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  #11  
Old Wednesday, September 09, 2009
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Default X Rays

X RAYS

Imagine that you could see right through your own skin. You could see the bones inside your body. You could watch food go down your throat when you swallow it. Imagine looking inside someone’s suitcase to see what’s inside. Does that sound impossible? Not when you know about X rays!

WHAT ARE X RAYS?

X rays are very powerful light rays that your eyes can’t detect. These light rays can slip through objects that visible light bounces off. We use X rays as a powerful tool to detect and discover things our eyes can’t see.

HOW WERE X RAYS DISCOVERED?

X rays were discovered by accident. In 1895, a man named Wilhelm Roentgen was experimenting with electricity in vacuum tubes in a black cardboard box. He noticed that a special screen he had nearby glowed when electricity went through the tubes. He experimented more and determined that invisible light rays from the tubes caused the screen to glow. These rays went right through the cardboard box! He named the invisible light rays he had found X rays.

WHAT ARE X RAYS USED FOR?

Just a few years after X rays were discovered, doctors were already using them to find bullets inside people who had been shot. Doctors later began to use X rays to find out if people are sick or have broken bones. Dentists use X rays to check up on people’s teeth.

An X-ray device called a CAT scan rotates around a person and creates a 3-D picture of the person’s insides on a screen. This device gives doctors clear views inside any part of the person’s body.

Scientists who study matter and energy often use X rays in their research. X rays help them see what things are made of. Many chemical elements were discovered using X rays.

Industries use X rays to test products and materials for flaws such as cracks in an airplane wing. X rays are also used to tell whether gems and works of art are real or fake. Border guards use X rays to look inside cars and containers. The X rays can find goods that are being smuggled from one country to another. Airports use low energy X rays to see inside luggage and check for dangerous items.

DOCTORS’ X RAYS

When a doctor takes an X ray of you, the X-ray machine shoots X rays at you. Most of the rays go through you and into a special film, which catches them. Some of the X rays that hit your bones, however, don’t make it through you. Bones absorb X rays more than other parts of your body. Because X rays absorbed by your bones never make it to the film, lighter areas appear on the film where your bones are! These lighter areas provide a picture of the bones.

X rays can be harmful. Doctors use X rays to kill cells that are harmful to people, such as cancer tumors. Because too many X rays can be harmful, doctors warn that X rays should be used only when necessary.
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Default Solar Energy

SOLAR ENERGY

Imagine a source of energy more powerful than a million electric power plants. And imagine that this energy source will never run out—at least not for a few billion years. This energy source is not imaginary. It’s the Sun! Solar energy shines down on us every day.

Solar energy is produced inside the Sun. It is the source of nearly all energy on Earth. This energy is stored in the ground, the oceans, and the wind. Even fossil fuels, such as oil and natural gas, come from ancient plant life that once soaked up sunlight. Today we use solar energy to heat buildings and produce electricity.

HOW DO WE USE SOLAR ENERGY?

You may have seen solar collecting plates on top of buildings. They are thin, flat boxes. The solar collectors capture the Sun’s energy. Sunlight heats air or water flowing through tubes in the boxes. The tubes carry the heat into the building.

Most of the Sun’s energy does not reach Earth’s surface. It is scattered and absorbed by the atmosphere, especially by clouds. That’s why you usually find solar-heated houses in areas that get lots of sunlight. Even in sunny places, it takes a lot of collecting plates to heat a house. Sometimes, not enough solar energy can be stored for use at night or on cloudy days. So the house needs an ordinary water heater and furnace, too.

There are different kinds of solar collectors. Concentrating collectors are much more powerful than flat-plate collectors. Concentrating collectors use curved mirrors to focus the Sun’s energy. They follow the Sun as it moves through the sky. They can produce temperatures high enough to boil water. They can be used to produce electricity.

ELECTRICITY FROM SOLAR ENERGY

We use small amounts of electricity from solar energy today. A photovoltaic cell is a kind of battery. It produces an electric current from solar energy. Tiny photovoltaic cells power watches and calculators. They provide electricity to satellites in space. Many photovoltaic cells linked together can produce enough electricity for an entire house.

Generating large amounts of solar power is more difficult. Power plants that burn oil or coal can produce electricity more cheaply than a solar power plant can. There are very few solar-energy power plants operating today.

