Eli5: how does the moon affect low and hightide of the ocean which is pretty heavy but does not cause other things which are lighter to start floating?
In: Physics
Google has all the answers, so here’s what I found. This is not my answer:
Everything on earth feels the gravity of the moon. Because the force varies with the square of the distance, it is relatively tiny on any tiny portion of the earth – like your balloon. The moon is approximately 60 earth radii away from Earth. But a tiny force applied everywhere is huge. As some have pointed out, the force is proportional to the mass of both objects. When one of said objects is all the free-standing water in the oceans of the world, that is a tremendous combined force and has tremendous effects. Because the water is fluid and relatively free to move, it reacts in aggregate to the attraction from the moon, and follows it. If we didn’t have large land masses, the bulge in the ocean that is the tide would simply follow the moon around the world without interruption. Instead, it sloshes back and forth in the big pools that are the ocean basins.
Your tethered balloon (do we assume that you meant helium-filled, and therefore floating? or did you mean air-filled and therefore hanging from its string? — I will arbitrarily assume that you meant helium-filled) does indeed feel a pull from the moon, no matter where the moon is in its orbit. And by the way, the moon is overhead of a given patch of earth during the night, only for part of the month. Another part of the month, the moon is visible in the sky during daytime. During the other two quarters of its orbit, the visible moon straddles night and morning, or afternoon and night. So the direction of pull is constantly changing throughout the day and night, and throughout the month.
BUT, due to the tiny mass of the balloon (less, even, than the same volume of air molecules), OTHER effects are much more detectable. For example, you specified that it was tethered to your roof. That means it is exposed to every outdoor breeze. That effect would completely drown out any observation you could make of the moon’s gravitational pull on that balloon.
Even if you got strangely lucky and the air was dead calm for as long as you cared to perform your measurements, the balloon and the land and buildings around it are still exposed to the sun. The balloon would expand in the sun’s warmth and would pull upward more strongly while it was being heated, which would swamp the minuscule attraction of the distant moon on such a low-mass object as that balloon. Then, the sun would set and the region would grow colder, and the balloon would shrink and sag. This change, too, would overwhelm any signal you could detect from lunar gravity. Just the heat of your body, standing near the balloon would be enough to set up convection currents in the air near the balloon.
[Source for the original author](https://www.quora.com/Why-doesnt-the-gravity-of-the-Moon-affect-anything-else-on-Earth-besides-the-tides-If-it-is-strong-enough-to-affect-oceans-shouldnt-it-pull-other-things-too-For-example-if-I-tie-a-balloon-on-the-roof-would-it-float-a-bit-higher-at-night)
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