Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jccc on 04/06/2014 09:27:28
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1. Gravity causes tide, would it also cause wind?
2. Making tide needs energy, would the moon lose momentum over time?
3. We put no work but tide keeps coming, is it perpetual motion?
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1. Gravity causes tide, would it also cause wind?
The gravitational field of the Sun and the Moon raise the level of the ocean, as a tide.
It's not so obvious to us, but the tide also raises the solid land (much less than it raises the water level), and it will also raise the top of the atmosphere.
But this very slow tidal movement of the air would be small compared to thermal differences due to sunlight, clouds and precipitation.
2. Making tide needs energy, would the moon lose momentum over time?
It takes about 29 days for the Moon to orbit the Earth once.
Most oceans on Earth have 2 tides per day.
So tides are mainly caused by the Earth rotating, rather than by the Moon orbiting the Earth.
Tides scrape on the ocean bottom and shorelines, which dissipates energy. This is slowing down the Earth's rotation very slightly, by about a second every year or two.
Angular momentum is conserved, so as the Earth slows down, the Moon slowly spirals further outward from the Earth.
3. We put no work but tide keeps coming, is it perpetual motion?
No. The energy comes from slowing the rotation of the Earth. Because the tide does not make the Earth speed up to it's original rotation rate, it is not perpetual motion.
Geologists have discovered fossils having both daily and annual growth rings. These fossils suggest that in the past, there were more days per year.
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Thank you much Sir!
Better than reading few books.
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The tidal forces are actually increasing the energy of the moon.
As evan_au pointed out, the Earth is transferring angular momentum to the moon, decreasing the rate at which the Earth rotates and increasing the altitude of the moon--for now...
As the moon's orbit increases, its orbital frequency decreases, but not nearly as quickly as the Earth's rotational frequency is decreasing. Eventually, the Earth's day will actually become longer than the orbital period of the moon. Once this happens, the moon will start falling back in again, and eventually hit the Earth (in a couple billion years, but before the sun becomes a red giant and scorches the Earth anyway)