SOLAR POWER IN THE FUTURE

It will become cheaper to produce electricity from solar energy as technology advances. Fossil fuels will become more expensive as they begin to run out. Solar-energy plants could become more common, once they can produce energy more cheaply than other types of power plants.

Photovoltaic cells can be used to power cars. So far, such cars are only experimental. But in 2003, a car was driven nearly 2,500 miles (about 4,000 kilometers) across Australia using only solar power.

Some scientists have proposed building solar-energy stations in space. These stations would collect energy from sunlight almost 24 hours a day. Then the energy could be beamed to Earth. But for now, such a system would be far too expensive to be useful.
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Default Internet

INTERNET

Do you play computer games over the Internet? Do you surf the Web? Do you send e-mail messages to your friends? You can get all kinds of information on the Internet. People use the Internet to work at home. Scientists use the Internet to help them do research. The Internet has made big changes in the way many people live and work.

WHAT IS THE INTERNET?

The Internet is a system that connects computer networks. The Internet links millions of computers all over the world. It allows your computer to get information stored on other computers far away. Some networks have only a few computers. Some networks have thousands of computers. Computers connect to the Internet through telephone and cable systems.

Many governments, big companies, and other organizations have intranets. The computers on an intranet are hooked up to the Internet. But only people who work for the organization that owns the intranet can use it. Other people on the Internet cannot see what is on the intranet computers.

WHERE DID THE INTERNET COME FROM?

The Internet grew out of a computer network called ARPANET. The United States military created ARPANET in the 1960s. From the 1970s until the late 1980s, the U.S. government only let a few scientists and people in the military use it. In the 1980s, the government let networks at universities join with ARPANET to create the Internet. The Internet grew quickly. Schools, libraries, local and state governments, companies, and families were on the Internet by the mid-1990s.

At first, it was hard to get information from the Internet. You could only see words and numbers on your computer screen. Then a British computer scientist named Timothy Berners-Lee created the World Wide Web in the 1980s.

THE WEB AND THE INTERNET ARE DIFFERENT

The difference between the Internet and the Web is sort of like the difference between highways and a delivery service. Delivery service trucks use highways to move packages from one place to another. The Web is like the delivery service. The Internet is like the highways. Information traffic from the Web travels over the Internet.

The Web is made of places called sites. People use special computer programs to make the sites. The sites are stored on computers called Web servers. Each site is made up of documents called Web pages. These Web pages can have text, pictures, sounds, and videos.

You need computer software called a Web browser to find and see Web pages. Each Web page has a URL (Uniform Resource Locator). The URL is like an address that the browser looks for. An example of a URL is: http://www.encarta.com/.

Many computer experts think that the Internet became so popular because of the Web. The Web is easier to use than the Internet by itself. By the end of 2000, more than 80 percent of all traffic on the Internet highway came from the Web.

THE INTERNET IS GROWING

Millions of people use the Internet every day. In 1981, only 213 computers were connected to the Internet. By 2003, more than 216 million computers were connected to the Internet.

No one knows for sure exactly how many people use the Internet. Computer experts thought that there were 61 million Internet users worldwide at the end of 1996. There may have been from 700 million to 900 million users by the end of 2003.

HOW TO GET ON THE INTERNET

You get on the Internet by joining a computer network. The network that you join is called an Internet service provider (ISP). America Online (AOL), Earthlink, and Microsoft’s MSN are popular ISPs. You pay a fee to the ISP just as you pay a phone company to use their telephone system.

The company that owns your ISP sends you software to install on your computer. The software lets you use the ISP’s network to get on the Internet. The ISP also gives you an e-mail address.

There are different ways to connect your computer to the ISP. You can hook up your computer with a modem and your home telephone line. This is called dial-up access. You can hook up to the ISP with a digital subscriber line (DSL) or a cable modem. A DSL uses the same wires as your telephone. A cable modem uses the same wiring that cable television uses. DSLs and cable modems bring Web pages to your computer screen much faster than a dial-up connection.

DSLs and cable modems are called broadband connections. Many computer experts think more people must get broadband connections in order for the Internet to continue growing.
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Default Satellites

SATELLITES

How does a television signal get to the other side of the world in seconds? What tells ships exactly where they are in the middle of the ocean? How do we get warning that storms are coming? Satellites do all these things and more.

