Lesson Move It!
Conservation of Energy & Energy Transfer in Crashes

Learning Objectives

After this lesson, students should be able to:

• Identify the difference between kinetic and potential energy.
• Explain how energy is transferred in an impact situation such as a car crash.

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

International Technology and Engineering Educators Association - Technology

State Standards

Introduction/Motivation

(Start with a class demonstration. Drop an easily breakable object, such as an egg, into a safe container, like a transparent bucket, making sure all students can see what happens.) Why did the egg break? What caused the object to move in the first place? (Let students explain what they know about this. Expect some to say that gravity caused the egg to fall and that eggs break easily.) We can understand why it broke by understanding mechanical energy.

What happens in a car accident? For a car moving at around 35 mph, the driver is also moving at 35 mph. However, in the event of an accident, the driver inside must slow down to zero mph as smoothly as possible. Otherwise, the result would be the same as flinging a body at 35 mph at a brick wall... ouch!

In today's lesson, we will look events, such as a car crash in this case, and examine the different ways energy is transferred in the systems. From an engineering standpoint, situations such as these are interesting because seatbelts, force limiters, airbags and even the chassis of a car all work together to keep the driver and passengers safe in a car accident.

Lesson Background and Concepts for Teachers

The US Department of Energy (DOE) provides basic information on energy suitable for students and is a good source of further information on the subject. http://www.eia.doe.gov/kids/energyfacts/science/formsofenergy.html

This lesson covers the law of conservation of energy. Energy can neither be created nor destroyed. This concept is further described on the DOE website. http://www.eia.doe.gov/kids/energyfacts/science/formsofenergy.html#conservation.

Potential Energy - Any energy that is stored is potential energy. Batteries, springs, and rubber bands are examples of stored or potential energy. In the associated activity Bombs Away! Egg Drop Experiment, height provides gravitational potential energy. For instance, a ball 1 foot above the ground possesses less potential energy relative to the ground than the same ball located 20 feet above the ground. Mechanical potential energy is energy related to the position or location of an object not the energy of motion itself. The amount of potential energy an object has is also related to its mass. For example, it is harder to lift a bowling ball 5 feet off of the ground than it is to lift a tennis ball 5 feet off of the ground.

Kinetic Energy - Energy associated with a moving object and is directly related to potential energy. Potential energy is transferred into kinetic energy when an object is let go from a particular height and begins moving. Any object in motion has kinetic energy. You may want to bring in various objects of different sizes and weights and drop them to demonstrate how the differing amounts of potential energy are transformed into various amounts of kinetic energy as the objects accelerate towards the ground. Similar objects of different weights demonstrate this most effectively as they will fall at the same speed but make different sounds as they hit the ground. A heavier object will make a louder sound than a lighter object. This sound can be a qualitative measure of how much kinetic energy the object has when it hits the ground.

In short, potential energy is energy that depends on the position or location of a mass, and kinetic energy is energy associated with the velocity of a mass. Students should be able to look at a system configuration and identify the types of energy associated with objects in the system and describe the probable energy transfer that can occur between different objects within the system. Remember, due to the law of conservation of energy, energy can neither be created nor destroyed. Mechanical energy in a car crash could be summed up as follows:

1. The car contains gasoline, which has energy stored as chemical potential energy.
2. The engine burns gasoline and changes it into kinetic energy. This is transferred to the car's axles in the form of rotation, causing the wheels to turn and the car to move.
3. When the car hits a stationary, immovable object (wall/pole/tree), the chassis crunching converts the kinetic energy of the car moving to heat energy due to the bending of the chassis material. The kinetic energy of the passenger must be dissipated in a smooth fashion. Seatbelts and airbags are the primary systems for this. When a passenger's head hits an airbag, kinetic energy is transferred to the airbag and then converted into heat as it stops moving. The seat belts hold the passenger in place and allow the front and rear portions of the chassis to dissipate the energy as smoothly as possible. Note: If you have trouble imagining how kinetic energy can be transferred to heat energy just by bending metal, try bending a paperclip repeatedly. If you use a larger paperclip and bend it repeatedly in the middle of a straight portion, you will be able to feel a little heat created from bending the metal. The same is true of a thick rubber band, but if this is used as a demonstration, be sure to warn students not to snap themselves in the face with the rubber bands.
4. The car crash scenario is further explained in this HowStuffWorks article on seatbelts, http://auto.howstuffworks.com/seatbelt.htm.

