Moon and Earth

The Moon does not smash into the Earth because...

A) The net force on it is zero.

B) It is beyond the main pull of Earth’s gravity.

C) It is being pulled by the Sun and planets as well as by Earth.

D) All of the above

E) None of the above

Solution: E. The Moon is pulled towards the center Earth because of the gravitational attraction between the two. This attraction supplies the centripetal force necessary to keep the Moon in orbit (circular motion). Of course it is more complicated than this because the Moon is also attracted to (and orbits) the sun (see this article from Rhett Allain). The important thing to recognize is that there can be a difference between the direction of the force (and acceleration) and the velocity. In uniform circular motion, they are perpendicular.