So I heard on NPR just now that we weigh more at low tide for reasons that have something to do with the moon and the change in our salt water content (the got if from the Almanac), but it doesn’t make sense to me. Anyone know?
PC
I don’t get the “salt water content” bit, but the other I think I get. At one high tide, the moon is more or less overhead, thus attracting you (very, very slightly). At the other high tide, the moon is at its farthest position from you, so any force vector it’s adding to the Earth’s gravitational pull is minimized. So at the low tides, neither of these case is true and you weigh (again, very, very slightly) more.
This page gives some good info. Seems the gravitational pull of the moon (F[sub]g[/sub]) minus the centifugal force (F[sub]c[/sub]) caused by the earth’s rotation about the common center of mass of these two bodies (barycenter) causes high tides on the side of the earth facing the moon, and high tides are caused on the OPPOSITE side of the earth by the centrifugal force caused by the earth’s rotation about the barycenter MINUS the smaller F[sub]g** (further from the moon). It could follow that the resultant forces that cause high tides would slightly overcome the effect of earth’s grativity on YOU, you would be lighter during high tides, and conversely heavier at lows.
The second point on salt water content doesn’t make sense to me.
UncleBill sounds right. You’d be lightest when the moon was directly above you, and heaviest when the moon was directly below you. Both situations would be high tides.
The only way I can see salt water content having anything to do with anything is if someone did a study and found that people eat more salt at low tides, making them retain water. Doesn’t sound very likely, though.
Actually, if the barycenter related centrifugal force and moon’s gravitational force work on your body as it does the water, you would be lightest when the moon is above you AND opposite you, both situations being high tides.