I've always been told that the moon's gravity causes tides, and now that we're starting our unit on gravity I finally decided to look up the actual physics behind the motion of earth's tides. (Disclaimer: most of this information is from the "Tide" article on Wikipedia) The basic overview is that the moon exerts a gravitational pull on earth which acts on both land and water, but because the oceans are fluid they respond in a much larger way to this force. Just for grins, I calculated the gravitational force of the moon on earth and using the equation G=(M1M2)/R^2 it works out to about 2E26 Newtons. For some perspective I converted this to pounds and got 4.5E25, which is 45 septillion pounds! However, the actual tidal force on any particle of water is the vector difference between the gravitational force of the moon on the particle and what the gravitational force of the moon on the particle would be if the particle were located at the earth's center of mass. Somehow this means that instead of following the inverse square law we learned about today, tidal force is proportional to the inverse cube of the distance from particle to moon. Since the ocean is so massive, despite a huge tidal force lunar tidal acceleration is just 1.1 × 10⁻⁷g Yet this is enough to cause tidal changes with amplitudes up to around 50 feet in areas like the Bay of Fundy in Canada, far exceeding the global average of about 1 meter.