WHAT ARE SATELLITES?

Satellites are objects in outer space that fly around planets in circular paths called orbits. Artificial satellites are made by people. Thousands of satellites are zooming around our planet right now.

The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Sputnik 1 broadcasted a steady signal of beeps. It circled Earth for three months and then fell back into the atmosphere and burned up. The atmosphere is the air that surrounds Earth.

HOW DO SATELLITES GET INTO SPACE?

Satellites need to reach a height of at least 120 miles (200 kilometers) to orbit. They also need to travel faster than 18,000 miles per hour (29,000 kilometers per hour). A satellite any lower or slower would soon fall back down to Earth. It takes a rocket to bring satellites up to that height and speed.

Most satellites are launched from the ground. Some small satellites can be launched from high-flying planes. This uses less fuel.

Other satellites are launched using a space shuttle or other piloted rocket. This way, astronauts on the space shuttle can make sure the satellite is working and gets into the right orbit.

WHAT ARE SATELLITES USED FOR?

Satellites are used for a great many things. Communications satellites beam TV, radio, and telephone signals all around the world. Navigational satellites help people know where they are and get where they are going. Weather satellites take pictures of clouds and storms from above to help make weather forecasts. Spy satellites look down and snoop on other countries. Other satellites help scientists to study Earth and other planets.

HOW DO SATELLITES WORK IN SPACE?

Space is a difficult place to be. You can’t plug in a cord in outer space, so satellites need to take a power source with them. It’s hard to get satellites pointed in the right direction because there’s nothing to turn them with. Satellites need to work in the freezing cold of Earth’s shadow as well as in the blazing heat of the Sun’s rays. They also need to be tough enough to survive collisions with tiny asteroids (space rocks)!

Most satellites use both power from the Sun and batteries to work. They catch the Sun’s energy using large flat solar panels. Satellites keep these panels pointed at the Sun. They use batteries when the Sun doesn’t shine on them.

Satellites can stay pointed in the right direction using small rockets called attitude thrusters. They can also use instruments called gyroscopes. Sometimes magnets on board the satellite can push against the magnetic field of Earth to aim the satellite correctly.

No air flows past satellites to cool them. To keep from getting too hot in the Sun, satellites have panels that open and close. This lets heat escape. Satellites often spin so the Sun doesn’t make one side so hot that it melts.

Satellites also need to be made from strong materials in case tiny asteroids hit them. They need materials that don’t become brittle in the cold and the harsh radiation of space.

DO WE GET SATELLITES BACK FROM SPACE?

When satellites stop working they are often left in orbit as so much space junk. Others drift too low to keep orbiting and burn up as they fall. Still others are brought back to Earth for repairs.

Nonworking satellites are sometimes sent down from orbit into the atmosphere to burn up on purpose. Space is very large, but still scientists need to be careful that satellites don’t crash into each other. They try to get rid of the broken ones.

HOW MANY SATELLITES ARE THERE?

Since Sputnik 1, more than 5,000 satellites from many countries have been launched. Artificial satellites now orbit the Sun, Mars, Venus, and other planets and their moons. Most satellites, however, orbit Earth. High above your head thousands of satellites circle the planet every day.
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Default Laser

LASER

Lasers are powerful enough to cut through steel. Lasers are delicate enough to use in eye surgery. Lasers “read” the information coded on compact discs (CDs). These are just a few things that lasers do.

WHAT IS A LASER?

A laser is a device that produces a beam of light and makes the beam more intense. A laser beam is very exact. It can travel a long distance without spreading out and losing its power.

Laser light is unlike sunlight or light from a light bulb. To understand the difference, think of a crowded city street. Thousands of people are walking along the sidewalk. Their clothes are of many different colors. They are walking in many different directions. Now think of a marching band in a parade. Everyone is wearing a uniform of the same color. They are all walking in the same direction, in step with one another.

Sunlight and light from lamps are like the crowd on the sidewalk. They are made up of many colors—all the colors of the rainbow. They spread out in all directions from their source. Laser light is like the marching band. It is light of a single color. It travels in a beam. It spreads out very little, even when traveling through outer space.

HOW ARE LASERS USED?

