Gravity is a force of nature that pulls any two objects toward each other. On Earth, gravity's pull gives objects—including you—weight. Yet in space, there is very little gravity, so objects weigh less. You might expect that doing certain tasks in space, such as lifting something heavy, would be easier. However, most early astronauts (people trained to travel in a spacecraft) found even the simplest of tasks almost impossible to do.

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

• Newton's third law of motion states that for every action, there is an equal and opposite reaction
• Engineers consider how forces act as they use the design process to solve problems

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
• Understanding problem/solution
• Establishing cause and effect
• Comparing and contrasting ideas

• reaction force
• counteract

In space, astronauts feel as if they are weightless—or free of the effects of gravity. In these "zero gravity" conditions, today's astronauts are able to perform experiments, fix broken satellites, and more. But in the early days of space flight, even as simple a task as turning a valve was hard to complete.

Click "View" on the left to watch this video segment about the first American astronaut to do work in space—during the 1966 Gemini 9A space flight.

According to the video, why did astronaut Gene Cernan struggle to complete his work assignment in space? Write your answer here.

In the 1600s, English scientist Isaac Newton explained that there were three laws that determine how objects move when gravity or some other force acts on them. Newton's third law of motion states that if one object pushes or pulls on a second object, the second one pushes or pulls back on the first. (Note: Newton's first two laws of motion are very different from the third law and are not important to understanding it.)

You might have seen or heard Newton's third law put another way: "For every action, there is an equal and opposite reaction." By "action" and "reaction," Newton meant that forces always occur in pairs, or twos: An action force is always met with a reaction force. By "equal" and "opposite," Newton meant that the two forces are equal in magnitude, or strength, but opposite in direction.

Here are some other common examples that show Newton's third law in action. The images at left may help make the idea more clear.

• A bird can fly because as its wings push against the air, the air pushes back against its wings.
• When a baseball bat strikes a baseball, the baseball hits back. The batter may feel a sting from the reaction force running through his or her hands.
• As the weight of a roadway on a suspension bridge pulls down on the vertical (up-and-down) cables, the cables pull back, providing the support it needs. In fact, if the cables pulled back with any less force, the roadway would fall!

Write down two or three other examples of Newton's third law of motion here.

Newton's third law applies everywhere in the universe—even in space. This video shows why Newton's third law made things difficult for astronaut Gene Cernan. His work outside the spacecraft is what astronauts call an "extra vehicular activity," or EVA. Engineers at NASA used Newton's third law to design solutions to help counteract, or cancel out, the effects of zero gravity for future EVAs. Click "View" on the left to watch the video.

Then show what you've learned by answering these three questions. Be sure to save all your answers before you go to the next screen.

In this activity, you will read more about how Newton's third law of motion affected Gene Cernan during his space walk. You will then highlight parts of the passage to answer two questions.

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 video again or reread the passage.

Astronauts in Outer Space

Video: 2m 09s

View

Working in Space

Video: 1m 20s

View

In this passage, you will learn more about the steps NASA's scientists and engineers took to overcome the challenges of working in space.

Click "View" on the left to read the passage.

When you finish the passage, briefly describe how NASA scientists and engineers developed their solution.

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: gravity, astronaut, zero gravity, reaction force, counteract

In this lesson, you have learned about Newton's third law of motion, how it created challenges for astronauts in space, and what scientists and engineers did in response to these challenges. 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 Newton's Third Law: Action-Reaction—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 form of the words) as part of your supporting details.