Newton’s Law of Gravity
Jean Brainard, Ph.D.
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Printed: January 23, 2017
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Newton’s Law of Gravity
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Newton’s Law of Gravity
Learning Objectives
• Describe Newton’s law of universal gravitation, and explain why it was so important.
• Identify factors that affect the strength of gravity between two objects.
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.
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.
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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 1.1.
FIGURE 1.1
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.
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
a.
b.
c.
d.
is 100 kg.
is constant.
depends only on their masses.
is greater when they are closer together.
Explore More
Watch the short video about Newton’s law of gravity and then answer the questions below.
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Chapter 1. Newton’s Law of Gravity
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/1453
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?
Resources
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/186748
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/186750
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References
1. User:Tó campos/Wikipedia. The moon is much closer to the Earth than the sun . Public Domain
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