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Clever folk out there, question about gravity (and tides)

nvingo

Well-known Member
OK so it's accepted that the 'gravity' of the Moon and the Earth, is what keeps the Moon in orbit, and that gravity is what causes the tides.

The pull of the Moon's gravity on the fluid (seawater) is what causes tide to 'rise'.

So at a point on the Earth's surface, an object is pulled downwards by Earth's gravity, and upwards by the Moon's gravity.

Is this effect measurable, by sensitive 'scales' - Newton balance or similar instrument, giving a different object weight (weight not mass - mass is constant) depending on the position of the Moon relative to the object being measured ?
 

phil t

Well-known Member
Also consider that gravity is not constant and varies (very small, but measurabley so) from place to place.
 

nheather

Distinguished Member
Every mass attracts other masses as defined by the universal gravity equation which is

F = G x (M1 x M2) / (R x R)

where M1 and M2 are the masses of the two objects (kg)
R is the distance (m)
G is the universal constant (about 6.7 x10(-11))

The gravity caused by earth dwarfs everything else because the mass is huge and the distance much closer than any other massive mass.

But yes, they can measure the force of attraction between two marbles sat close to each other.

The moon is the next closest large mass and is so significant it cause the tides. The other planets and stars and meteors also have an affect but because the distance is much greater the force of attraction is small.

With that universal equation if you plug in the mass and radius of the earth then you get the equation that we are familiar with in GCSE physics and maths

F = mg

Where g = 9.8 m/s/s

Cheers,

Nigel
 

nheather

Distinguished Member
It says mass is constant but weight is not constant, and that is to do with variations in gravitational attraction.

I noticed that after Christmas I weighed more. Since my mass is a constant, it must be some freak change in gravity. So no need for any of this dieting nonsense I just need to wait until gravity settles back down to normal values :)

Cheers,

Nigel
 

imightbewrong

Distinguished Member

DPinBucks

Distinguished Member
OK so it's accepted that the 'gravity' of the Moon and the Earth, is what keeps the Moon in orbit, and that gravity is what causes the tides.

The pull of the Moon's gravity on the fluid (seawater) is what causes tide to 'rise'.

So at a point on the Earth's surface, an object is pulled downwards by Earth's gravity, and upwards by the Moon's gravity.

Is this effect measurable, by sensitive 'scales' - Newton balance or similar instrument, giving a different object weight (weight not mass - mass is constant) depending on the position of the Moon relative to the object being measured ?
It's not quite a simple as that, because it doesn't explain why there are two high tides per day; one almost 'under' the Moon, and one on the other side of the Earth.

The reason is that the sea under the Moon is 4,000 miles nearer the Moon than the centre of the Earth is, and 8,000 nearer than the sea on the other side.

That means that the three points are trying to take different orbits around the Moon (the Earth actually orbits the Moon, although much less so than the Moon orbits the Earth). Because the sea is a fluid, that causes it to bulge away from the Earth in trying to take its own path. But it can't bulge very far because the Earth's gravity is too strong.

There are also tides in the rocks of the Earth' not just in the sea. The effect is almost too tiny to measure, but it's there.
 

nvingo

Well-known Member
Ok, so the planets with their gravities all orbit the Sun, does the Sun have 'tides' of its gaseous fluid? Probably much less significant due to the Sun's mass.
 

imightbewrong

Distinguished Member
Ok, so the planets with their gravities all orbit the Sun, does the Sun have 'tides' of its gaseous fluid? Probably much less significant due to the Sun's mass.
You might be interested in the Parker Solar Probe: Parker Solar Probe - Wikipedia

See how it (pink) launches in an elliptical orbit from Earth (dark blue) around the Sun (yellow, duh) and then gets into a tighter and tighter orbit each time Venus (light blue) passes giving it a gravity assist, ending up being closer to the Sun than anything eva. Science!

Animation_of_Parker_Solar_Probe_trajectory.gif
 

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