Hands-on Activity: Equal & Opposite Thrust in Aircraft: You’re a Pushover!

Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

A line drawing shows two ice skaters demonstrating Newton's third law of motion. They are facing each other with their hands in front of them pushing on the other person's hands.
Figure skaters demonstrating Newton's third law of motion.
copyright
Copyright © 2006 Benjamin Crowell, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Skaters_showing_newtons_third_law.svg

Summary

Through this activity, Newton's third law of motion is demonstrated, which is the physical law that governs thrust in aircraft. Guided by a worksheet, students do several activities —pushing on walls, releasing air from balloons—that show that for every action there is an equal and opposite reaction. They also calculate the missing mass or acceleration based on the the third law of motion equation: mass of object 1 x acceleration of object 1 = mass of object 2 x acceleration of object 2. They relate their understanding of the third law of motion and thrust to crashed cars and airplane movement.
This engineering curriculum meets Next Generation Science Standards (NGSS).

Engineering Connection

Engineers apply their understanding of scientific principles, such as Newton's laws of motion and mathematics to design, analyze and improve their inventions. Engineers use algebra and basic math skills to determine exact values for variables such as velocity, temperature, acceleration, energy and mass. Using these real measurements and calculations enables engineers to design efficient, safe and successful aircraft.

Pre-Req Knowledge

Students should have a firm grasp of mass before this activity, but acceleration may be a new concept. For this activity, it suffices to define acceleration as how fast or slow something speeds up or slows down; for example, when a car passes on the highway or the Earth's gravity.

Learning Objectives

After this activity, students should be able to:

  • Explain that two objects pushing off of each other experience the same force.
  • Relate the amount of air pressure with acceleration.
  • Use algebraic methods to describe Newton's third law of motion.

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Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

  • Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Use variables to represent quantities in a real-world or mathematical problem, and construct simple equations and inequalities to solve problems by reasoning about the quantities. (Grade 7) Details... View more aligned curriculum... Do you agree with this alignment?
  • Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
  • Knowledge gained from other fields of study has a direct effect on the development of technological products and systems. (Grades 6 - 8) Details... View more aligned curriculum... Do you agree with this alignment?
  • Solve real-world and mathematical problems involving the four operations with rational numbers. (Grade 7) Details... View more aligned curriculum... Do you agree with this alignment?
  • Solve linear equations and inequalities in one variable, including equations with coefficients represented by letters. (Grades 9 - 12) Details... View more aligned curriculum... Do you agree with this alignment?
Suggest an alignment not listed above

Materials List

Introduction/Motivation

Thrust is based on Newton's third law of motion. It states that for every action there is an equal and opposite reaction. This means that when you push something, it pushes you back. That is the reason you brace yourself to push something heavy, such as a huge snowball for making a snowman. You could not just stand straight up and push against a wall. You would fall over backwards! It would be just the same as if someone standing in front of you had pushed you over.

An airplane moves forward by pushing the air molecules around it. The air will not push the airplane forwards on its own, and the airplane certainly cannot tell the air to push it forwards; the airplane pushes air backwards. It does this by increasing the pressure of the air in the combustion chamber and pushing it out the back of the engine. When the action of the airplane pushing air out of the back of the engine occurs, the opposite reaction is that the airplane moves forward. You can also think of the propellers colliding with the air molecules, causing them to move opposite of the blade. When air is moving fast, it creates a region of low pressure, which attracts more air into it. The propeller continues to "feed" this low-pressure zone.

Newton's third law of motion can be written as: mass of object 1 x the acceleration of object 1= mass of object 2 x the acceleration of object 2. Or more specifically as: m1 x a1 = m2 x a2.

