> From: [EMAIL PROTECTED]
>
> Newton's law of gravity is F = G Ma Mb / r^2
My understanding is that the *law* of gravity as such is the inverse square
law. i.e. that as the distance from the attractive body to the other
attractive body is doubled then the force exerted by gravity falls to a
quarter of it's original value.
Is that what your above representation says Glenn? Yes, except for an
alarming absence of a pair of ultimately superfluous but reassuring brackets
after the "G" and around "Ma Mb/r^2", I think it is - but you do need to
explain that the 'a' and the 'b' are in this case there to distinguish the
two objects: since 'a' is standardly used to denote 'acceleration' in
Physics algebra, and this might cause some confusion.
I think one standard representation of this inverse square law of
gravitation is (imagine that my "2" is superscript for square):
F = G*((m*m1)/r2)
Where F is centripetal force, G is the gravitational constant (problematic,
but observed for the most part to be an acceleration of 10 meters per second
per second) m is the mass of the attracted object and m1 is the mass of the
attractive (or rather second attracted) object, / is the divide sign, * is
for times, and r is the radius or distance from the attractive body in
meters.
Of course the gravitational constant G is arrived at by observing conditions
at the earth's surface on the assumption that everywhere on the globe r=
some fixed number of meters from the earths "center of gravity" whatever
that is (this is a problematic assumption - even the oceans are "hilly", the
earth is not of an even density, and you have to take into account the
rotation of the earth and the squashed orange shape, so on and so forth
until you even encounter relativity problems measuring times for
accelerations).
Despite problems the very necessary assumption is made that in cases where
r=the surface of the earth you don't need to factor in G, and upon this
axiomatic assumption depends the possibility of setting this gravitational
constant G, which is, accordingly, an observation, not a law. (an important
point).
Because you don't or can't factor in G at the surface of the earth the law:
F=mv2
(where v is the velocity of the object attracted) seems to work ok as a
determinate of the force of gravity on earth (G), as indeed it also does as
a determinate of the force exerted by a string on a conker as you twirl it
about your head.
I suppose that the point to leave you with is that the inverse square law is
a fact about *how* gravitation behaves: it is not an *explanation* for why
we gravitate. This is additionally confirmed by the fact that the 10m/s2
assigned to G is assigned by observations, not by any law. As far as I
remember from my school experiments we all used to get figures varying
around the 9.9 mark - and, like scientists, we then had to get together and
take an average or set some consensus figure as to what G is. It really is
as imprecise as that.
Elephant
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