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# 4.15: Newton's Law of Gravity

Difficulty Level: At Grade Created by: CK-12
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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

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. You can learn more about Newton’s law of gravity and how he developed it in the video at this URL:

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 the Figure below.

### Summary

• 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.

### Vocabulary

• law of universal gravitation: 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.

### Practice

Watch the short video about Newton’s law of gravity at the following URL, and then answer the questions below.

1. What equation did Newton use to represent the force of gravity between two objects? What does each letter in the equation stand for? Which letter stands for a value that never changes?
2. Based on the equation, how does the force of gravity between two objects change when the mass of one of the objects doubles?
3. If the distance between the two objects doubles, how does this affect the force of gravity between them?

### Review

1. What is Newton’s law of universal gravitation?
2. Describe the relationship between the masses of two objects and the force of gravity between them.
3. If two objects each have a mass of 10 kg, then the force of gravity between them
1. is 100 kg.
2. is constant.
3. depends only on their masses.
4. is greater when they are closer together.

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### Vocabulary Language: English

law of universal gravitation

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.

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## Concept Nodes:

7 , 8
Date Created:
Nov 01, 2012