A photograph shows a sedan automobile after an indoor crash test with its front end crumpled where it hit a wall head on.
Figure 1. Safety testing an automobile.
copyright
Copyright © safercar.gov http://www.safercar.gov/staticfiles/DOT/safercar/ncapmedia/images/2013/v07108P012.jpg

(Show Figure 1 to the class—a photograph of a car crash test.) Look at this photograph. Do you think the car is pushing more on the wall or the wall is pushing more on the car? (Answer: They are pushing on each other exactly the same amount!) Why do you think the car is crunched up, but the wall is not? (Answer: Both the car and wall had the same amount of force applied, but since the wall is so heavy, and designed to absorb impact without hurting, it was less affected. The wall has "dampers" that permit it to move a small amount, enough to absorb all the energy from the car. The car on the other hand, likely flies back five feet, because it is free to move and weighs less).

Procedure

Before the Activity

  • Try the demonstrations and review the worksheet.
  • Make copies of the You're a Pushover Activity Worksheet.
  • For Part 1, arrange for the use of an area with wall access where students can push on the wall, such as the outdoor sides of the school building, hallway, classroom or gym.

With the Students

Part 1: You're a Pushover!

  1. Distribute the worksheet to students.
  2. Take the class to an area where they can each stand in front of a section of wall.
  3. Direct students to push on the wall as hard as they can. Watch how students instinctively brace themselves before pushing. Ask them to observe and describe what happened. Expect them to notice that they did not fall over and the wall did not move. Tell them this is an example of a slow collision.
  4. Now direct students to stand upright and flat-footed, close to the wall. Have them push as hard as they can. What happens? (Answer: Students aree pushed backwards by the wall.)
  5. Have students answer the Part 1 questions on their worksheets.

Part 2: Pushing on Air!

  1. Hand out a balloon to each student. Guide them through the exercise as follows.
  2. Blow up the balloon just a little bit. (Note: Demonstrate how to first stretch the balloon while it's deflated to make the first blow up easier.)
  3. Gently let the air out of the balloon without letting go of the balloon. Feel the stream of air as it comes out. This sensation is caused by the air molecules colliding with your hand. When they do, the skin on your hand is pushed inwards a small amount, and the air molecules bounce off backwards. Both your skin and the air molecules are affected equally, but since your skin weighs so much more, it moves less.
  4. Blow up the balloon until it is almost full.
  5. Gently let the air out again and feel the stream of air. Ask students to observe and describe what happened. Expect students to feel the air come out much faster because the pressure inside the balloon is greater.
  6. Blow up the balloon a little bit and let it go. Do the same with the balloon filled all the way. Notice how much faster the balloon accelerates when it is full.
  7. Recap: This demonstrates Newton's third law of motion: every action has and equal and opposite reaction. The force of the air leaving the balloon is equal to the force of the balloon moving forward. A small balloon travels more slowly and a shorter distance than a big balloon because a big balloon releases more air!

Part 3: Gotta Be Equal

  1. Remind students that Newton's third law of motion states that for every action there is an equal and opposite reaction. When you swat the air with your hand, you feel some resistance, correct? Since you are moving the air molecules with your hand, your hand is also being moved by the air molecules. Always equal and opposite! It just may not feel equal because heavier things are moved less.
  2. Direct students to calculate the missing mass or acceleration on the worksheet, based on the equation for Newton's third law of motion.
  3. Give students some time to complete the worksheet.

Attachments

Safety Issues

Make sure students do not push too hard against the wall as they might not catch themselves before they fall over backwards.

Make sure students do not blow too much air in the balloons, as they might pop and cause injury.

Troubleshooting Tips

Watch that students act responsibly with the balloons. You may want to conduct this activity outside or in a gym to give them sufficient space to let their balloons fly.

Assessment

Pre-Activity Assessment

Voting: Ask a true/false question and have students vote by holding thumbs up for true and thumbs down for false. Tally the votes and write the number on the board. Give the right answer.

  • True or False: Thrust is based on Newton's third law of motion. (True)
  • True or False: Newton's third law of motion states that for every action there is an unequal but similar reaction. (False: The law states that for every action there is an equal and opposite reaction.)
  • True or False: When air molecules collide with each other (such as the compressed air zooming out of a balloon and hitting the air molecules near the balloon's outlet), they just sit still. (Answer: False, they collide and move away from each other.)
  • True or False: If a big truck and small car going the same speed (both driven by remote control) crashed into each other, the small car would move away from the crash faster. (Answer: True)

Activity Embedded Assessment

Worksheet/Pairs Check: Have students record their Part 1 and Part 2 observations on the You're a Pushover! Activity Worksheet and answer the worksheet questions. Have students compare their answers with their peers. Collect the worksheets and review their answers to gauge their depth of comprehension.

