Introduction:
Newton's Third Law: Action-Reaction

You might think that some tasks would be easier to do in space than on Earth. For example, because there is very little gravity in space, objects weigh less. So, lifting and moving a heavy object becomes a relatively easy job. Why is it, then, that early astronauts found it incredibly difficult to simply turn a screw? Newton's third law of motion provides the answer.

The purpose of this activity is to help you understand Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.

Next: Action-Reaction

In the 1600s, English physicist and mathematician Isaac Newton described 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.

Here is a real-world example of Newton's third law: When gravity causes a book to push downward on a table, the table pushes back upwards on the book, so that neither the table nor the book moves up or down.

Can you think of other examples? Write your examples in the box below, then click "save notes" before moving on to the next page.

Next: Newton's Third Law in Action

You might have seen or heard Newton's third law put another way: "For every action, there is an equal and opposite reaction." Here it gets a little more detailed. By "action" and "reaction," Newton was suggesting that forces always occur in pairs: 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 size, but opposite in direction—that is, 180 degrees from one another. A strong or weak push or pull on an object is met with an equally strong or weak push or pull.

Here are some other examples that demonstrate Newton's third law in action:

• When you punch an object, the object applies equal force back on your fist. You may experience this reactive force as pain.
• Likewise, when a baseball bat strikes a baseball, the baseball hits back. The batter feels a jolt running through his or her hands.
• A bird can fly because as its wings push against the air, the air pushes back against the wings.
• As the weight of a roadway on a suspension bridge pulls down on the vertical 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!

Next: Newton's Third Law of Motion: Watch This!

Newton's third law of motion does not only apply to conditions here on Earth. Watch this video segment about the first work assignment in space.

Note: You will hear the following words in the video segment. Click on each word to see its definition.

• cosmonaut
• counteract
• EVA
• reactive force
• weightlessness
• zero gravity

Next: Read About It!

Think about what you have learned about Newton's third law of motion. In this activity, you will read about how this law affected Gene Cernan during his space walk.

The following words will appear in the reading. Click on each word to see its definition.

• friction
• weightless

Click "View" on the left to begin the activity. Be sure to print your notes before you exit the activity. Your notes will not be saved.

If you want, you can watch the video again.

Newton's Third Law of Motion

QuickTime Video
Length: 3m 26s

Next: Write About It!

You have explored Newton's third law of motion and how it created challenges for astronauts in space. Now you will learn about the steps NASA's scientists and engineers took to overcome these challenges. Then you will have the chance to share your ideas with others.

• Print out your notes. (After you print, your notes will be erased. Make sure you are done before printing.)
  
  
• Print out and read How to Turn a Bolt in Space (PDF).
• Then, select and complete a writing assignment on page 2 of the printout.

Note: The following words will appear in the reading. Click on each word to see its definition.

• friction
• weightless