Gravity • Orbital Speed • Orbital Period • Kepler’s Third Law
In this adventure, students use Newton’s law of gravitation to compute a single powerful number, GMsun, and then use it to calculate orbital speed, orbit size, and orbital period. The first activity focuses on numerical work for Mercury, Earth, and Neptune. The second activity uses planetary tables and charts to look for patterns across the Solar System.
The story for this adventure begins with an old human mystery: What is fire? The punch line is that fire is, in one beautiful sense, sunlight stored in trees and later released. This connects naturally to the next question of the adventure: how strongly does the Sun control the Solar System?
Students compute GMsun from the gravitational constant and the mass of the Sun. Then they use that number to calculate:
for Mercury, Earth, and Neptune.
Students examine the full planetary data tables and charts and explain how orbital speed, orbit size, and orbital period change as distance from the Sun increases. This leads to the relationship between T and r3/2, and ultimately to Kepler’s Third Law.