Because lasers produce such exact beams of light, they are very useful tools. Some lasers are so powerful they can drill holes in diamonds. These lasers can cut a piece of steel to an exact shape needed for a machine part. Powerful lasers can produce temperatures of 10,000° Fahrenheit (5500° Celsius) and higher. This ability makes them useful in factories for joining together large pieces of metal.

Much smaller lasers “read” price tags on products. At the supermarket, you’ve probably seen the checkout clerk run foods over the laser scanner. Small lasers in compact disc (CD) players read the information coded on CDs. This information is then played back as music. Lasers even carry telephone conversations. Laser beams send thousands of telephone calls through thin glass threads called optical fibers.

In hospitals, doctors use lasers for delicate operations such as repairing damaged eyes. The military uses lasers to guide airplanes and missiles. And laser beams are used to create colorful light shows. These are just a few of the ways we use lasers.

WHEN WERE LASERS INVENTED?

Famous scientist Albert Einstein first suggested the idea of a laser in 1917. In the 1950s, scientists began making the idea work. American scientist Gordon Gould suggested the name laser in 1957. It was short for Light Amplification by Stimulated Emission of Radiation. The first working laser was built in 1960. It was built by another American scientist, Theodore Maiman. The development of lasers advanced rapidly during the 1970s and 1980s.

Scientists today are using a huge and powerful laser to study how atoms join together in the Sun to release energy. This laser was built at Lawrence Livermore National Laboratory in California. It takes up a building the size of a football stadium.
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Default Radar

RADAR

Radar was once a secret military weapon. In 1940 and 1941, radar helped Britain defend itself against German bombers during World War II. Today, radar has many uses. It prevents airplanes from colliding in midair. Radar is even found in many kitchens. Microwave ovens use radar technology to heat food.

WHAT IS RADAR?

Radar uses radio waves to locate airplanes, ships, storm clouds, and other large objects. A radar system sends out radio waves. These invisible waves move at the speed of light—about 186,000 miles (300,000 kilometers) a second. When the waves strike an airplane or other object, some of the waves bounce back. The waves that come back provide information to the radar operator. They tell how far away the object is, how fast it is moving, and what direction it is moving in. The word radar comes from RAdio Detection And Ranging.

WHEN WAS RADAR DEVELOPED?

Radar was developed during the 1920s. Scientists worked on military uses of radar in top-secret laboratories during the late 1930s and early 1940s. Radar became an important weapon during World War II. Radar stations on England’s coast located German bombers before the planes reached England. British fighter planes then attacked the bombers. Radar also helped locate German submarines before they could attack American and British ships.

HOW IS RADAR USED?

Airports use radar to keep track of planes in the air. In the control tower, air traffic controllers study radar displays that look like computer screens. Tiny dots on the screens show where planes are. The dots move as the planes move. Radar prevents collisions in the air and helps pilots land planes safely at airports. Airplanes carry radar to warn pilots if their plane is too close to another plane.

Have you seen a weather map on television? Did the weather reporter say it was a radar map? Radar locates rain, storm clouds, and moving masses of air. It can help predict thunderstorms and tornadoes.

Satellites orbiting Earth produce radar maps for predicting weather. Scientists have made pictures of Earth’s surface by using radar in satellites. During the 1990s, the Magellan spacecraft used radar to map the surface of the planet Venus. Magellan’s radar penetrated the thick clouds that surround Venus.

Police officers use radar to detect cars and trucks that are going faster than the speed limit. Stopping people from speeding improves traffic safety. But some drivers buy devices that detect radar waves. The devices warn drivers to slow down before they are caught speeding.

The military builds stealth aircraft and stealth missiles that can dodge enemy radar. Other aircraft and missiles are guided by radar systems.

Radar guns in baseball parks track the speed of pitches. That’s how the announcer knows the pitcher has thrown a 98-mile-per-hour fastball.
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Default Paper

Paper

What do a dollar bill, a cardboard box, and a book have in common? They’re all made from paper, of course!

Paper is one of the world’s most important and useful products. Without it, there would be no newspapers, magazines, writing paper, or greeting cards. There would be no paper bags or boxes, paper money, gift-wrapping, or toilet paper. Take a look around you. How many things can you see that are made from paper?

WHAT IS PAPER MADE FROM?

Paper is made from tiny fibers from plants. You can see the fibers at the edge of a torn piece of paper.