Other situations can be analyzed similarly. Determine the types of energy present at the beginning of a scenario and then determine what happens to that energy. For more information on types of energy, look at the Department of Energy page: http://www.eia.doe.gov/kids/energyfacts/science/formsofenergy.html#forms

HowStuffWorks provides another good discussion of the terminology important in this lesson and activity, specifically, a discussion of the terms mass, force, energy and kinetic energy. http://science.howstuffworks.com/fpte.htm

Associated Activities

• Bombs Away! Egg Drop Experiment - Students are challenged to design devices to protect a falling egg, representing a disaster relief supply package drop.

  Watch this activity on YouTube

Lesson Closure

Review with the class what was learned in the lesson:

• Energy comes in different forms; in this lesson, we looked at the mechanical energy of impact.
• Potential and kinetic energy are forms of mechanical energy.
• In general, why does a person in a car have MUCH more kinetic energy than a person on a bicycle? A person riding in a car usually has a greater velocity. This is a challenge for automotive engineers whose jobs involve providing adequate safety to car passengers.
• Seat belts, airbags and force limiters are all devices designed to reduce the force due to rapid deceleration (slowing down) that people feel when in car accidents.

Vocabulary/Definitions

acceleration: The rate of change of velocity with respect to time. The measure of how fast the velocity of an object increases or decreases.

energy: The capacity to do work. There are different types of energy including mechanical, heat, electrical, magnetic, chemical, nuclear, sound, or radiant. The energy dealt with in this lesson and associated activity will be primarily mechanical energy since it is the energy of motion.

force: Anything that tends to change the state of rest or motion of an object. Force is represented by two quantities; its magnitude and direction in space. The magnitude of a force is represented by quantities such as pounds, tons, or Newtons. Direction in space refers literally to the direction a force is applied. This means that force is a vector and requires two (2) pieces of information to define it completely. When a number of forces act simultaneously on an object, the object moves as if acted on by a single force with a magnitude and direction that are the sum of the applied forces.

impact: The striking of one object against another; collision.

kinetic energy: The energy possessed by an object because of its motion.

mass: A measure of how much matter an object contains, or the total number of particles in an object. Mass is not weight. Weight is the force caused on a mass by gravity. Therefore, your mass would not change on different planets, but your weight would. For instance, you would weigh about 1/6th of your body weight now if you were on the moon.

potential energy: The energy of a particle or system of particles resulting from position, or condition. Gravitational potential energy is based on how high off of the ground an object is while other forms of potential energy can include a spring, a battery, or fuel.

vector: A quantity that has both magnitude and direction. Examples of vector quantities include velocity, weight and force. Alternatively, speed and mass are NOT vector quantities and can be represented by their magnitude.

velocity: A vector quantity whose magnitude is a object's speed and whose direction is in the object's direction of motion. Velocity is different from speed because velocity describes a direction as well.

Assessment

Question: Ask students to identify the potential and kinetic energy of a skateboarder or a snowboarder on a half-pipe.

Real-World Challenge: Ask students how they might transport supplies to a hard to reach disaster area, or a trapped military team, with no roads or landing airstrips for planes or helicopters. After a bit of discussion, introduce (if it has not been considered yet) the idea of dropping supplies out of airplanes. Ask students to use their imaginations to determine what kind of problems could occur when dropping supplies (damaged supplies, supplies landing on people, supplies not arriving where they are supposed to, food bags exploding, etc). Let students know that the U.S. military and disaster relief groups have been dealing with the same issues! Ask students what kind of energy and energy transfers cause these problems. The associated activity, Bombs Away! Egg Drop Experiment, asks students to design and build a device to allow supplies to land safely and accurately in a pre-determined location.

Lesson Extension Activities

• Find a Safe Car - Research the safety options on your family car or cars people driving in your community. Refer to https://www.nhtsa.gov/ratings as a helpful tool to compare automotive safety options. Are the cars you researched safe? Why or why not?
• How do Seat Belts Work? - Research how seat belts work in conjunction with force limiters to protect drivers and passengers.
• How do Airbags Work? - Research airbags. When were the first airbags used? Do airbags provide adequate safety?
• How are supply drops made? - Have students research how the military and relief groups drop supplies.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.