Mechanical energy is the most easily understood form of energy for students. When there is mechanical energy involved, something moves. Mechanical energy is a very important concept to understand. Engineers need to know what happens when something heavy falls from a long distance changing its potential energy into kinetic energy. Automotive engineers need to know what happens when cars crash into each other, and why they can do so much damage, even at low speeds! Our knowledge of mechanical energy is used to help design things like bridges, engines, cars, tools, parachutes, and even buildings! In this lesson, students will learn how the conservation of energy applies to impact situations such as a car crash or a falling object.

This lesson covers one of the fundamental principles of engineering and applied physics, the Law of Conservation of Energy, a key concept in many areas of engineering. During the associated activity, Bombs Away!, students use energy concepts just as engineers do to design a device to cushion impact and protect an egg when dropped. force mass velocity potential energy kinetic energy acceleration gravity conservation of energy Students should be able to identify the difference between kinetic and potential energy. Students should be able to explain how energy is transferred in an impact situation such as a car crash. Explain to the students that you are going to do a demonstration. Drop an easily breakable object (e.g. an egg) into a safe container, like a bucket, making sure all the students can see what happens (you might consider using a clear bucket). Ask the students why the object broke. Ask the students what caused the object to move in the first place. They will likely answer that gravity caused the egg to fall and say that eggs break easily. After they have explained what they know about this, tell them that we can understand how and why it broke by understanding mechanical energy. Next, ask the students what they think 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 0 mph as smoothly as possible. Otherwise, the result would be the same as flinging a body at 35 mph at a brick wall…ouch! The purpose of the lesson will be to look at an event (here a car crash) and examine the different ways energy is transferred in the system. From an engineering standpoint, such a situation is interesting because seatbelts, force limiters, airbags, and even the chassis of the car all work together to keep the driver and passengers safe in a car accident. 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. The following website provides a brief explanation of how the law of conservation of energy applies to a pendulum without friction. http://library.thinkquest.org/2745/data/lawce1.htm. 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, 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: The car contains gasoline which has energy stored as chemical potential energy. 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. 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 passengers 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. The car crash scenario is further explained in the 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 Force is 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. 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. 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. 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. 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. 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. The energy possessed by an object because of its motion. The striking of one object against another; collision. The rate of change of velocity with respect to time. The measure of how fast the velocity of an object increases or decreases. Bombs Away! Energy comes in different forms; in this lesson, the mechanical energy of impact was examined. 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 job involves providing proper safety to passengers riding in a car. Seat belts, airbags, and force limiters are all devices designed to reduce the force due to rapid deceleration (slowing down) that a person feels when in a car accident. Ask students to identify the potential and kinetic energy of a skateboarder or a snowboarder on a half-pipe. For more information on half-pipes, see About.com. 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 United States 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, asks students to design and build a device to allow supplies to land safely and accurately in a pre-determined location. Find a Safe Car - Research the safety options on your family car or cars you see people driving in your area. The website, www.crashtest.com, may be 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 the driver and passengers. How do Airbags Work? - Research airbags. When were the first airbags used? Do airbags provide adequate safety? How do they do supply drops? - Have students research how the military and relief groups drop supplies. Crashtest.com Crashtest.com: www.crashtest.com Crashtest.com "How Force, Power, Torque, and Energy Work" "How Force, Power, Torque, and Energy Work", How Stuff Works: www.howstuffworks.com "How Force, Power, Torque, and Energy Work" "How Crashes Work" "How Crashes Work", How Stuff Works: www.howstuffworks.com "How Crashes Work"