# When do you use G=F/m and when do you use F=(Mm)/(r^2)

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#1
case in point:
Attachment 973590
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2 months ago
#2
(Original post by Abzino16th)
case in point:
Attachment 973590
The attachment won’t load, and I think you also meant F=Gmm/r^2 as opposed to F=mg. Note the difference in capital and lower case g. Little g is the gravitational field strength, whereas big G is the gravitational constant. You use g=F/m when you want to find the field strength at a point, or find the weight a mass has. Newton’s law of gravitation is used to calculated the force of attraction between 2 objects, not the field strength (force per unit mass)
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#3
(Original post by TM3141)
The attachment won’t load, and I think you also meant F=Gmm/r^2 as opposed to F=mg. Note the difference in capital and lower case g. Little g is the gravitational field strength, whereas big G is the gravitational constant. You use g=F/m when you want to find the field strength at a point, or find the weight a mass has. Newton’s law of gravitation is used to calculated the force of attraction between 2 objects, not the field strength (force per unit mass)
Sorry about the attachment how about now, also typing is difficult for me so I’ll just write it for you:
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#4
(Original post by TM3141)
The attachment won’t load, and I think you also meant F=Gmm/r^2 as opposed to F=mg. Note the difference in capital and lower case g. Little g is the gravitational field strength, whereas big G is the gravitational constant. You use g=F/m when you want to find the field strength at a point, or find the weight a mass has. Newton’s law of gravitation is used to calculated the force of attraction between 2 objects, not the field strength (force per unit mass)
I’m referring mainly to questions 1b and 2a which equations do I use and how do I tell what to use, the answers I got don’t make sense
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2 months ago
#5
(Original post by Abzino16th)
I’m referring mainly to questions 1b and 2a which equations do I use and how do I tell what to use, the answers I got don’t make sense
Gravitational attraction is always given by F=Gm1m2/r^2 so you should use that equation. The other one is to define the field strength at a given point relative to the centre of mass
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#6
(Original post by TM3141)
Gravitational attraction is always given by F=Gm1m2/r^2 so you should use that equation. The other one is to define the field strength at a given point relative to the centre of mass
For question 4a I used newtons law however for b it looks as though that was the scenario I was meant to use it. Notice how it the masses are quoted under it?

I’m a little confused, I don’t think I’m doing this correctly. Here’s what I wrote anyways.
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2 months ago
#7
For question 4a I used newtons law however for b it looks as though that was the scenario I was meant to use it. Notice how it the masses are quoted under it?

I’m a little confused, I don’t think I’m doing this correctly. Here’s what I wrote anyways.
Part a looks correct. The only problem is the rocket mass is 1500kg not 1000kg. Then for part b it is just to realise the pull from the Earth is stronger so it is towardds the Earth and as both 'pulls' are acting in the same plane, you can just subtract the magnitudes from part a yes.
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