How hard does the Earth pull on the Moon? We can figure this out even if we don't know anything about Newton's Law of Universal Gravitation (yet). Let's simplify things by pretending that the Moon orbits the Earth in a circle at a constant speed -- it's not a terrible approximation.

- What is the average distance between the Earth and the Moon?
*(Hint: look in back covers of your textbook)* - What is the mass of the Moon?
- How long does it take the Moon to complete one orbit around the Earth?
- What is the centripetal force that the Earth must exert on the Moon?

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.