distinguish their meanings, and apply them to a described case.

Orbit: An orbit results from motion influenced by gravity around another body or shared center of mass. Elliptical orbit: An ellipse is the curved orbital shape described by Kepler's first law for ideal two-body bound motion. Gravity supplies the acceleration that bends an orbiting body's path. Without that acceleration, inertia would carry the body along a straighter trajectory rather than holding it above Earth. Orbital motion does not require an engine continually pushing in the direction of travel. Distance and the masses involved influence motion. Mass remains an intrinsic property while weight changes with the local gravitational field; apparent weightlessness does not mean gravity is absent. Application: A satellite repeatedly travels around Earth. This most directly illustrates orbit. A planet's distance from the Sun varies along its orbit. This most directly illustrates elliptical orbit.

Objective: Explain orbital period and mass versus weight, distinguish their meanings, and apply them to a described case.

Orbital period: An orbital period is the time required to complete one orbit. Mass versus weight: Mass measures matter or inertia; weight is gravitational force and changes with local gravity. Gravity supplies the acceleration that bends an orbiting body's path. Without that acceleration, inertia would carry the body along a straighter trajectory rather than holding it above Earth. Orbital motion does not require an engine continually pushing in the direction of travel. Distance and the masses involved influence motion. Mass remains an intrinsic property while weight changes with the local gravitational field; apparent weightlessness does not mean gravity is absent. Application: A planet completes one revolution in its measured year. This most directly illustrates orbital period. An astronaut has the same mass but less weight on the Moon. This most directly illustrates mass versus weight.