Gravity

A force which pulls two objects with mass toward each other.

Gravity represents the attraction between objects. All objects with mass are affected by gravity. Gravity acts like a magnet — pulling objects together. What causes gravity is not really known.

The Earth has gravity. Gravity holds everything close to this planet. Trees, water, animals, buildings, and the air we breathe are all held here by gravity. All of the planets, their moons, and the stars in the universe have gravity. Even our own bodies have gravity. The Earth's gravity is far stronger than our own so we don't notice the gravity our bodies possess.

Gravity is affected by the size and proximity of objects. The Earth and the moon have a stronger pull on each other than the Earth and, say . . . , Jupiter because the Earth and moon are closer to one another. Likewise, Earth exerts a stronger pull than the moon because it is larger, so there is more pull on our bodies here on the Earth than we would feel on the moon if we were astronauts visiting there. We don't actually "feel" gravity. We only feel the effects of trying to overcome it by jumping or falling.

Mass is the "stuff" that matter is made of. People often confuse mass with weight. But weight is actually the result of gravity pulling on the mass. Gravity's pull on an object decreases with distance from it. So, a climber on top of Mt. Everest weights a little less than at sea level. If a spaceship travels far enough from Earth, it will eventually escape Earth's pull completely.

We measure mass in grams. We measure weight in ounces and pounds. Your mass would stay the same if you could travel from planet to planet, but your weight would vary depending on how the gravity of that planet pulls on you. Check out how much you would weigh on another planet.

When one space object revolves around another, it is referred to as an orbit. The Earth orbits the sun. Our moon orbits the Earth. Many of the other planets in our solar system also have moons that orbit them. It is gravity that holds the Moon in orbit around the Earth, and gravity that enables the Sun to hold onto its family of planets.

Earth now has many man-made objects orbiting it. Satellites are placed in orbit around the Earth to assist with telephone calls, television broadcasts, and other forms of communication. Astronauts have sometimes left debris orbiting our earth too. Gloves, tools, and other junk continuously orbit our earth, floating for an unknown period of time in space.

Two forces — centripetal and centrifugal — work to keep the planets and their moons in orbit around each other.

If a person drops a 10-pound ball and a 5-pound ball off a building at the same time, which one will hit the ground first? Will gravity pull harder on the 10-pound ball?

This was the subject of a famous story about the Italian scientist Galileo Galilei, who is said to have tried dropping two objects from the Leaning Tower of Pisa to find the answer to this question. You might be surprised to find out that the two objects hit the ground at almost the same time.

Gravity works the same on all objects unless wind resistance gets in the way. So if one of those balls had been attached to a parachute, it would have slowed the ball down. Amazing science! Read more about Galileo's story.

How strong can gravity get? Imagine gravity pulling on everything and not letting it escape — not even light! This is the essence of a black hole. The gravity in a black hole pulls everything back in toward its center which may be as small as a single atom. This makes a black hole invisible because even light is unable to break away from its effects. Scientists believe there may be millions of black holes in the Milky Way Galaxy.

So how do astronauts float in space if the pull of gravity is so strong? Astronauts don't actually float. They are being pulled by the Earth's gravity just the same as the rest of us.

But they are also orbiting the Earth, or moving sideways. This is known as centrifugal force. This sideways movement actually is pulling them away from the Earth at the same time that the Earth is pulling them down, so it appears as if they are floating.

Microgravity — is that like small gravity? Not really. Let's explain it this way. If something is falling near the earth, or normal gravity, it accelerates at 32.2 feet per second per second or 32.2 ft / sec².

Objects in space not only fall toward the Earth but also sideways or around the Earth. So if you see a video of an astronaut using a tool and that tool seems to float, it is because the astronaut and the wrench are both falling at the same time. This is a free fall or microgravity. Visit NASA's site for additional information on this concept.

The Earth's tides are caused by the moon's gravitational pull on the oceans. Tides are the rise and fall of the ocean level as related to the shoreline. High tides occur when the Earth and the moon are facing each other, and the moon is exerting its greatest pull on the ocean waters.

To a lesser degree, the moon also pulls on the rivers, lakes, and land. Find out more about tides by visiting this page on How The Moon Affects Ocean Tides.

