# Why doesn't the moon make waves on rivers and smaller lakes?

Asked by buster (10222) May 1st, 2008

A lake can be glass smooth if the winds not blowing. Why doesn’t the pull of the moon have the same affect on all bodies of water?

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Some lakes do have waves, especially larger ones, which leads me to suspect that it has to do with a fundamental property of how gravity works: larger bodies have larger gravitational forces acting on one another. So the gravitational pull between a puddle and the moon is relatively weak, whereas the gravitational pull between the ocean and the moon is much stronger simply because the ocean is so much larger:

Mathematically, you can find the force of gravity between two objects by:

F=G(m1)(m2) / r^2

Where G is the universal gravitational constant, m1 is the mass of one body, m2 is the mass of the second body, and r is the distance between the two. So you can see why the force of gravity, and consequently the size of the waves, would be much stronger as the body of water grew larger.

That’s my best guess.

nikipedia (27499)

thats a good answer. can you tell me why waves are a lot larger on hawaiis north shore than say panama city beach florida?

buster (10222)

Its also because of ocean currents and winds. Lakes and rivers usually do not hve the intense winds that the open ocean can have. Also, there arent any strong currents on a lake, such as the gulf stream in the ocean.

eambos (8894)

Scratch my last answer. I should have done my homework. As usual, wikipedia > nikipedia:

http://en.wikipedia.org/wiki/Ocean_surface_wave

nikipedia (27499)

That’s right Eambos. You should feel the wind on the North Shore too. Grew up in Kailua myself and really didn’t like going to the North Shore with my surfer friends. I always got sucked in by the tide cause I was so small and the current so strong. :-P

cooksalot (1426)

It’s the tides that are affected by the moon, not waves. Waves are generated through a completely distinct mechanism.

The tidal affect of the moon on lakes and rivers is smaller due to smaller volume.

soundedfury (2536)

I feel that I deserve at least a partial donut, C. Gravity still plays a part, according to the “Science of Waves” section. Perhaps a very small donut. A donut hole?

nikipedia (27499)

Sure you should. After all when the tide rises no matter how windy or no wind the waves do get bigger. Trust me they do.

cooksalot (1426)

There are waves big enough to surf sometimes in the Great Lakes. See: http://en.wikipedia.org/wiki/Lake_surfing

bearfair (403)

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