Lesson: May the Force Be With You: Weight

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

An aircraft showing the four forces acting on it: thrust, drag, lift, weight.
Forces acting on an aircraft.

Summary

The purpose of this lesson is to help students understand the relationship between the mass and the weight of an object. Students will study the properties of common materials and why airplanes use specific materials.

Engineering Connection

One of the most important aspects of aircraft design that an engineer must take into consideration is weight. Every additional part or piece added to an airplane adds weight that makes it harder for the airplane to overcome the force of gravity to fly. So, when engineers design an airplane, they minimize its weight in their choice of parts and materials, while still assuring that it is strong and safe.

Educational Standards

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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.

Suggest an alignment not listed above

Learning Objectives

After this lesson, students should be able to:

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

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.

There are many criteria engineers use for choosing a material. Ask the students to name a few considerations an engineer must take into account when choosing a material for a design. (Answers: weight, strength, resistance to heat, conductivity, cost, manufacturability [how easily the material can be cut, molded, shaped, connected, etc.], and aesthetics.) Ask the students to imagine if 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 enormous and the weight of the bridge 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 way less expensive — a tremendous savings to the project. The correct choice of materials is also very important in an airplane. Ask the students what material properties might be important to engineers when they design an airplane. (Answers: Weight is one of the most important properties since an airplane needs to be as light as possible to get off the ground, 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 beneficial for building airplanes.

Advances in materials science are a 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. Next, 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" there 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 they do on the moon.

A diagram of the four forces of flight acting on an airplane, represented by four opposing arrows.  An arrow pointing straight upward shows lift, an arrow pointing right (or forward) shows thrust, an arrow pointing downward shows weight, and an arrow pointing left (or backwards) shows drag.
Figure 1. The four forces of flight: lift, weight, thrust and drag.
copyright
Copyright © Tom Rutkowski, University of Colorado, Boulder, 2003.

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.

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 speed of the airplane or by increasing the area of the wings. Increasing the speed of an airplane is difficult because the plane will experience more drag and use more fuel. Increasing the surface area of the wings 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 out of aluminum, which is lighter than steel and stronger than plastic or wood.

Engineers may develop new alloys of metals 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 will generally be strong and a good conductor. Engineers can 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 the some of the most important factors in designing airplanes. Other important criteria are the cost of the materials and the appearance of the materials. Engineers try to make their airplane designs look good and fly well. One important property of a material that engineers must pay special attention to is the heat resistance of the material. 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 has high heat resistance. Low heat conductivity means that heat does not pass easily from one side of the material to the other. This is like a thermos that is used to keep 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. A ceramic also is very heat resistant, which means that it can withstand incredibly intense temperatures. If ceramic tiles were not used on the space shuttle, the heat would ultimately melt it!

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.

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.

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 the students what material properties may be important to engineers in developing new airplanes. (Possible answers: strength, weight, heat resistance, cost, and appearance of the materials.)

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, there is a net upward force called lift.)
  • How does Bernoulli's Principle create lift? (Answer: Because the top of a wing is longer than the bottom, air traveling across the top of the wing 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.

  • 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.)
  • An engineer must take material properties into consideration when designing something. (True. Some material property considerations include weight, strength, resistance to heat, conductivity, cost, manufacturability and the aesthetics of the material.)
  • If a 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.)
  • The correct choice of materials is not very important in building an airplane. (False. The correct choice of materials is very important in an airplane in order to keep it up in the air while flying.)

Lesson Summary Assessment

Inside/Outside Circle: Have the 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 there is a net upward force 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: 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.)
  • 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 the flight of an airplane? (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 what materials from which 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 the cost of the materials and the appearance of the materials.)

Sales Pitch!: Students pretend to be salespersons who are 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.

  • Materials to research include: metals, alloys, ceramics, composites, polymers.

Lesson Extension Activities

A fun extra activity is to look at the relationship of weight and mass on other planets. The attached worksheet "How much do I weigh" has the students make a simple calculation of their weight on other planets using their mass on earth.

Students can research and learn more about materials and their properties. There are web sites to explore the properties of various materials further. Students may start with: http://www.engr.sjsu.edu/WofMatE/Structure.htm

To make a historical connection, have students research the history on airplane materials presented in this lesson (i.e., Otto Lilienthal and William Henson and John Stringfellow) as well as other interesting materials that engineers have tried to use on airplanes in the past.

References

Nahum, Andrew. Flying Machine. Alfred A. Knopf, Inc., New York, 1990.
Guyford, Stever H., Haggerty, James J. Flight. Time Inc., New York, 1969.
http://www.engr.sjsu.edu/WofMatE/Structure.htm
http://www.nyu.edu/pages/mathmol/textbook/weightvmass.html

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 a grant 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.

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