Lesson May the Force Be with You:
Weight

Learning Objectives

After this lesson, students should be able to:

• Explain the difference between weight and mass.
• State that weight is one of the four main forces acting on airplanes.
• Explain why larger airplanes are a challenge for engineers to design.
• Explain that physical properties that are important to material selection in airplanes.
• Explain that engineers can design new materials to fit their needs.

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.

NGSS: Next Generation Science Standards - Science

Common Core State Standards - Math

International Technology and Engineering Educators Association - Technology

State Standards

Introduction/Motivation

Have you ever wondered why some things that serve the same purpose are made of different materials? Some of you may have helped in the kitchen and noticed that mixing bowls are made of metal, plastic, wood and glass. Why do we need so many different types of bowls? The key is the different physical properties of each material.

Engineers consider many criteria when choosing materials. What are a few considerations engineers must take into account when choosing materials for designs. (Possible answers: weight, strength, resistance to heat, conductivity, cost, manufacturability [how easily the material can be cut, molded, shaped, connected, etc.], and aesthetics.) Imageine: What is someone tried to make a bridge for cars out of paper. What might happen? (Answer: The bridge would collapse because paper breaks at a much lower stress than other materials, such as steel and concrete.) Engineers could make a bridge out of titanium, which is strong and extremely light, but the cost would be high and bridge weight is generally not very important. In the long run, engineers would use a material that is just as strong as titanium, such as steel, but is less expensive — a tremendous savings to the project.

The optimum choice of materials is also very important in airplane design. What material properties might be important to engineers when they design airplanes? (Possible answers: Weight is an important property since airplanes need to be as light as possible to get off the ground; other important properties to consider are strength, manufacturability, resistance to heat [important in the engine] and cost.) Airplanes have been made of some very interesting materials in the past. Otto Lilienthal, a German inventor, used cotton to cover the wings of his first gliders. Two Englishmen, William Henson and John Stringfellow, used silk to make propellers for models of their steam-powered airplanes. Through trial and error, it was soon discovered that these two materials were not ideal for building airplanes.

Advances in materials science result of engineers devoting their lives to the study of materials. Some engineers study the structure of matter on the atomic level and determine how different atoms will interact. Then they determine how they will put the atoms together in a specific way. Finally, they fabricate and test their new materials. Keep your eyes open! You never know when you might see one of these new materials in an everyday product.

Lesson Background and Concepts for Teachers

What Is Weight?

Weight is the force exerted on all objects by gravity. However, in order to talk about weight, we must first understand mass. Mass is a measure of how much "stuff" is in an object. The mass of an object is independent of where the object is. For example, a person with a mass of 100 kilograms on Earth will still have a mass of 100 kilograms on the moon. The force of gravity on an object, on the other hand, determines weight. Since gravity on Earth is about six times that of the moon, a person on Earth weighs six times more than s/he would on the moon.

How Does Weight Affect Flight?

Weight is the force that pulls an airplane back towards the Earth. Weight must be overcome by lift (as seen in Lesson 2 of the Airplanes unit) in order to achieve flight. The force of lift must be greater than the weight of an airplane for the airplane to climb (see Figure 1). Refer to the associated activity Physics Tug of War to have students look at the effects of weight on motion as explained by Newton's second law.

Why Use Different Materials?

In order for engineers to design bigger airplanes, they must increase lift to hold the extra weight. They can do this by increasing the airplane speed or by increasing the wing surface area. Increasing the airplane speed is difficult because the plane will experience more drag and use more fuel. Increasing the wing surface area is also difficult because this adds extra weight to the plane, which is the problem the engineers are trying to solve in the first place! A solution engineers often use is to replace parts of the plane with lighter materials. For example, today most airplanes' bodies and wings are made of aluminum, which is lighter than steel and stronger than plastic or wood.

Sometimes engineers design new metal alloys when they cannot find a material that meets their needs. Alloys are mixtures of metals that have properties of the individual metals in the mixture. If an alloy contains a metal that is a good conductor of electricity and a metal that is strong, the alloy itself is generally strong and a good conductor. Engineers also design different types of plastic and ceramic materials.

Engineers choose the materials they use for certain projects based on many factors. Strength and weight are some of the most important factors in designing airplanes. Students can gather their own observations of these two properties compared over different materials in the associated activity Bend That Bar. Other important criteria are material cost and  appearance. Engineers strive to make their airplane designs look good and fly well. One important material property that engineers must pay special attention to is heat resistance. A ceramic material is used on the space shuttle to shield the inside from the high temperatures created on the surface of the shuttle during reentry. This material has very low heat conductivity and high heat resistance. Low heat conductivity means that heat does not pass easily from one side of the material to the other, which is similar to how a thermos keeps hot coffee warm. The walls of the thermos have low heat conductivity, which means the heat does not travel easily from the coffee to the outside of the thermos. This traps the heat inside and keeps the coffee warm for longer periods of time than without using a thermos. Ceramics also tend to be very heat resistant, which means that they can withstand incredibly intense temperatures. If ceramic tiles were not used on the space shuttle, the heat would ultimately melt it!

Associated Activities

• Bend That Bar - Students make observations about the strength and weight of different materials.
• Physics Tug of War - Students look at the effects of weight on motion. They learn Newton's 2nd Law of Motion by examining the forces necessary to move different weights.

