http://www.ck12.org Chapter 4. Motion and Forces
FIGURE 4.35
Orbital Motion of the Moon
Just as Earth orbits the sun, the moon also orbits Earth. The moon is affected by Earth’s gravity more than it is by
the gravity of the sun because the moon is much closer to Earth. The gravity between Earth and the moon pulls
the moon toward Earth. At the same time, the moon has forward velocity that partly counters the force of Earth’s
gravity. So the moon orbits Earth instead of falling down to the surface of the planet.
TheFigure4.36 shows the forces involved in the moon’s orbital motion around Earth. In the diagram, v represents
the forward velocity of the moon, and a represents the acceleration due to gravity between Earth and the moon. The
line encircling Earth shows the moon’s actual orbit, which results from the combination of v and a. You can see an
animated version of this diagram at:
http://en.wikipedia.org/wiki/File:Orbital_motion.gif
