Physics 197 Lecture 17: Introduction to Orbits

• School: WUSTL
• Department: Physics
• Course: Physics 197
• Professor: Ferrar

Document Summary

All planets move in ellipses, with the sun at one focus. A line from the sun to the planet sweeps out equal areas in equal times. If a is the planet"s semimajor axis (one-half of the ellipse"s greatest width), then t^2 is proportional to a^3, where t is the planet"s period. Consider an isolated (or freely falling) system of two interacting objects with masses. M and m. a nonrotating reference frame connected to the system"s center of mass (cm) will be inertial in either case (as long as we ignore external gravitational interactions in the freely falling case) The orbit of either object around the system"s center of mass must lie in a plane perpendicular to the xed angular momentum vector. As an object moves through an angle d in a tiny time dt, its radius vector from the system"s cm sweeps out an area da = 1/2(r^2)(d )