Universal Gravitational Constant help!

[axp107]

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Everytime I see a passage on PS relating to Space/Spaceships/Orbits/Planets, I simply freak out. Can anyone offer general tips and things to remember when doing passages on space/spaceships.

1) F = GMm/r^2

Is big M always the mass of the earth.. or is it the Mass of "any" central planet that other planets are orbiting around

2) I REALLY don't have a conceptual understanding of space and gravity on other planets in general..

Is gravity in "space" = to 0? why else do you float... but on the moon for example, there is gravity right?

3) Take a look at this question...

How much would a 100kg man weigh on the moon?
If you are given the Mass, Radius, and Orbital radius (whatever that means) of the Earth and Moon..

Is big M in this case the mass of the moon?..... then what is "r".. is it the radius of the moon or the Orbital radius of the moon (whatever that means)

 

[streetlight]

Everytime I see a passage on PS relating to Space/Spaceships/Orbits/Planets, I simply freak out. Can anyone offer general tips and things to remember when doing passages on space/spaceships.

1) F = GMm/r^2

Is big M always the mass of the earth.. or is it the Mass of "any" central planet that other planets are orbiting around

2) I REALLY don't have a conceptual understanding of space and gravity on other planets in general..

Is gravity in "space" = to 0? why else do you float... but on the moon for example, there is gravity right?

3) Take a look at this question...

How much would a 100kg man weigh on the moon?
If you are given the Mass, Radius, and Orbital radius (whatever that means) of the Earth and Moon..

Is big M in this case the mass of the moon?..... then what is "r".. is it the radius of the moon or the Orbital radius of the moon (whatever that means)

F = ma = mg
= m (GM/r^2)
= (100)(G)(Mmoon)/(radius of moon)^2

That's right...i think?

 

[axp107]

Nvm.. I had figure that out..

but its mainly the concepts about outer space that I don't under=stand.

 

[gridiron]

Everytime I see a passage on PS relating to Space/Spaceships/Orbits/Planets, I simply freak out. Can anyone offer general tips and things to remember when doing passages on space/spaceships.

1) F = GMm/r^2

Is big M always the mass of the earth.. or is it the Mass of "any" central planet that other planets are orbiting around

2) I REALLY don't have a conceptual understanding of space and gravity on other planets in general..

Is gravity in "space" = to 0? why else do you float... but on the moon for example, there is gravity right?

3) Take a look at this question...

How much would a 100kg man weigh on the moon?
If you are given the Mass, Radius, and Orbital radius (whatever that means) of the Earth and Moon..

Is big M in this case the mass of the moon?..... then what is "r".. is it the radius of the moon or the Orbital radius of the moon (whatever that means)

Hey! Newton's law of gravitation is based on the fact that all objects exert some attractive force of gravitation on one another. G is the constant of proportionality. The equation you have above is the force of gravitation attraction of an object (m) on earth to earth (M)--this isn't always true as it can explain the gravitational interaction between any planetary body and object. M isn't always the mass of the earth, but if you are dealing with a passage in which the planetary body is earth it is convention to assign M the mass of the planetary body. Thus, M is dependent on the planetary body in question. It doesn't have to be a planetary body it just depends on the passage. If you are dealing with two objects you usually assign m1 and m2 for the mass of object 1 and 2.

There is gravity on the moon but the value is smaller than earth--the reason why you float. You have two equations to describe the force of gravity acting on a object of mass m---the equation you have above and F=mg. Those two equations mean the same. When you set them equal to one another, what you solve for is the force of gravity on a planetary body of mass M and radius r. To solve for gravity on another planetary body of which you know the dimensions relative to earth--you must know the value of gravity on something--you take a ratio of the gravity on earth to the ratio of the gravity on the other body whose dimensions relative to earth you know. Let me explain:

Fg on earth = GMe/re^2 (1) e=earth

You know that equals to 9.8. To find the gravity on moon given the dimensions of the moon relative to earth just take a ratio of (1) to the dimensions of the moon relative to earth. You will be left with some number either a integer or fraction and you multiply that by 9.8. I hope this helps and good .