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Physics, Astronomy & Cosmology / Re: the direction of gravity?
« on: 16/01/2017 00:20:08 »Be careful here. It depends on the definition of "weight" you are using. If you are using it as synonymous to "mass", as we tend to commonly do on Earth, then you can say this. If you are using it as a measure of downward force such as measured by a spring scale, then you can say that its slower rate of fall on the Moon is due to its lesser weight, as its lesser weight is due to the smaller gravitational force of the Moon.The rate it falls is determined by the rocks mass/weight? If I lifted and dropped that same rock while standing on the moon would the rock drop slower to the ground? If the answer is yes, then my question is why? What is determining the direction the the rock travels once I let it go, me lifting the rock, or the earth and the moon?No,the rate the rock falls does not depend on its weight.
On Earth, we tend to use mass and weight some what interchangeably. when we say something "weighs" 1 kg, it means the same as it masses 1 kg. But we really are talking about two different units from two different unit systems. The first is a kg(f) and is a unit of force in the Gravitational MKS system and the second is a kg(m), a unit of mass in the Absolute MKS system ( there is also a lb(f) and lb(m) in the corresponding FPS systems. Since at 1 g a 1 kg(m) mass exerts a 1kg(f) force downward while at rest on a surface We tend to intermix the two. Technically, this is incorrect as we are mixing units from different systems. Technically, if we are using kg(m), then you should give the weight in Newtons. There is no name for the unit of mass if you are using kg(f) and it is a derived unit of kg-sec^2/m ( in the FPS systems the mass unit that goes with the lb(f) is called a slug, and the force unit that goes with the lb(m) is the poundal)
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If you dropped the same rock on the Moon it would ,.as you suggest , fall there more slowly.More accurately, the combination of the mass of the object and your distance from its center. The moon is 1/81 the mass of the Earth, but also has just a bit only 1/3 its radius. being closer to the Moon's center counters it lower mass somewhat, so you end up with gravity force ~1/6 that on the surface of the Earth.
This is because because the Moon has less mass * than the Earth and the mass of an object such as the Earth or Moon determines the strength of its Gravity Field.
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Once you stop holding the rock it moves in the direction of the "centre of mass" of the Earth or Moon until it meets an obstacle such as the surface of the Earth -or your toe ,perhaps where its journey is "held up" .
* broadly speaking ,the Moon "weighs less" than the Earth and so attracts other objects like your rock less powerfully.