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Q & A: Size Does Matter Sometimes

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Most recent answer: 10/22/2007
Does the amount of oxygen and gravitational force be determined by the planet’s siz?
- Anonymous
As Newton figured out, the force of gravity between two objects is given by F = G*M*m/(R*R), where G is a constant, M and m are the masses of the two objects, and R is the distance between the centers of the objects. You can use this formula to figure out the force on an object on the surface of any planet. If you plug in the objects mass for m, the planets mass for M and the radius of the planet for R, you will get the force. A common way to re-write this is F = mg, where g = G*M/(R*R), and happens to also be the acceleration of an object dropped near the planets surface (sometimes called the surface gravity).

If you go to a different planet, you will get a different answer value for "g". If the planet is bigger it will usually work out that g (which depends on the mass and the radius of the planet) will be bigger too. In other words, the gravity on the surface of a bigger planet is usually bigger than the gravity on the surface of a smaller planet.

Now, what about oxygen. This depends on two things. first of all there must be some way for oxygen to form in the first place (like plants, for example, or certain other chemical reactions). Second, the gravity of the planet has to be big enough to keep the oxygen atoms from just flying away from the surface of the planet into space. This may sound odd, but just think of oxygen (or any other gas) atoms as little baseballs. If you throw a baseball on the moon, it can go a lot farther than if you throw it on the earth, This is because the gravity on the surface of the moon is not as string as the gravity on the surface of the earth. If the moon were really small, you would be able to throw a baseball fast enough that it would never come back down. (This is called escape velocity). Its the same with gas atoms like oxygen. They all move pretty fast. If they are on a planet where the gravity is really weak, they would all eventually escape and there would be none left.


(published on 10/22/2007)

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