by Mark Hughes
You are shrinking right now. Every day you are taller in the morning than you are at night. In fact, at the end of each day you are about 1/2 inch (1.25 centimeters) shorter than when you got up that morning. If you don't believe it, have someone measure you when you get up in the morning and then again before you go to bed.
This isn't because your heavy backpack has weighed you down. It happens because of a force called gravity. As you walk around during the day, gravity is pulling you down, or more correctly, toward the center of Earth. Lying down to sleep at night gives your spine a chance to stretch back to your full height.
Pulling Us Down
You might be wondering why gravity just shrinks you a little instead of flattening you like a pancake. The reason is that gravity's strength depends on an object's mass. Mass means how much matter—or stuff—an object has inside it. The Moon is much smaller than Earth, which means that the Moon's gravity is less than Earth's. The gravity on the Moon is actually about one-sixth of the gravity on the Earth. Watch a video of astronauts on the Moon, and you can see them bouncing around in ways that would be impossible at home. If you did want to be the size of a pancake, get on a ship to Jupiter or the Sun; both have lots of mass, which means their gravitational pull is pretty intense.
Does size equal mass?
Keep in mind that size does not mean bigger mass. Imagine two balls that are about the same size, like a soccer ball and a bowling ball. Will they have the same mass? No.
An easy way to think about mass is to consider how much matter or "stuff" you can find inside the object. The amount of matter in the object will affect how heavy it is, meaning gravity has plenty of matter to grab onto. In the case of the soccer ball, air molecules make up most of its interior. The outside of the ball looks solid, but cut it in half and the ball will look like an empty shell. Whereas the inside of a bowling ball is usually completely solid, so if you cut it in half you will end up with two solid pieces. The bowling ball has more matter inside it, making it denser than the soccer ball. Therefore, its mass will be greater than the soccer ball.
The same thing can be seen in the solar system. Take Saturn and Earth for example. Physically, Saturn is much larger than Earth. So, in this case imagine Saturn as the soccer ball and the Earth as a baseball. It is easy to see the size difference between the soccer ball and baseball. Cut the soccer ball in half and you see its shell and hollow interior. Saturn is very much like that, since the inside of the planet does not have rock but is instead made up of the gases hydrogen and helium. Slice the baseball in half and you will see a core made of cork surrounded by layers of rubber. Like the baseball, the Earth is not a hollow core but has layers of air, water, and rock wrapping around a core of iron. If Earth and Saturn were the same physical size, then the Earth would have more mass than Saturn. However, since the physical size of Saturn is nearly 10 times larger than the Earth, Saturn's overall mass is bigger. Earth, however, is more dense than Saturn.
Gravity is Never Pushy
Gravity always pulls, it never pushes. For instance, Earth pulls on the Moon, but the Moon also pulls on the Earth. Imagine a giant playing a game of tug-of-war with an elf. You would think the giant could yank the cord, pull over the elf, and win the game. Not so fast, because the elf also has mass and is strong enough to resist the giant but does not have enough strength to pull away completely. This is just like the relationship between Earth and the Moon, the Sun and the planets, and anything else with mass.
While Earth keeps the Moon in orbit, the Moon does its own pulling. You can see the effect of the Moon on Earth in the ocean's tides. On every coastline of the planet, you can watch the level of the water rise and fall; this is due to the Moon's pull on Earth.
Sir Isaac's Observation
The first person to realize something strange was going on when objects fell was Sir Isaac Newton. He lived from 1643 to 1727 and was a very smart man. Newton watched the world around him and began to see patterns in nature. One pattern became his theory of gravity. The word gravity comes from an older word—gravitas—which means "heavy". Newton noticed that gravity is all around us, between any two objects no matter their size.
For instance, Earth is very large compared to your computer and your backpack. Keep in mind that your computer and backpack both have mass and create their own gravity, but Earth is much bigger. Earth is so big that it easily overpowers the gravity made by your items. This means that any object that falls on Earth, no matter its size, will fall at the same speed. Drop your computer and backpack off a bridge at the same time and both will accelerate, which means speed up, at the same rate. The only thing that would slow them down is air resistance. You can feel air resistance if you open a window in a moving car and stick out your hand. The speed of the car will determine the strength of the air hitting—resisting—your hand.
Gravity is all around us. It keeps us safely on Earth's surface instead of floating out into airless space. Gravity keeps the Moon around Earth, and keeps Earth moving around the Sun. Life on Earth would not be the same without it.
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