You can make paper from many types of plant fibers. Papermakers use fibers in straw, leaves, bamboo, sugar cane, and bark. Long ago, most papermakers used the fibers in cotton and linen rags. Today, most paper is made from wood fibers.

The most important trees used for making paper are softwood trees. Softwoods include pine, fir, hemlock, and spruce. The long fibers in softwoods are ideal for making many kinds of paper.

After paper is used, it can be reused, or recycled, to make new paper.

HOW IS PAPER MADE?

Paper is made in two stages. The first stage is to remove the fibers from the wood. This is done by grinding the wood or cutting it into chips that are softened with chemicals. The wood fibers are then mixed with water to make a souplike substance called pulp.

The second stage is to spread out the pulp, press it flat, and dry it. This makes the fibers stick together in thin sheets. Some paper is still made by hand. But most paper is made by machines at factories called paper mills.

WHAT ARE THE DIFFERENT TYPES OF PAPER?

Different kinds of pulp make different kinds of paper. Pulp made by grinding is called groundwood pulp. It’s inexpensive to make, but the grinding breaks the wood fibers into very short pieces. Groundwood pulp is used to make cheap papers, such as newsprint.

Pulp made using chemicals is called chemical pulp. The chemicals separate the fibers from each other but do not break them. Chemical pulp is used to make stronger, longer-lasting paper for use in fine books and magazines.

The best writing paper and stationary comes from cotton and linen rag fibers. Thin rag fibers are long, strong, and make very durable paper.

Fiber from recycled paper is used to make paper towels, napkins, and tissue. Paper for printing is treated with special chemicals so the paper won’t absorb ink and cause fuzzy lines that are hard to read.

HOW DOES A PAPERMAKING MACHINE WORK?

The main part of a papermaking machine is a wide belt made of tightly woven wire mesh. The belt moves in a loop, and it keeps moving all the time.

Pulp is poured evenly onto the belt at one end of the machine. As the belt moves along, water drains from the pulp. The fibers remain, leaving a mat of wet paper. The belt goes through metal rollers that squeeze out more water.

Now the paper is strong enough to be lifted off the belt. It passes between heated rollers that dry it completely. Finally, the paper is pressed tightly between cold metal rollers that make it smooth. The finished paper is wound onto large rolls or cut into standard sizes.

WHO INVENTED PAPER?

The ancient Chinese invented paper about 2,000 years ago. Chinese papermakers used fibers from tree bark and old rags. The art of paper-making spread out from China about 500 years later. It finally arrived in Europe about 900 years ago.

The invention of the printing press in the 1400s made books popular, and the demand for paper increased. All paper was handmade until 1798. That’s when a Frenchman named Nicholas Robert invented a papermaking machine that could make paper in continuous rolls.

Before the invention of paper, ancient people used many different surfaces for writing. They wrote on clay, wood, stone, and metals. More than 4,500 years ago, the ancient Egyptians made a paperlike material from a plant called papyrus. Papyrus reeds were cut into flat slices, layered, moistened with water, and pressed into sheets. The English word paper comes from the word papyrus.
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Default Dams

DAMS

Beavers build them from sticks. Landslides create them from trees, mud, and debris. Humans make them from earth and concrete. These structures are dams. Dams hamper the flow of water in a river or stream.

Landslides don’t mean to create dams. They do so by dumping a lot of earth and other stuff in a river. Scientists think beavers build dams for protection. Beaver dams capture water in front of the lodges in which beavers live. Beavers can hide from their enemies in this deeper water. A dam also protects the beaver lodge by slowing the river’s speed.

WHY DO WE BUILD DAMS?

We build dams to control water. A dam built across a river or stream stops the water’s flow. Water collects in a lake behind the dam. The lake stores water for people to use later. The lake, or water storage area, is called a reservoir.

The water in reservoirs travels in pipes to people’s homes for drinking water. It can flow through canals for farmers to use in watering their crops. People also sail boats and swim in reservoirs.

Many dams use reservoir water to produce electricity. Water flows into large machines called turbines inside the dams. The turbines power other machines that generate electricity. Electricity produced in this way is called hydroelectric power.

Some dams are built to prevent flooding. During the rainy season, the reservoir stores the river’s extra water. During the dry season, the dam sends the reservoir water back into the river.

