Michael Volk, age 10, of Gary, Indiana forhis question:
Why does Earth have more gravity than other planets?
How much you weigh depends upon the weight of the planet Earth. This is called surface gravity, which is the weight of an object on the surface of a planet. It works because gravity is a two way pulling force between your planet and your body. It so happens that, for its size, our Earth is the weightiest of the nine planets. So you might expect its surface gravity to be the strongest. But because of some very complicated reasons, this is not so.
On the surface of the earth, a pound of sugar weighs one pound and a 100 pound boy weighs one hundred pounds. Naturally. On the surface of faraway Neptune, that same boy would weigh 112 pounds and on dazzling Saturn he would weigh 117 pounds. On giant Jupiter, a 100 pound earthling would weigh a thumping 264 pounds.
Since weight is related to surface gravity, obviously some of the other planets have more gravity than the earth has. Actually, this is a very complicated story and we need three special words to understand it. These words are volume, mass and density. Together, they explain why surface gravity is different on different planets.
When scientists talk about the size of a planet, they use the word volume. Volume is the amount of space an object fills. The volume of the earth fills a sphere about 8,000 miles wide. Neptune's volume is 60 times bigger, Saturn's volume is 734 times greater than the earth's. If giant Jupiter were a hollow ball it would have enough space to swallow 1,312 earth sized planets.
Scientists refer to a planet's weight as mass, which is almost though not quite the same. Mass is the amount of matter packed into a volume. As we know, a sack of coal weighs more than a sack of feathers. This is so because the coal is denser. Its molecules of matter are more densely crowded together. So for its size, it weighs more than feathers.
A certain quota of gravity is a built in part of every speck of matter. A planet's gravity depends upon its mass, its density and its volume. And in mass, volume and density, no two planets are alike. Hence, on each one of them the surface gravity must be different. For its size, the earth is the densest and the most massive. But because its volume rates as one of the smallish planets, its surface gravity is not the greatest.
It helps to compare a planet's density to the weight of an equal amount of water. For example, the earth weighs 5 times more than an earth sized ball of water. Its density is 5.5. Giant Jupiter's density is only 1.3. But, though made of such light weight materials, there is so much of it that its total mass is more than 318 times greater than the earth's. Jupiter's surface gravity is more than 2,11 times stronger than the earth's because its volume and total mass are so much greater.
Even though the big planets are made of lighter materials, their surface gravity is stronger because their huge volumes make them more massive. Naturally the other smaller planets also have their quotas of gravity. For example, Mars is smaller and less massive than the earth. Its surface gravity is therefore much less. If a 100 pound earth boy landed on Mars, his weight would be reduced to 38 pounds.