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

Deadinside

Member
Joined
Mar 4, 2020
Messages
96
Gender
Undisclosed
HSC
N/A
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
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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.
 

Deadinside

Member
Joined
Mar 4, 2020
Messages
96
Gender
Undisclosed
HSC
N/A
so the questions are the same? And it doesn’t matter whether you add it or just use the value?
 

Drdusk

Moderator
Moderator
Joined
Feb 24, 2017
Messages
2,025
Location
a VM
Gender
Male
HSC
2018
Uni Grad
2023
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.
 

Deadinside

Member
Joined
Mar 4, 2020
Messages
96
Gender
Undisclosed
HSC
N/A
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
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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.
 

Deadinside

Member
Joined
Mar 4, 2020
Messages
96
Gender
Undisclosed
HSC
N/A
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
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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.
 

Deadinside

Member
Joined
Mar 4, 2020
Messages
96
Gender
Undisclosed
HSC
N/A
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
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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.
 
Last edited:

idkkdi

Well-Known Member
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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

Moderator
Moderator
Joined
Feb 24, 2017
Messages
2,025
Location
a VM
Gender
Male
HSC
2018
Uni Grad
2023
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
Joined
Aug 2, 2019
Messages
2,454
Gender
Male
HSC
2021
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.
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top