What is mechanical energy? How do we use it? Energy Energy

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1 You probably already have some idea what energy is. is easy to recognize. Yet it can be hard to describe. Where do you think you ve seen energy today? How do you think you will use energy tomorrow? Let s find out more! What is mechanical energy? How do we use it? is the ability to do work. is also needed to cause something to change. Neither statement gives a clear idea of what energy really is. The best way to understand energy is to look at each of the forms of energy all around you. One type of mechanical energy is the energy of motion. Another type of mechanical energy is stored energy. When you lift a book over your head, you give it stored mechanical energy. If you drop the book, the stored energy is changed to energy of motion as the book falls. This roller coaster is another example of the two kinds of mechanical energy. The car follows the tracks up and down over the hills. As this happens, mechanical energy changes back and forth between stored energy and energy of motion. Both kinds of mechanical energy are all around us. When a ball is thrown into the air, it has energy of motion. It also gains stored energy as it rises. When someone pulls back on the chains of a playground swing, the swing moves higher. The car has stored energy when it is stopped. When it is moving, it has mechanical energy. Think about where it has the most of each type. As the swing moves higher, it gains stored energy. As the swing comes back down, the stored energy changes to energy of motion. Stretching a rubber band gives it stored energy as well. What happens when you release the rubber band? The rubber band gains energy of motion as it flies through the air. 1

2 Raindrops fall from a cloud and run downhill into a stream. Water in the stream goes over the top of a waterwheel. This causes the waterwheel to spin. How many examples of stored energy and energy of motion can you find in these events? Remember that the stored mechanical energy of an object depends on its height. Think about the top of the waterwheel. What type of energy does the water have there? What is heat energy? How do we use it? Everything on Earth is made of particles much too small to see. These particles are always moving. The faster they move, the more heat energy they have. As something gains heat energy, its temperature rises. We burn fuels to release heat energy. Some common fuels are gasoline, coal, natural gas, and wood. We use the heat they release to heat our homes and cook our food. Heat from burning gasoline changes into mechanical energy. We use this energy to move our cars. Some fuels can produce the very high temperatures needed to make steel, glass, and bricks. particles: the tiny building blocks of all things We use heat from burning natural gas to cook food. 2

3 Temperature is not the same as heat energy. Remember, heat energy is caused by the motion of tiny particles. Temperature measures how fast the particles are moving. A small puddle and a big lake will have the same temperature when their particles are moving at the same speed. When particles begin to move, the lake will give off more heat energy than the puddle because the lake has more particles. What is light energy? How do we use it? Another form of energy is light energy. Some common forms of light energy are the Sun, light bulbs, and flames in a fire. Light travels in waves we cannot see. These waves can travel through glass, water, and even empty space. The light we see is made up of all the colors of the rainbow. When the colors are mixed together, the light appears colorless. When the colors are separated, we see a rainbow. This rainbow is made up of light waves we can see. Due to the size of the light waves, some light energy cannot be seen. However, these invisible waves are still useful. The energy in a microwave oven uses waves we cannot see, but that can cook our food. Doctors and dentists can see inside our bodies using X-ray machines. X-rays are other waves that we cannot see but that we know are there. 3

4 Mirrors can do strange things. Find two large mirrors. Place them about three or more meters apart. The mirrors should exactly face each other. Stand between the mirrors. When you look into one mirror, you should see the other mirror. How many reflections of yourself can you see? What do you think is happening to the light waves as they move between the mirrors? Getting Technical: Optical Fibers Today, people use lots of optical fibers to send messages. These fibers are not much thicker than a human hair. They are very small tubes made of glass or plastic. When light enters one end of the fiber, it passes down the tube because it is reflected back and forth inside the walls. This works even if the tube is bent. You can think of an optical fiber as a light pipe. Signals can be sent through a fiber as pulses of light. Phone conversations and TV programs can be changed into light pulses. These pulses can be sent long distances through optical fibers. This has many advantages over sending electrical signals through metal wires. Optical fibers are lighter and smaller than metal wires. They can carry much more information. They also use less energy. What is sound energy? How do we use it? Sound, like light, travels in waves. The length of a sound wave determines the sound s pitch. Pitch is a measure of how high or low a sound is. A whistle is high pitched. A bass drum is low pitched. Do you think sound can bounce? It sure can! Think of an echo if you yell into a canyon. That echo is just your sound that bounced back to you. Optical fibers transmit light. Sound energy and light energy also have some important differences. Light travels much faster than sound. That is why we see a lightning flash before we hear the thunder. Also, light can travel through empty space, but sound cannot. Sound needs a material like air or water to carry it. Suppose you were in a space station traveling through empty space. If a supply rocket from Earth pulled up beside the station, you would never hear it coming. You can see sound waves using special tools. Long waves produce sounds with different pitches than short waves. 4

5 Sound is how most people communicate in everyday life. Strings located in our throats vibrate. This causes air particles to vibrate. These vibrations are sound waves carried through the air. Sound waves enter other people s ears and cause their eardrums to vibrate. These vibrations send signals to our brains. Our brains recognize the vibrations as words. What Do You Know? The short story below describes five forms of energy. As you read the story, draw a line from each energy form to the place in the story that describes it. Draw at least one line for each energy form. You may draw more lines if you see an energy form mentioned more than once. Stored Mechanical Mechanical of Motion It was a dark and stormy night. Lightning flashed and thunder boomed. Deep in the mountains, a girl and her parents huddled around their campfire for warmth. Suddenly a bolt of lightning struck the top of a nearby mountain. The lightning strike loosened a large boulder on top of the mountain. The boulder began rolling down the steep slope toward the family s camp. Look out! shouted the girl. Heat Light Sound 5

6 Exploring Echoes Take your child to a surface that will produce an echo. The surface should be as vertical and flat as possible for example, a sheer cliff, a high wall, or a tall building and the spot should be deserted so that ambient sounds do not interfere with the echo. (You can also try an empty gymnasium or cafeteria.) With your child, stand about 100 meters from the surface (or as far from the surface as you can, if 100 meters is not possible). Instruct your child to clap or to shout loudly, and listen for the echo. (To produce a sharper, more consistent sound, you may wish to bang two drumsticks together rather than clap or shout.) Ask your child to explain what causes the echo. Your child should know sound is produced by waves of energy; as the energy moves through the air, it causes air particles to vibrate. If the waves strike a surface, they will reflect, or bounce, back in the direction they came from. Next, stand directly in front of the surface with your child. Ask your child to predict whether you will hear an echo if you clap or shout (or bang the sticks together) while standing this close to the surface. Then, instruct your child to make the sound and listen you will not hear an echo. Walk backward a step at a time, making the sound each time, until you can hear the echo. Ask your child to try to explain why you need to be a certain distance from the surface to hear the echo. (After you hear a sound, the sound lingers in your ear for about 0.1 seconds. During this very brief time, you cannot distinguish between the original sound and an echo. Sound waves travel through air at approximately m/s, depending on the temperature of the air. So, you will hear an echo only if the sound wave travels more than 1/10 of this distance between the source of the sound and your ear.) Here are some questions to discuss with your child: 1. What is similar about the ways that light and sound travel? 2. What is different about the ways that light and sound travel? 3. What causes an echo? (Or simply: What is an echo?) 4. Why do you have be a certain distance from a reflecting surface to hear an echo? 6