When an object floats on top of the water, gravity is still in play, pulling objects toward the earth. For an object to float, it must displace enough water to make up the same mass as the object itself. Once that happens, the remaining mass then sits on top of the water.

For example: if a boat has a mass of 45 grams, it will displace 45 grams of water, and if that has happened before the whole thing has sunk below the surface level of the water, the boat floats. A ball of clay may sink right away, but if you flatten the clay out into the shape of a raft, it moves aside water equal to its mass and floats.

Sir Isaac Newton (1642–1727) is the scientist credited with explaining gravity. According to the story, Newton was in his garden when he saw an apple fall from a tree. He began to wonder why the apple had fallen to the earth instead of just floating away. He came up with the idea that some unseen force must attract the apple towards the Earth. He named this force "gravity."

You would think that gravity would have been a well-known concept before Newton. After all, most of us played with gravity when we were toddlers in high chairs, dropping our spoons on the floor just to make mom come and get them. And no doubt kids from centuries ago played similar games with their mothers. But it wasn't until Newton published his ideas in a book in 1687 that the scientific world understood how gravity works as a universal physical law.

In addition to his work on gravity, Newton is known for his Three Laws of Motion, which explain the relationship between an object and the forces acting on it and how it moves in response to those forces.

Balloons often seem to defy gravity. We've all lost a balloon when we have accidentally let it go and watched it drift away in the sky. These balloons are filled with helium. Helium is a gas that is lighter than the other gases that make up our air. Because this gas is so light, it tries to float to the top of our atmosphere and it takes our balloon with it. The poor balloon is not strong enough to hold on to this helium for the ride to the top and so it ends up popping long before its journey is done.

Hot air balloons work on similar science. Hot air naturally tends to rise upward because it is less dense than cooler air. A hot air balloon has a heat source that fills the balloon's interior with warm air, allowing it to rise. To come down, the pilot allows the air to escape or to cool down, and the balloon descends.

Two forces working together keep the planets and their moons in orbit around each other. "Centripetal" and "centrifugal" are terms scientists use to explain these forces. They work together to keep objects moving at a steady speed in a circular path.

The word centripetal is from a Latin word meaning "towards the center." It means that the direction of force is inward, coming from the center. You might think of it like a weight tied to a string that you swing around your head. The force is coming from the tension in the string which is pulling on the weight from the center of the large circle you are creating with your motion. Another example of centripetal force is the moon orbiting around the earth, where the force comes from gravity pulling it towards the center.

Centrifugal means "fleeing from the center" in Latin. It is the outward push that a person in a car going around a curve feels toward the outside of the turn. Other examples are mud flying off of a spinning tire, or children being pushed outward on a merry-go-round as it spins. The force the children feel is centrifugal force pushing them outward.

Flying has been done by birds for millions of years. Yet flight is still an amazing science, as it seems to be about the ability of heavy objects to break away from gravity. For birds, it is all about the structure of their bones, wings, and feathers. For humans that don't have those structural positives, we have to design machinery that mimics the science that enables birds to fly.

Airplane wings are designed after bird wings with a special curve in an effort for air to "lift" the plane. As a wing moves through the air, the curve of the top of the wing speeds up the air as it flows across. This reduces the pressure of the air over the top of the wing. Air on the underside then pushes upward. This is known as Bernoulli's Principle, for the famous scientist Daniel Bernoulli, who determined that the higher the velocity of a liquid, the lower the pressure it exerts. This also applies to air and is what allows kites, airplanes, space shuttles, and even plastic discs to fly. It helps fans blow air and propellers to push boats through the water.

The gravity of the earth is intense enough to make escaping its pull a real challenge for scientists when it comes to heading out into space. A rocket or other craft must reach a very high speed, or velocity, in order to break away from the pull of the earth. Escape velocity is the minimum speed an object must attain in order to leave the gravitational pull of another body. In most cases, this is referring to the earth. That was the first body of mass that scientists could learn how to leave. Later on, it was the moon. Perhaps someday we will learn to leave other bodies in our solar system. For Earth, escape velocity is about 7 miles per second or 25,000 miles per hour.

Visit these other Science Trek websites to learn more about related topics.

• Astronauts
• Astronomy
• Flight
• Force and Motion
• Planets
• The Moon
• The Sun