# Thread: how do i calculate gravity? :(

1. Newbie
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Sep 2014
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## how do i calculate gravity? :(

if i wanted to calculate the force of gravity using the formula Fg = G (m1*m2)/(d^2)
in order to find out the pull that, say, Saturn has on a person at the earths surface, would i use the mass of the earth as M1 or the mass or the person as m1 or would I just calculate distance from the surface of the earth rather then distance from the centre of the earth? Im guessing I would have to do two calculations to account for maximum distance and minimum distance and display the answer as a range of values?
thanks so much for any answers.... theyre a big help
r

2. In your calculatio, M1 is the mass of the person, M2 is the mass of Saturn.

The distance d will vary as the earth turns, as you point out, but will also vary through the year, as earth and Saturn can be on opposite sides of the sun. Use a spreadsheet, and calculate all the range of values.

3. Newbie
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Hi, thanks so much for the reply... does this mean that the mass of the earth itself is irrelevant in the calculation? By what youre saying, the gravitational pull of the earth would act on the person, but not alter the actual quantifiable gravitational pull of saturn on the person except in practical terms... ie . that it applying a far greater counteracting forcr. Is this right?

Originally Posted by grapes
In your calculatio, M1 is the mass of the person, M2 is the mass of Saturn.

The distance d will vary as the earth turns, as you point out, but will also vary through the year, as earth and Saturn can be on opposite sides of the sun. Use a spreadsheet, and calculate all the range of values.

4. Member
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Aug 2014
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Originally Posted by grapes
In your calculatio, M1 is the mass of the person, M2 is the mass of Saturn.
Be careful though - it's very important not to reverse the two.

5. Originally Posted by rckelly
... does this mean that the mass of the earth itself is irrelevant in the calculation? By what youre saying, the gravitational pull of the earth would act on the person, but not alter the actual quantifiable gravitational pull of saturn on the person except in practical terms... ie . that it applying a far greater counteracting forcr. Is this right?
That is my question too. I guess it depends on what you are looking for.

My thought would be to compute the pull of gravity between the person and the Earth (although; we already know that), and then subtract the pull of the person to Saturn. (assuming Saturn is directly overhead of the person)

6. Originally Posted by rckelly
Hi, thanks so much for the reply... does this mean that the mass of the earth itself is irrelevant in the calculation? By what youre saying, the gravitational pull of the earth would act on the person, but not alter the actual quantifiable gravitational pull of saturn on the person except in practical terms... ie . that it applying a far greater counteracting forcr. Is this right?
Depends upon the configuration of the planets. Sometimes it is counteracting, sometimes it is augmenting, sometimes the forces act at right angles to each other.
Originally Posted by OnePlus
Originally Posted by grapes
In your calculatio, M1 is the mass of the person, M2 is the mass of Saturn.
Be careful though - it's very important not to reverse the two.
M1 and M2 are symmetrical in the formula, so it doesn't make a difference which is which. The calculated force is the force that Saturn pulls on the person, and the force that the person pulls on Saturn.

7. Originally Posted by rckelly
Hi, thanks so much for the reply... does this mean that the mass of the earth itself is irrelevant in the calculation? By what youre saying, the gravitational pull of the earth would act on the person, but not alter the actual quantifiable gravitational pull of saturn on the person except in practical terms... ie . that it applying a far greater counteracting forcr. Is this right?
Grapes already answered the second part of this (that the force from Earth isn't always counteracting the pull from Saturn). But what he left implied, I'll state explicitly: yes, the size of the gravitational force you feel from Saturn would be the same if you were floating in space rather than standing on the Earth, as long as your distance from Saturn was the same.

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