http://www.ck12.org Chapter 13. Forces
FIGURE 13.22
Aiming at the center of a target is likely to result in a hit below the bull’s eye.Orbital Motion
The moon moves around Earth in a circular path called anorbit. Why doesn’t Earth’s gravity pull the moon down
to the ground instead? The moon has enough forward velocity to partly counter the force of Earth’s gravity. It
constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet.
As a result, the moon keeps orbiting Earth and never crashes into it. The diagram inFigure13.23 shows how this
happens. You can explore gravity and orbital motion in depth with the animation at this URL: http://phet.colorado
.edu/en/simulation/gravity-and-orbits.
FIGURE 13.23
In this diagram, "v" represents the forward
velocity of the moon, and "a" represents
the acceleration due to gravity. 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.
