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Gravity, the Earth’s Safeguard


Mankind lives under the influence of gravity. Not only apples from the trees, but all objects fall down from above. We try to jump high up in the air, but we don’t stay there even for a few seconds; we fall right back down, far from reaching the sky. We don’t feel it because we always live under the influence of gravity, but we are always caught up by gravity. Even at this moment, gravity constantly works for the trees, chairs, air, etc.

Mankind has not been sparing endless efforts to get away from gravity for a long time. In 1783, the Montgolfier brothers travelled in the sky in a hot-air balloon. About 100 years later, the first powered airplanes were invented by the Wright brothers. Now 100 years have passed again, and modern civilization which has improved even more has invented supersonic or gigantic airplanes. Mankind who took off the ground started dreaming to travel in space which is completely out of the earth’s gravity. Their desire resulted in designing a rocket and making a manned spaceship and the International Space Station.

What is gravity?

What is gravity? Gravity is force that pulls objects to the center of the earth. To know about gravity, we need to understand the law of universal gravitation. The universal gravitation is gravitation that works between all objects of the universe. The universal gravitation is proportional to mass, and inversely proportional to the square of the distance, which means its strength increases the bigger the mass is and the closer it is.

Let’s think about the apple tree which Newton got the law of universal gravitation from. The apples pull the earth, and the earth pulls the apples. The strength of the earth pulling the apples and the strength of the apples pulling the earth are the same according to the law of action and reaction. However, since the mass of an apple is much smaller than that of the earth, the apples fall to the earth.

The universal gravitation doesn’t only work between the earth and the apples, but between the earth and the moon, between the sun and the earth, and between galaxies. Newton conceived a concept of universal gravitation by expanding the gravity of the earth to the universe. Universal gravitation and gravity are basically the same concept because universal gravitation just added meaning that it is a general law of the universe.

We call the size of gravity weight. People often get confused between mass and weight. However, mass is a value that does not change, while weight changes, depending on gravity that applies to the object. For example, a man who weighs 60 kg [132 lb] on the earth weighs 10 kg [22 lb] on the moon, because the gravity of the moon is 1/6 of the earth.

Weightless space, which is so different from the earth

Most people think that it is gravity-free in space. However, there are many celestial bodies in space and there is gravity everywhere. Therefore, zero gravity, or weightlessness, is not a gravity-free state, but a state with very weak gravity because it is far from celestial bodies or a state where one doesn’t feel gravity because many forces are offset.

In space where there is zero gravity, an object floats around instead of falling to one direction, and our bodies float though we don’t make effort, and we can easily spin around just by giving a little effort. Liquid floats around as round drops by surface tension. So we can’t drink water without using a straw. As there is no convection currents, candles easily go out, and it is not easy to boil water or oil by using fire. In space, we cannot make noodles or fries which are common foods on the earth.

In a state of weightlessness, many changes occur in the body. The spacemen get about 3% taller than they are on the earth. The National Aeronautics and Space Administration [NASA] made spacesuits to fit the heights of the spacemen according to their heights measured on the earth for their first spaceflight. However, as their life on space continued, their spacesuits became too small for them to the point where they couldn’t freely move. It was because they got taller as there was zero gravity which had impact on the spine.

On top of that, the waist becomes thinner as the internal organs go up, and the muscles become weak as there is no need to beat gravity. As calcium comes out of the bones, there is more danger of osteoporosis, and about 1% of calcium of the entire bones is lost while spending about a month in space.

The faces of the spacemen who are in space change to be very round. It’s called “moon-face.” One of the science tasks of the Korean Astronaut Program was “research on the changes of the spacemen’s faces.” As there is zero gravity, blood spreads evenly, and blood moves up and the face gets swollen. On the other hand, the legs become thin as the legs lose much blood.

In a state of zero gravity, the production of the red blood cells decreases that the blood becomes watery and the blood’s supply of oxygen drops dramatically. Moreover, there is no difference between up and down, and so they come to lose sense of direction and suffer from space sickness. Those who go out to space undergo a strict training on the earth to be prepared for this situation, but they sometimes suffer from side effects such as walking problem even after they return to the earth.

Importance of gravity

The movie Gravity describes space realistically. This movie lets the viewers feel the importance of gravity instead of a sense of freedom from gravity. When the characters of the movie are carrying out their duties in space, the spaceship they took gets destroyed by space debris1 that rushes toward it at a great speed. Then the main character, who is left alone in space where there is zero gravity to hold her body or air to breath in and out, fights against zero gravity and struggles to return to the earth.

1. Space debris: the collection of defunct man-made objects in space. They are of many kinds such as old or broken satellites, tools that spacemen lose while working in space, and even a piece of paint. The space debris rotates around the earth in a certain orbit. Since it is moving at a great speed of 7 km/s [4.3 mi/s], even one little fragment has a tremendous power.

On the earth, an object stops immediately even if a force is applied to it. It is because friction is made as the object collides with air and meets the surface by gravity. On the contrary, in space, we cannot do even simple things such as changing the direction of the body or moving or stopping. Space is a weightless world where you cannot feel gravity contrary to the earth, and it is close to vacuum where there is almost no substance. For this reason, once an object starts moving with a force in the space, it tries to keep its state forever. In other words, once you are bounced off toward the dark space, you will travel space forever and never be able to come back.

When viewing the whole universe, we can realize the importance of gravity all the more. Just as the moon is held by the earth’s gravity, the planets in the solar system are orbiting around the sun, held by the sun’s gravity. Gravity is playing the role of holding all celestial bodies in their places. If gravity suddenly disappears, the moon that revolves around the earth will leave the earth, and all the atmosphere that is held by gravity will disappear as well. Starting from the planets close to the sun, the planets will leave the influence of the sun’s gravity and be bounced off to the faraway space, and galaxies will also leave their orbits. If gravity, which holds the whole universe, disappears, the universe will expand quickly and everything will be scattered in the end.

The world without gravity is not a space of freedom which mankind has dreamed of, but it is an extremely dangerous space where men cannot survive. Gravity doesn’t bind men, but it is a safeguard of the spaceship called Earth, which travels in space at the speed of 30 km/s [18.6 miles/s]. We can move freely according to our will on the earth without flying away into space, thanks to gravity. Nevertheless, mankind still dreams of perfect freedom, not bound by gravity, and of the infinite universe, standing here on the earth.

Science Teachers’ Meeting, 101 Science Questions That Even Science Teachers Wonder (in Korean), Book Mentor, 2010
Go Ho-gwan, Our Destiny in a World Without Gravity (in Korean), Science Donga (March, 2013)
Oh Ga-hee, Thank the Earth’s Gravity! (in Korean), Science Donga (November, 2013)
Kim Hyeong-ja, Why Do Physicists Risk Their Necks for Gravity? (in Korean), Weekly Chosun (November 5, 2013)