Gravity

Lesson Objectives

• I can define gravity.

• I can describe Earth’s gravity and its effects.

• I can define the relationship between gravity and weight.

Introduction

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Long, long ago, when the universe was still young, an incredible force caused dust and gas particles to pull together to form the objects in our solar system. From the smallest moon to our enormous sun, this force created not only our solar system, but all the solar systems in all the galaxies of the universe. The force is gravity.

Guided Learning

Defining Gravity

Gravity has traditionally been defined as a force of attraction between things that have mass. According to this conception of gravity, anything that has mass, no matter how small, exerts gravity on other matter. Gravity can act between objects that are not even touching. In fact, gravity can act over very long distances. However, the farther two objects are from each other, the weaker is the force of gravity between them. Less massive objects also have less gravity than more massive objects.

Earth’s Gravity

You are already very familiar with Earth’s gravity. It constantly pulls you toward the center of the planet. It prevents you and everything else on Earth from being flung out into space as the planet spins on its axis. It also pulls objects that are above the surface—from meteors to skydivers—down to the ground. Gravity between Earth and the moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth and the other planets moving around the much more massive sun.

Q: There is a force of gravity between Earth and you and also between you and all the objects around you. When you drop a paper clip, why doesn’t it fall toward you instead of toward Earth?

A: Earth is so much more massive than you that its gravitational pull on the paperclip is immensely greater.

Gravity and Weight

Weight measures the force of gravity pulling downward on an object. The SI unit for weight, like other forces, is the Newton (N). On Earth, a mass of 1 kilogram has a weight of about 10 Newtons because of the pull of Earth’s gravity. On the moon, which has less gravity, the same mass would weigh less. Weight is measured with a scale, like the spring scale shown in the figure below. The scale measures the force with which gravity pulls an object downward. Watch this video to delve a little deeper into weight and gravity.

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You may have heard a story about Isaac Newton coming up with the idea of gravity when an apple fell out of a tree and hit him in the head. The story isn’t true, but seeing how things like apples fall to Earth helped Newton form his ideas about gravity, the force of attraction between things that have mass. Of course, people had known about the effects of gravity for thousands of years before Newton came along. After all, they constantly experienced gravity in their daily lives. They observed over and over again that things always fall toward the ground. However, it wasn’t until Newton developed his law of gravity in the late 1600s that people knew gravity applies to everything in the universe that has mass.

Newton’s Law of Universal Gravitation^[3]

Newton was the first one to suggest that gravity is universal and affects all objects in the universe. That’s why Newton’s law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you—your desk, your book, your pen. Even tiny molecules of gas are attracted to one another by the force of gravity. Watch this video about Newton’s law of gravity and how he developed it.

Q: Newton’s law of universal gravitation had a huge impact on how people thought about the universe. Why do you think it was so important?

A: Newton’s law was the first scientific law that applied to the entire universe. It explains the motion of objects not only on Earth but in outer space as well.

Factors That Influence the Strength of Gravity

Newton’s law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them.

• Objects with greater mass have a stronger force of gravity between them. For example, because Earth is so massive, it attracts you and your desk more strongly that you and your desk attract each other. That’s why you and the desk remain in place on the floor rather than moving toward one another.
• Objects that are closer together have a stronger force of gravity between them. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and the sun. That’s why the moon circles around Earth rather than the sun. You can see this in this figure.^[4]

Review

• Newton’s law of universal gravitation states that the force of gravity affects everything with mass in the universe.
• Newton’s law also states that the strength of gravity between any two objects depends on the masses of the objects and the distance between them.
• Gravity has traditionally been defined as a force of attraction between things that have mass. The strength of gravity between two objects depends on their mass and their distance apart.
• Earth’s gravity constantly pulls matter toward the center of the planet. It also keeps moons and satellites orbiting Earth and Earth orbiting the sun.
• Weight measures the force of gravity pulling on an object. The SI unit for weight is the Newton (N).

Weight

The measure of the force of gravity acting upon an object.

Gravity

        Gravity is the force of attraction between things that have mass.

Law of universal gravitation

The law of universal gravity is a law stating that gravity is a force of attraction between all objects in the universe and that the strength of gravity is greater when masses of objects are greater or distances between objects are shorter.