HOW BIG ARE DAMS?

If you’ve ever visited a large dam, you know it is an amazing sight. Dams are some of the biggest structures ever built.

The Hoover Dam on the border of Nevada and Arizona is as tall as a 72-story building. The Grand Coulee Dam in the state of Washington contains enough concrete to build a sidewalk all the way around the Earth.

Few dams are this big, however. Most dams are small structures less than 10 feet (3 meters) tall.

WHAT ARE DAMS MADE OF?

Many large dams are made of concrete. Some are made of packed earth or rocks. Because these materials are not as strong as concrete, dams made of earth or rocks must be very thick. The Tarbela Dam in Pakistan is made of earth and rock. It contains more than 15 times as much material as the Grand Coulee Dam.

Dams must be strong enough to withstand the pressure of water against them. Dams also must be cared for and repaired. A dam that breaks can cause disaster. In 1889, a dam in Pennsylvania broke and let loose a wall of water. The water submerged the town of Johnstown, knocking down houses and killing more than 2,000 people.
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Default Rockets

ROCKETS

You hear a rumble and a roar. A blast of fire shoots out of a big rocket. The rocket heads up into the air. Maybe the rocket is carrying a satellite into orbit around Earth. Maybe the rocket is carrying a space probe to another planet!

Only a big rocket can make it into outer space. No other machine is as powerful as a rocket.

WHAT DO ROCKETS LOOK LIKE?

A rocket looks like a long tube. Most rockets have fins on the back end to help them fly straight.

Rockets that carry fireworks can be short, only a few inches long. They are usually made of cardboard.

Rockets that go into space are huge. They are made mostly of metal.

WHAT MAKES ROCKETS GO?

Rockets burn fuel. Many different chemicals can be used as rocket fuel. The burning fuel makes hot gases. The gases blow out of the bottom end of the rocket. The hot gases shooting downward make the rocket go upward.

You can see how a rocket moves by blowing up a balloon. Hold the end of the balloon tightly so the air cannot get out. Then let go. The air rushes out of the opening in the balloon. The air rushing out makes the balloon fly around.

A rocket, like a balloon, has a small opening. The opening in a rocket is called a nozzle. Hot gases blasting out of the nozzle make the rocket go.

HOW DO WE USE ROCKETS?

People use rockets to carry things through air and space. Different kinds of rockets carry different things.

Sounding rockets carry instruments to measure air pollution, rays from space, and weather. Lifesaving rockets carry ropes to ships stranded offshore. Distress rockets signal for help.

The most powerful rockets carry satellites and spacecraft into space. Many spacecraft use smaller rockets called thrusters to move around once they’re in space.

Rockets can also be used as weapons. The rocket weapons are called missiles. Most of the rockets made are missiles.

WHAT IS A MISSILE?

Missiles are rockets that carry bombs. The British used rockets carrying bombs against the United States in the War of 1812. The national anthem of the United States even has a line about the rockets: “And the rockets’ red glare ….”

Guided missiles have steering systems that guide them to destroy their targets. The smallest guided missiles can be carried by soldiers. The biggest guided missiles are huge. They can carry nuclear bombs around the world.

LAUNCHING A MISSILE

Missiles can be launched (fired into the air) from the ground, from airplanes, from ships, and even from submarines. Missiles can also be launched from bombproof underground tubes called silos.

Soldiers on battlefields launch small missiles out of tubes that they can carry on their shoulders. Special trucks carry ground-to-air missiles that aim at airplanes. Special racks underneath fighter planes carry air-to-air missiles.

HOW DO YOU LAUNCH A SPACE ROCKET?

Big rockets are launched from launch pads. A rocket stands on the pad next to a tall tower. The towers have elevators to take workers up and down. Gigantic tractors called crawler transporters bring big rockets or the space shuttle to a launch pad.

Controllers count the seconds before launch as they finish checking everything. “Five, four, three, two, one ….” Bridges that connect the tower to the rocket swing away. “Ignition!” The rocket engines fire. The spacecraft lifts off into the sky.

Firing one rocket does not always provide enough power to send a spacecraft far from Earth. The most powerful rockets often have different stages. Stages are separate rockets stacked on top of each other.

HOW DO ROCKET STAGES WORK?

