# Fg between earth and the moon radius? (1 Viewer)

##### Member
i got two different answers from two different textbooks and I’m so confused. Is the force exerted by earth on the moon the same as Fg between earth and the moon? Cause for the force exerted by earth on the moon the radius was 3.84*10^20 but for Fg between earth and moon i got 3.9 x 10^8 for the radius... they seem similar but i had to add the distance between earth and moon with radius of earth and radius of moon which is 3.84x10^20.. can someone pls explain this?

#### idkkdi

##### Well-Known Member
i got two different answers from two different textbooks and I’m so confused. Is the force exerted by earth on the moon the same as Fg between earth and the moon? Cause for the force exerted by earth on the moon the radius was 3.84*10^20 but for Fg between earth and moon i got 3.9 x 10^8 for the radius... they seem similar but i had to add the distance between earth and moon with radius of earth and radius of moon which is 3.84x10^20.. can someone pls explain this?
10^8 is negligible in an addition with 10^20.

#### tito981

##### Member
10^8 is a lot (stressing the word a lot) smaller than 10^20, so adding these numbers will be as useful as having just 10^20.

##### Member
so the questions are the same? And it doesn’t matter whether you add it or just use the value?

#### Drdusk

##### π
Moderator
i got two different answers from two different textbooks and I’m so confused. Is the force exerted by earth on the moon the same as Fg between earth and the moon? Cause for the force exerted by earth on the moon the radius was 3.84*10^20 but for Fg between earth and moon i got 3.9 x 10^8 for the radius... they seem similar but i had to add the distance between earth and moon with radius of earth and radius of moon which is 3.84x10^20.. can someone pls explain this?
Your wording is very confusing. I really don’t understand what you’re asking.

##### Member
Your wording is very confusing. I really don’t understand what you’re asking.
if i have to find the force exerted by earth on the moon, will i get the same answer as Fg between earth and the moon?

#### idkkdi

##### Well-Known Member
if i have to find the force exerted by earth on the moon, will i get the same answer as Fg between earth and the moon?
you have conceptually misunderstood something.

you can only have a force acting on an object. There is no such thing as a force, without an object, between the earth and the moon, but there is a field. A field is measured in terms of acceleration.

Now for a field between earth and the moon, you would simply do a summation of the fields produced by the earth and the moon.

##### Member
you have conceptually misunderstood something.

you can only have a force acting on an object. There is no such thing as a force, without an object, between the earth and the moon, but there is a field. A field is measured in terms of acceleration.

Now for a field between earth and the moon, you would simply do a summation of the fields produced by the earth and the moon.
Oh okay thank youuu. I got confused because the teacher just wrote Fg between earth and the moon

#### idkkdi

##### Well-Known Member
Oh okay thank youuu. I got confused because the teacher just wrote Fg between earth and the moon
if your teacher wrote Fg, that is not notation for field strength, in which case, i have no idea how to help you.

##### Member
if your teacher wrote Fg, that is not notation for field strength, in which case, i have no idea how to help you.
So the question is wrong and should be g because he added the distance and the radius of the two planets?

#### idkkdi

##### Well-Known Member
So the question is wrong and should be g because he added the distance and the radius of the two planets?
if it was fg then it is as I said before.

Fg = Gm1m2/r^2

The distance between the centre of two planets, which is technically what r is, is equal to the radius of earth + distance between earth and moon's closest edges + radius of moon. Here is why your textbook just ignores radius of earth and radius of the moon. Adding 3.9 x 10^8 to 3.84 x 10^20 is redundant because that just makes 384000000000390000000, which you just write as 3.84 x 10^20.

here's where me and dusky got confused. Fg between the earth and moon can be slightly ambiguous in that you normally don't describe forces as between x and y but rather as x on y or y on x. in the case where you describe a force as between two things, direction is ambiguous and not usually indicated.

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• #### idkkdi

##### Well-Known Member
if it was fg then it is as I said before.

Fg = Gm1m2/r

The distance between the centre of two planets, which is technically what r is, is equal to the radius of earth + distance between earth and moon's closest edges + radius of moon. Here is why your textbook just ignores radius of earth and radius of the moon. Adding 3.9 x 10^8 to 3.84 x 10^20 is redundant because that just makes 384000000000390000000, which you just write as 3.84 x 10^20.

here's where me and dusky got confused. Fg between the earth and moon can be slightly ambiguous in that you normally don't describe forces as between x and y but rather as x on y or y on x. in the case where you describe a force as between two things, direction is ambiguous and not usually indicated.
actually, on second thought, you should probably write direction as 'towards each other'.

#### Drdusk

##### π
Moderator
if it was fg then it is as I said before.

Fg = Gm1m2/r

The distance between the centre of two planets, which is technically what r is, is equal to the radius of earth + distance between earth and moon's closest edges + radius of moon. Here is why your textbook just ignores radius of earth and radius of the moon. Adding 3.9 x 10^8 to 3.84 x 10^20 is redundant because that just makes 384000000000390000000, which you just write as 3.84 x 10^20.

here's where me and dusky got confused. Fg between the earth and moon can be slightly ambiguous in that you normally don't describe forces as between x and y but rather as x on y or y on x. in the case where you describe a force as between two things, direction is ambiguous and not usually indicated.
Yeah I was literally scratching my head thinking maybe I was cooked or something from studying.

EDIT: My confusion was something completely different. I didn't get what question OP was asking

#### idkkdi

##### Well-Known Member
if it was fg then it is as I said before.

Fg = Gm1m2/r^2

The distance between the centre of two planets, which is technically what r is, is equal to the radius of earth + distance between earth and moon's closest edges + radius of moon. Here is why your textbook just ignores radius of earth and radius of the moon. Adding 3.9 x 10^8 to 3.84 x 10^20 is redundant because that just makes 384000000000390000000, which you just write as 3.84 x 10^20.

here's where me and dusky got confused. Fg between the earth and moon can be slightly ambiguous in that you normally don't describe forces as between x and y but rather as x on y or y on x. in the case where you describe a force as between two things, direction is ambiguous and not usually indicated.
wrote r instead of r^2 the first time mb.