In general, the force of gravity between two objects is:
F = G*M*m/r^2
Where M is the mass of one object, m the mass of the other object, r the
distance between them, and G a universal gravitation constant given as G =
6.673*10^-11 N m^2/kg^2.
Near the earth's surface where r=6400 km, and the Mass of the earth=6*10^24
kg, you can set:
G*M/r^2 = (6.673*10^-11 N m^2/kg^2) * (6*10^24 kg) / (6.4*10^6 m)^2 =
9.8 m/s^2
So you set g = G*M/r^2 = 9.8 m/s^2 and just write
F = mg where g = 9.8 m/s^2
It works anywhere, including the moon, you just get a different "g" when you
plug in a different mass of the planet and radius at the planet's surface.
End of sermon,
Nat
> I thought that gravity on the moon is 1/6 of that on earth,
>
> Han
>
> ----- Original Message -----
> From: "Joseph B. Reid" <[EMAIL PROTECTED]>
> To: "U.S. Metric Association" <[EMAIL PROTECTED]>
> Sent: Saturday, February 17, 2001 1:30 AM
> Subject: [USMA:11059] Re: Mass and Force
>
>
> > In USMA 11043 Gene Mechtly wrote:
> >
> > >On Thu, 15 Feb 2001 [EMAIL PROTECTED] wrote:
> > >> ...
> > >> ... on the moon 1 lbm = 0.16 lbf (appr)...
> > >
> > > Sorry, Han. That is pure garbage. Mass is not equal to force
> > >by *any* unitless multiplication factor, exactly or even approximately.
> > > The relationship is f=m.a; Newton's Second Law in most simple
> > >form. You forgot the acceleration!
> > >
> > >>...while the natural constant 9.8 m/s2 is universally the same.
> > > Wrong again, Han; g is a *variable* not a constant!
> > > The constant 9.80665 m/s2 is a constant invented by humans
> > >(not a natural constant) to lock "gravitational" units to "absolute"
> > >units, unambiguously, by Newton's Second Law.
> > >
> > >> In the Technical system we had 1 kgm = 1 kgf at sea level, but on the
> moon,
> > >> again, 1 kgm = 0.16 kgf (appr).
> > > Still more garbage, Han.
> > > I'm sorry to be so blunt since this conflicts with my
> high regard
> > >for your great contributions to the advancement of SI.
> > >
> > >Gene.
> >
> >
> > I quite agree with what Gene wrote. I think Han really meant
> that at sea
> > level at 45° latitude kilogram-mass and kilogram-force are equal
> > numerically by definition. But on the moon a gravitational
> force of x kgf
> > = 0,16x kgm.
> >
> >
> >
> > Joseph B. Reid
> > 17 Glebe Road West
> > Toronto M5P 1C8 Tel. 416 486-6071
> >
> >
>
>
>
