On Earth, when an object falls, it falls toward the ground because of the force of gravity between Earth and the object. A force is a push or a pull that can make an object speed up, slow down, or change direction. The force of gravity makes an object gain speed as it falls.

Gravity is not the only force that affects objects on Earth. In this lesson, you will explore gravity and see how other forces also affect an object's motion.

Goals
Here are the big ideas you will learn about in this lesson:

• Things on or near Earth are pulled toward it by gravity.
• A force can cause changes in the speed or direction of an object.
• When more than one force acts on an object, how the object's motion changes depends on the strength and direction of the forces.

Strategies
You will be using many reading and writing strategies to complete this lesson. The key strategies that you will focus on are:

• Determining important information
• Constructing summaries
• Using text features
• Comparing and contrasting ideas
• Making inferences

• force
• gravity
• resistance

Gravity is a force of nature that pulls objects toward each other. It's important to understand gravity because it is a major part of everyday life. Gravity keeps you standing on the ground by pulling you toward Earth's center. Gravity gives you weight and causes objects to fall. Any object on or near Earth experiences the force of gravity.

In this video, you will learn more about how the force of gravity pulls objects toward Earth's center and how it changes the speed of a moving object. Watch the video to see how the acceleration caused by gravity would affect a person falling through an imaginary tunnel that passes through the center of Earth.

Click "View" on the left to watch the video.

As you saw in the video, gravity causes the falling man to gain speed as he travels toward the center of Earth. The acceleration caused by gravity on Earth is about 9.8 meters per second (m/s) per second. That means that every second, the speed of a falling object increases by 9.8 m/s.

The man keeps moving toward the other end of the tunnel. However, once he passes the center of Earth, gravity continues to pull him toward the center, which is now in the opposite direction from how he is moving. As a result, gravity causes the man to slow down as he moves from the center toward the other end of the tunnel.

In two to four sentences, describe how gravity affects the speed at which the man falls through the tunnel.

Galileo, the 16th-century Italian scientist, studied gravity and the motion of objects. It is said that in one of his experiments, he dropped two objects of different sizes from the Tower of Pisa to see if they would fall at the same rate. Although he may not have actually done this, his writings describe what he thought would happen: if gravity is the only force acting on the objects, they would reach the ground at the same time.

During the Apollo 15 mission in 1971, the Falcon spacecraft landed on the Moon, and an astronaut demonstrated Galileo's experiment. Watch as the astronaut drops a falcon feather and a hammer to see if they reach the ground at the same time. What do you think will happen?

Click "View" on the left to watch the video.

How did this experiment support Galileo's findings about falling objects? Write your answer in two to three sentences.

Gravity is not the only force that affects the motion of an object falling toward Earth. When an object moves through the air, air molecules push against it in the opposite direction with a force called air resistance. When forces act on an object in opposite directions, they counteract each other. As a result, air resistance reduces the effect of gravity and therefore affects how fast the object falls.

As you saw in the video, when an astronaut on the Moon releases two objects at the same time from the same height, both objects accelerate at the same rate and hit the surface of the Moon at the same time. This experiment would not have the same result if it were done on Earth because of the air in our atmosphere. But because there is hardly any atmosphere on the Moon, the only force acting on the objects is the Moon's gravity.

So, when more than one force acts on an object, what happens?

• Two forces acting on an object in the same direction combine into one stronger force. If two people push a large rock in the same direction, they can push it with more force together than alone.
• Two forces acting on an object from opposite directions counteract each other. If two people push a large rock from opposite sides, it will move in the direction of the stronger push.
• When forces are equal and opposite to one another, they are balanced and there is no effect on the motion of the object. If two people push a large rock with equal force from opposite sides, it will not move at all.

Now read about how the forces of gravity and air resistance counteract each other. Click "View" on the left to begin the reading.

How does air resistance affect how fast a feather falls? Write your answer in two to three sentences.

The videos you have seen and the text you have read explored how gravity and other forces affect how an object moves.

Now show what you have learned so far about gravity and air resistance by answering these three questions. Be sure to save all your answers before you go to the next screen.

You will now check your understanding of the vocabulary words for this lesson by placing them in sentences. Click "View" on the left to begin the activity.

Before moving to the next screen, choose two words from the vocabulary list and write a new sentence for each word.

Vocabulary words: acceleration, balanced, counteract, force, gravity, resistance

As you saw and read on the previous screens, the force of gravity causes an object to accelerate toward the ground. However, the force of air resistance counteracts gravity and affects how fast an object falls. In this activity, you will compare the forces of gravity and air resistance. You will sort the labels into two columns based on whether they describe gravity or air resistance.

Click "View" on the left to begin the activity. Your teacher will review the answers with you.

Then, before moving on to the final assignment, you may want to watch this lesson's videos again or reread the passage.

Gravity at Earth's Center

Video: 1m 46s

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Galileo's Experiment on the Moon

Video: 0m 48s

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Counteracting Forces

Document

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In this lesson, you have learned that the force of gravity causes all objects on Earth to fall toward Earth's center. You have also learned that a force from the opposite direction, such as air resistance, can counteract gravity. You will now write an essay that shares what you have learned and what you think about it.

Begin by reviewing your notes. Click "review my work" above and view or print out the screen that pops up.

Final Assignment Instructions

• Click Forces of Gravity and Air Resistance—Final Assignment (PDF).
• Choose one of the topics in the PDF and write an essay.
• Organize your response so that it includes at least three supporting details that you have learned from the reading passage, video(s), and other materials in this lesson.
• Use at least three vocabulary words (or a form of the words) as part of your supporting details. For example, for the word jump, you could use jumps, jumped, or jumping.