  Watch this activity on YouTube

Lesson Closure

Ask students to explain the difference between mass and weight. Make sure they understand that mass is how much matter is in an object, while weight is the force that an object is attracted to another object due to gravity. Discuss some possible solutions to the problems of developing larger airplanes, and how each of these solutions affects flight. Ask what material properties may be important to engineers in developing new airplanes. (Possible answers: Material strength, weight, heat resistance, cost and appearance.)

Vocabulary/Definitions

alloy: A mixture or solid solution of two or more metals, the atoms of one replacing or occupying positions between the atoms of the other. Brass is an alloy of zinc and copper.

ceramic: A hard, brittle, heat-resistant and corrosion-resistant material made by shaping and then firing a nonmetallic mineral, such as clay, at a high temperature.

mass: The mass is the measure of how much material is in an object. The mass of an object is not dependent on gravity and, therefore, is different from, but proportional to, its weight.

material science: The observation, identification, description, experimental investigation, and theoretical explanation of materials and their properties.

matter: That of which anything is composed. Matter is anything that has mass and exists as a solid, liquid, gas, or plasma.

weight: The force with which a body is attracted to Earth or another celestial body, equal to the product of the object's mass and the acceleration of gravity.

Assessment

Pre-Lesson Assessment

Discussion Question/Answer Review: Solicit, integrate and summarize student responses.

• What is lift? (Answer: When the air pressure below a wing is greater than the air pressure above the wing, the net upward force is called lift.)
• How does Bernoulli's principle create lift? (Answer: A wing is designed so the top is longer than the bottom; then when air travels across the top of the wing it moves faster and exerts less pressure than air beneath the wing. The result is a net force up, or lift.)
• Some of you may have helped in the kitchen and noticed that we mix things in bowls that are made of metal, plastic, wood and glass. Why do we need so many different types of bowls? (Answer: The key is the different physical properties of each material.)

Post-Introduction Assessment

Voting: Ask true/false questions (see examples below) and have students vote by holding thumbs up for true and thumbs down for false. Count the number of true and false, and write the number on the board. Give the right answer.

• True or false: Airplanes have been made of some very interesting materials in the past. Otto Lilienthal, a German inventor, used paper to cover the wings of his first gliders. (False. He used cotton.)
• True or false: Engineers must take material properties into consideration when designing something. (True. Some material property considerations include material weight, strength, resistance to heat, conductivity, cost, manufacturability and aesthetics.)
• True or false: If a highway bridge was made out of paper, it would collapse. (True. Paper breaks at a much lower stress than other materials, such as steel and concrete.)
• True or false: The choice of materials is not very important in building airplanes. (False. The smart choice of materials is very important in airplanes in order to keep them up in the air while flying.)

Lesson Summary Assessment

Inside/Outside Circle: Have students stand in two circles such that each student has a partner. Three people may work together if necessary. The outside circle faces in and the inside circle faces out. Ask the students a question. Both members of each pair think about the question and discuss their answers. If they cannot agree on an answer, they can consult with another pair. Call for responses from the inside or outside circle or the class as a whole.

• What are the four forces affecting airplane flight? (Answer: Lift, weight, thrust and drag.)
• What is lift? (Answer: When the air pressure below a wing is greater than the air pressure above the wing, the net upward force is called lift.)
• Which of the four forces did we learn about today and in which direction does it affect flight? (Answer: Weight, and in a downward direction.)
• What is mass? (Answer: Mass is the measure of how much material is in an object. The mass of an object is not dependent on gravity and, so it is different from, but proportional to, its weight.)
• What is weight? (Answer: The force with which a body is attracted to Earth or another celestial body, equal to the product of the object's mass and the acceleration of gravity.)
• How does weight affect airplane flight? (Answer: Weight is the force that pulls an airplane back towards the Earth. Weight must be overcome by lift (as seen in Lesson 2 of the Airplanes unit) in order to achieve flight. The force of lift must be greater than the weight of an airplane for the airplane to climb.)
• What affects the weight of an airplane? (Answer: The materials from which the plane is constructed.)
• What must engineers consider when choosing the materials to build an airplane? (Answer: Engineers choose the materials they use based on many factors: strength and weight are the some of the most important factors in designing airplanes. Other important criteria are material cost, appearance and durability.)

Sales Pitch!: Students pretend to be salespersons trying to sell their "material" to a large airplane manufacturer. Have student groups/pairs research different materials and create a persuasive poster or flyer as well as a 10-minute sales pitch of their findings for the next class. Materiails to research include: metals, alloys, ceramics, composites, polymers.

Lesson Extension Activities

A fun extra activity is to figure out the relationship of weight and mass on other planets. Use the How Much Do I Weigh Worksheet, which asks students to make simple calculations of their weight on other planets using their mass on Earth.

Assign students to research and learn more about materials and their properties. Many websites are available that explore the properties of various materials. Have students start with this website: http://www.engr.sjsu.edu/WofMatE/Structure.htm.

To make a historical connection, have students research the history of airplane materials presented in this lesson (that is, Otto Lilienthal, William Henson and John Stringfellow) as well as other interesting materials that engineers have used for airplanes in the past.

References

Copyright

Contributors

Supporting Program

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.