Rockets headed for space must go really fast, about 25,000 miles per hour (40,000 kilometers per hour). They must go fast enough to overcome Earth’s gravity. Gravity is the force that pulls you back down to the ground when you jump up. Using more than one rocket stage is the best way to go really fast.

The bottom stage fires, uses up its fuel, and drops off. Then the next stage fires. Each stage takes the spacecraft faster and higher. The huge Saturn V rocket that sent Apollo astronauts to the Moon had four stages.

WHO INVENTED ROCKETS?

Chinese people probably invented rockets more than 1,000 years ago. By the end of the 13th century people in Asia and Europe also knew how to make rockets.

American physicist Robert H. Goddard researched new, more powerful kinds of rockets during the early 1900s. A German inventor named Wernher von Braun helped Germany make missiles during World War II (1939-1945). After World War II, von Braun helped the Americans make rockets. The Soviet Union also made rockets. Soviet scientists launched the first satellite into space in 1957.

The United States, Russia, and other countries made bigger and more powerful rockets. Rockets have launched spacecraft to the Moon and most of the planets.

Space engineers are working on better rockets. They are testing rockets that use nuclear power. They are trying to build rockets that get their power from beams of light.
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Default Simple Machines

SIMPLE MACHINES

You ride your bicycle to school. You run the flag up the flagpole. You use a shovel to dig. In all of these actions, you are using simple machines.

When you think of a machine, you probably imagine something that is made of metal and has moving parts, such as a car or washing machine. However, anything that changes the force, or effort, needed to do work is a machine.

There are four kinds of simple machines: levers, pulleys, wheels and axles, and inclined planes. No matter how complicated any machine is, it is made up of these four simple machines.

WHY ARE SIMPLE MACHINES USEFUL?

Simple machines help us by reducing the force necessary to move something. They reduce the force by increasing the distance over which the force is applied. If you want to open the lid of a paint can, for example, you wedge a screwdriver into the edge and pry the lid up. To lift the edge of the lid just a tiny bit, you must move the handle of the screwdriver a much greater distance.

LEVER

How easy do you think it would be to lift a friend 3 feet (1 meter) off the ground? Pretty difficult? But there is a simple way. Put your friend on a seesaw.

A seesaw is a type of lever. It is a rigid bar or plank with a central point, called a fulcrum. If you push down on one end of the lever, you get a force pushing up at the other end. The closer the load is to the fulcrum, the easier it is to lift the load. So if your friend weighs more than you, he or she should sit nearer to the middle of the seesaw.

Other examples of levers are nutcrackers, scissors, shovels, and tweezers. Even your arm is a lever—its fulcrum is your elbow.

PULLEY

A pulley is a wheel with a groove in the edge to guide a rope or cable. A single pulley does not decrease the force needed to lift something. It only changes the direction of the force. The pulley on a flagpole, for instance, lets you raise the flag by pulling down instead of up.

Two pulleys combined can decrease the force necessary to move something. Builders use a system of pulleys to lift heavy items such as bricks to the tops of buildings.

WHEEL AND AXLE

A bicycle wheel turns around a rod at its center, called an axle. Imagine trying to spin the front wheel of a bike by twisting the axle. It would be much easier to spin the wheel by grabbing its outside edge. By applying force on a wheel, you move the load at its axle. It takes less force (over more distance) to turn a wheel than it does to turn its axle directly.

Wheels and axles are found in many everyday objects, such as doorknobs, bathroom taps, and ceiling fans.

INCLINED PLANE

If you wanted to raise a car 3 feet (1 meter), how would you do it? Lifting the car straight up would be extremely difficult. Driving it up a ramp would be easy. A ramp is a slanted surface called an inclined plane.

Inclined planes decrease the effort needed to lift a load by increasing the distance over which the load is moved. Instead of moving the car 3 feet straight up, the car moves several feet forward as it goes up the ramp—farther, but easier.

A wedge is a double inclined plane. When you split wood, you apply downward force. The wedge-shaped axe head changes redirects the force sideways against the wood.

SIMPLE MACHINES WORK TOGETHER

All complex machines are made of combinations of simple machines. A car includes hundreds of simple machines. Even a can opener contains three. The hinged handle is a lever. The turning knob is a wheel and axle. And the sharpened cutting disk is a wedge.
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