Lesson: Form vs. Function

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

A drawing of an old boxy car (left) and a sleek car that is plugged into a wall (right).
Students explore energy efficiency of cars
Copyright © (left) Department of Ecology, State of Washington, and (right) US Environmental Protection Agency http://www.ecy.wa.gov/news/envirotips/tips_winter.htm http://www.epa.gov/climatechange/kids/solutions/technologies/vehicles.html


Students take a closer look at cars and learn about some characteristics that affect their energy efficiency, including rolling resistance and the aerodynamics of shape and size. They come to see how vehicles are one example of a product in which engineers are making changes and improvements to gain greater efficiency and thus require less energy to operate.

Engineering Connection

More than one-third of the greenhouse gases released into the Earth's atmosphere come from the transportation sector. Engineers continually strive to make vehicles more fuel efficient so as to reduce their carbon footprints and lower their operating costs. Some aspects of car design that directly impact vehicle fuel efficiency include reducing size and weight, improving aerodynamics, and reducing rolling resistance. These changes result in more fuel efficient cars that are appealing in appearance and reliable in performance.

Learning Objectives

After this lesson, students should be able to:

  • List characteristics that affect a car's energy efficiency.
  • Describe how rolling resistance affects a car's energy efficiency.
  • Explain how a car's shape and size are related to aerodynamics.

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A car's energy efficiency depends on multiple things, including its weight, rolling resistance, and aerodynamics. A car that is energy efficient can travel farther on the same amount of gas (vs. a less energy efficient car), which saves the driver money on his/her fuel expenses and reduces the amount of carbon dioxide emissions released into the atmosphere.

Engineers are constantly looking for ways to make cars more energy efficient by improving their designs, among other technologies to reduce overall fuel consumption.

Lesson Background and Concepts for Teachers

The aerodynamics of a car depends on how much air the car has to move out of the way as it travels along. Engineers first design a car and then test its aerodynamics by placing it in a wind tunnel and measuring the amount of air that is missing from the tunnel (we call this displacement). A wind tunnel is a big machine that blows forced air from one end to the other; similar to a blow dryer blowing through the center tube of a roll of paper towels. Smoke streaks (or trails of smoke, such as from a camp fire) are used to visualize the air moving over the car.

Often, engineers mimic (or copy) aerodynamic animals, such as fish and birds, when coming up with the design concepts for the body of a car (the part of the car where the driver and passengers sit). These animals move through air and water easily and with little energy because they are sleek and have no sharp corners or flat surfaces facing the wind. This allows the wind to flow smoothly over the surfaces, creating little turbulence (or, air movement) as the animal moves, as shown in Figure 1. When an object creates a lot of turbulence as it moves through the air, the amount of friction the object feels from the air increases. For example, a semi-truck with a curved wind-blocking shield on the front moves more easily through the air than a semi-truck with no wind blocker, as it has a flat front. The truck with the wind blocker is more energy efficient. This explains how the aerodynamics of a vehicle affects the energy efficiency of a vehicle, and more so at high speeds than at low ones.

The shape of a car affects the aerodynamics. A smoother car, creates less drag (less interruption with air molecules), which is considered more aerodynamic.
Figure 1. As an object moves through the air, it pushes aside air molecules. This is called drag, which affects a car's aerodynamics.
Copyright © NASA http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.html

The amount of energy a car uses can also be affected by the rolling resistance of a car's tires on the road surface. This friction is necessary so the tires stick to the ground and roll (without friction, the tires would just endlessly spin without moving the car forward), but too much friction makes it stick too much and makes it hard to move the car forward or backward. You need to be able to find the balance between the amount of resistance, or friction, needed to keep the vehicle on the road and the ability to move efficiently without it flying off the road or being "stuck" to the road. Engineers design tires that increase a car's energy efficiency by rolling smoothly while making sure they are sticky enough to stay safely on the road, especially through corners and wet or icy conditions. The tread and rubber compound of the tires affect the rolling resistance of the car and also affect the car's handling. The rolling resistance plays a larger part in the energy efficiency of the car at lower speeds.

Finally, a car's weight affects its fuel efficiency. The weight determines how much energy it takes to accelerate (speed up) the car at all, and it also affects how much energy it takes to move the car up a hill. Energy use increases with greater weight when moving an object against gravity, so keeping the weight as low as possible reduces the overall energy use. Engineers use new materials, such as carbon fiber, to reduce the weight of cars and design them so they are lighter. It is a balancing act between creating a safe car and a light one. The weight plays into the energy efficiency of a car more at low speeds when the car is accelerating (just starting to speed up and move) than when it is moving at a steady speed.

All these factors contribute to a car's energy efficiency, with some of them being more important at high speeds and others being more important at low speeds. Aerodynamics affect the energy efficiency at high speeds, and rolling resistance and weight have a larger affect at low speeds.


aerodynamics: The ability of an object to cut through air efficiently.

energy efficiency: Being able to do more with less energy.

rolling resistance: The force of friction acting on a rolling object by the ground to slow it down.

weight: A measure of heaviness of something; how much something weighs.

Associated Activities

  • Cars: Engineering for Efficiency - Students design and build model race cars out of everyday materials and race them down a track to determine how weight affects the energy efficiency of their car's designs.

Lesson Closure

Cars are something that students have contact with everyday. As an important part of the world's energy and carbon budget, cars provide an example of how engineers are working to solve the energy and climate change crisis. In the associated activity, Cars: Engineering for Efficiency, students explore the different aspects of energy efficiency by using gravity as a "fuel" and seeing firsthand how energy losses affect a car's energy efficiency.


Group Discussion: Talk about what sorts of cars students are familiar with. Ask about their sizes, shapes, tires and capacity. Ask whether the car is heavy or light, aerodynamic or boxy, able to carry many people or just a few.

Alternatively, take the class out to the school parking lot. Look at cars and discuss whether they look energy efficient and why. If the classroom has a view of a busy street, do this while watching cars drive by. Make a chart on the board with the data gathered and have students look for common themes.

Drawing: At the end of the lesson, have students sketch different car bodies. Have each student design one that is energy efficient and one that is not.

Lesson Extension Activities

Show students a picture of a semi-truck without a fairing. Ask them to think of ways to make the truck more aerodynamic. At the end, show them a picture of a truck with a fairing. Compare the two pictures and discuss the value and effectiveness of the wind-deflecting device.


Canright, Shelley. What is Aerodynamics? June 1, 2011. National Aeronautics and Space Administration. http://www.nasa.gov/audience/forstudents/k-4/stories/what-is-aerodynamics-k4.html
"Drive a Fuel-Efficient Car. Why? How?" Winter Tips, Department of Ecology, State of Washington. http://www.ecy.wa.gov/news/envirotips/tips_winter.htm
Green Vehicles and Making Cars More Fuel Efficient - A Student's Guide to Global Climate Change. Last updated April 13, 2011. US Environmental Protection Agency. http://www.epa.gov/climatechange/kids/solutions/technologies/vehicles.html


Eszter Horanyi and Janet Yowell


© 2009 by Regents of the University of Colorado.

Supporting Program

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


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.