Post Activity Assessment

Math Problems: Have students work through a series of mass and acceleration problems on the You're a Pushover! Activity Worksheet. Later, review their answers to gauge their depth of understanding.

Figure Drawing/Discussion: Have student volunteers label some of the forces of flight on objects drawn on the classroom board.

  • Draw a simple balloon on the board. Ask student volunteers to come up to the board and draw an arrow that represents thrust. What is giving the balloon thrust? (Answer: The air from inside the balloon makes it move forwards.) What arrow is going to be opposite of thrust? (Answer: Drag.) Let students know that they will be learning about drag next. Next, ask a volunteer to draw an arrow that represents weight. (Answer: This arrow will be pointing down.) Next have a volunteer draw an arrow that represents lift. (Answer: This arrow will be pointing up.)
  • For a little more difficult example, draw a simple rocket on the board. Ask student volunteers to come up to the board and draw an arrow that represents thrust. What is giving the rocket thrust? (Answer: the rocket fuel gives it thrust just as air from the balloon makes it move forwards.The gas molecules flying out of the rocket collide with the air molecules and the ground, causing it to push away in the opposite direction...UP!) What arrow is going to be opposite of thrust? (Answer: Drag.) Let students know that they will be learning about drag next.
  • Next ask a volunteer to draw an arrow that represents weight. (Answer: This arrow points in the same direction as drag, pointing down.) Next have a volunteer draw an arrow that represents lift. (Answer: This arrow points in the same direction as thrust, pointing up.) Ask students why the arrow goes in the same direction as thrust. (Answer: Because the rocket's forward movement is up.) Ask if anyone can think of other examples of the four forces of flight and what arrows they would draw on them.

Activity Extensions

If small scooters are available, such as the low square type with four weels used in gym class, or even a few skateboards, roller blades or rolling office chairs. Have students sit on the scooters and push off of each other with their feet. This enables them to see and feel how larger students do not move as far as smaller students. Take care that kids do not fall off the scooters and hurt themselves if they lose their balance or push off too hard. If just a few rolling items are available, conduct as a class demo.

Another great activity to explain the concept of Newton's third law of motion is balloon rockets (such as the Action-Reaction! Rocket activity in the Mechanics Mania unit. See Figure 2 for an illustration of that activity's setup.) Set up balloon rocket stations, or strings, at different distances. Have students try to figure out how to get their rockets to go a specified distance.

A schematic drawing of the action-reaction rocket activity setup shows a taught string tied between the high backs of two desk chairs. Along the string a "rocket," composed of an inflated balloon in a plastic bag, hangs attached and below a plastic straw that is strung on the string, enabling the "rocket" to move along the string with the thrust of the air leaving the balloon.
Figure 2. Action-reaction rocket activity setup.
copyright
Copyright © 2005 Ashleigh Bailey, ITL Program, College of Engineering, University of Colorado Boulder

Activity Scaling

  • For younger students, to help them understand the problem, do the worksheet math on the board first before they try to do the division on their own.
  • For younger students, it may be easier to explain Newton's third law of motion (for every action, there is an equal and opposite reaction) in more elementary-friendly terms, such as, "when you push something, it pushes you back."
  • Give older students a problem in which the mass is known for two objects pushing off of each other as well as the distance that one of the objects moves. Challenge them to use this known information to solve for the unknown distance the other object moved.

Contributors

Tom Rutkowski; Alex Conner; Geoffrey Hill; Malinda Schaefer Zarske; Janet Yowell

Copyright

© 2004 by Regents of the University of Colorado

Supporting Program

Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder

Acknowledgements

The contents of this digital library curriculum were developed under grants from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation (GK-12 grant no. 0338326). However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: February 8, 2018

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