gravity math

ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au

You must be on a different planet:

radius in meters-: 6,392.42

That would make the circumference about 40,164.73 meters. I've certainly
driven well over 40km in a roughly straight line without circumnavigating
the globe!

"no way" wrote in message
...
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to
check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if
i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au

You must be on a different planet:

radius in meters-: 6,392.42

That would make the circumference about 40,164.73 meters. I've certainly
driven well over 40km in a roughly straight line without circumnavigating
the globe!

"no way" wrote in message
...
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to
check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if
i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au

"no way" writes:
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to check
against.

I have the Formula:
Gravity=GM/r^2

I've always seen this as the force between two objects M and m.
Force=GMm/r^2, you have two different masses and must find the product

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

Whoa! 6000 meters, that's about 4 miles for us old folks!
That's an awfully small earth. Maybe you meant 6392 kilometers?
That will make a difference of 10^3^2 in your result.

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Suppose for a moment I am 50 kilograms mass.

Force = 6.667*10^-11 newton M^2/kg^2 * 50 kg * 6*10^24 kg/(6.392*10^6 M)^2
which is 489 Newtons force pulling me against the planet.
(Everything on the right hand side except Newtons cancels)

And if we do not have M*m in the equation your result will have units
of Newtons/kg with the 50 removed from the above equation.

So, problem was you had crushed the entire mass of the planet into a
ball that was only a few miles in diameter. Same problem happens
when you crush a star down into a tiny ball, all that mass close to
you makes for a huge force.

I really hope I haven't made a mistake in any of this.

"no way" writes:
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to check
against.

I have the Formula:
Gravity=GM/r^2

I've always seen this as the force between two objects M and m.
Force=GMm/r^2, you have two different masses and must find the product

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

Whoa! 6000 meters, that's about 4 miles for us old folks!
That's an awfully small earth. Maybe you meant 6392 kilometers?
That will make a difference of 10^3^2 in your result.

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Suppose for a moment I am 50 kilograms mass.

Force = 6.667*10^-11 newton M^2/kg^2 * 50 kg * 6*10^24 kg/(6.392*10^6 M)^2
which is 489 Newtons force pulling me against the planet.
(Everything on the right hand side except Newtons cancels)

And if we do not have M*m in the equation your result will have units
of Newtons/kg with the 50 removed from the above equation.

So, problem was you had crushed the entire mass of the planet into a
ball that was only a few miles in diameter. Same problem happens
when you crush a star down into a tiny ball, all that mass close to
you makes for a huge force.

I really hope I haven't made a mistake in any of this.

Here's a quick check using Mathematica:

g = GravitationalConstant
(6.673*^-11*Meter^2*Newton)/Kilogram^2

m = EarthMass
5.9742*^24*Kilogram

r = EarthRadius
6378140*Meter

G = (g*m)/r^2
(9.799700137089136*Newton)/Kilogram

Convert[G, Meter/Second^2]
(9.799700137089136*Meter)/Second^2

(? Looks like the Radius may have been from the equator)

As a quck check:

AccelerationDueToGravity
(9.80665*Meter)/Second^2

Check your "Radius"

HTH
Dana


"no way" wrote in message
...
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to
check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if
i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au

Dana wrote:

Here's a quick check using Mathematica:

g = GravitationalConstant
(6.673*^-11*Meter^2*Newton)/Kilogram^2

[snip]

G = (g*m)/r^2
(9.799700137089136*Newton)/Kilogram

Convert[G, Meter/Second^2]
(9.799700137089136*Meter)/Second^2

Conventionally the gravitational constant is represented by a capital
G, while the standard acceleration of gravity at sea level on earth
is the small g. Switching them as you have done above is likely to
confuse others.

--
Odysseus

Dana wrote:

Here's a quick check using Mathematica:

g = GravitationalConstant
(6.673*^-11*Meter^2*Newton)/Kilogram^2

[snip]

G = (g*m)/r^2
(9.799700137089136*Newton)/Kilogram

Convert[G, Meter/Second^2]
(9.799700137089136*Meter)/Second^2

Conventionally the gravitational constant is represented by a capital
G, while the standard acceleration of gravity at sea level on earth
is the small g. Switching them as you have done above is likely to
confuse others.

--
Odysseus

Here's a quick check using Mathematica:

g = GravitationalConstant
(6.673*^-11*Meter^2*Newton)/Kilogram^2

m = EarthMass
5.9742*^24*Kilogram

r = EarthRadius
6378140*Meter

G = (g*m)/r^2
(9.799700137089136*Newton)/Kilogram

Convert[G, Meter/Second^2]
(9.799700137089136*Meter)/Second^2

(? Looks like the Radius may have been from the equator)

As a quck check:

AccelerationDueToGravity
(9.80665*Meter)/Second^2

Check your "Radius"

HTH
Dana


"no way" wrote in message
...
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to
check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if
i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au

"no way" wrote in message
...
ok = I'm trying to accurately compute the Surface gravity of a world.

I've gotten some figures for our solar system, so I have known data to
check
against.

I have the Formula:
Gravity=GM/r^2

Now, on Earth, it should be about 9.8 meters per second.

Earth
mass in kilograms: 5,974,000,000,000,000,000,000,000
radius in meters-: 6,392.42

NOW - Here's where I'm getting lost. G.
G is the Universal Gravitational Constant.

I have something he
G=6.667x10E-11 newton M^2/kg^2

ooooookaaaayy...
I'm lost. I know it there is a different way to compute it, depending if
i'm
using grams and centimeters, or kilograms and meters, but when I plug the
above into a spreadsheet, I get some HUGE number for the force of gravity
for Earth.

Anyone have any ideas on what I should do?

Thanks!

-Joel

traveller at dhimaging dot com dot au



Hi Joel,

In the southern hemisphere the radius of the earth is about 6.37*10^6 m.


--
Greg

(remove BALL from address to reply)