A roller coaster works the same way as a bicycle coasting down a hill. When you ride your bike to the top of a hill, you pedal to get there. Then, to coast down the hill, you take your feet off the pedals and glide down the other side. If the slope is steep enough, you can go very fast. Similarly, a roller coaster is only powered at the beginning of the ride, when the coaster, or train, is pulled up the first hill. When it goes over the top of the hill, the weight of the train itself, pulled downward by gravity, is what keeps the entire unit moving.
There are no cables that pull the train around the track. This conversion of potential energy (stored energy) to kinetic energy (the energy of motion) is What drives the roller coaster, which often reaches 60 miles (96.5 kilometers) per hour. Running wheels guide the train on the track, and friction wheels control the train’s movement to either side of the track. A final set of wheels keeps the train on the track even if it is upside down. Air brakes stop the car as the ride ends.
Although a helicopter doesn’t have wings like an airplane, it uses the same principle of lift to rise and maneuver in the air. The blades of a helicopter’s propeller-like top rotor are shaped just like a plane’s wings—flat on the bottom and rounded on the top—and are likewise adjustable. Instead of rushing forward through the air like a plane does to gather enough lift to fly, a helicopter moves only its (three to six) rotor blades, which are attached to a central shaft driven by an engine.
The rotor blades slice through enough air—creating the changes in surrounding air pressure that produce lift—to achieve flight. Adjusting the angle at which the rotor blades are set helps control a helicopter’s lift and manner of flight. Because the angle of the rotor is adjustable, too, a helicopter has far greater maneuverability than an airplane: besides moving up, down, and forward, it can fly backward and hover in the air.
The word computer first appeared in the seventeenth century as the job title of a person who did calculations as an occupation.
Although slide rules were sometimes called computers, it wasn’t until the 1940s, with the development of massive electronic data machines, that the human occupation of computing became obsolete. These mechanical devices became known as computers.
NASA’s space shuttle, also called the Space Transportation System (STS), takes off from Earth like a rocket but lands like an aircraft. It cannot fly to the Moon, but is used to orbit Earth, where the crew can do scientific work, place satellites in orbit, and visit orbiting space stations. Usually five to seven crew members ride the space shuttles, which have all been launched from Kennedy Space Center in Florida. Six shuttles have been built: the first orbiter, Enterprise, was built in 1974 for testing purposes. Five others have gone into space: Columbia, Challenger, Discovery, Atlantis, and Endeavour.
The space shuttle Challenger disintegrated 73 seconds after launch in 1986, and Endeavour was built as a replacement. Columbia broke apart during re-entry in 2003. NASA announced that the space shuttle would no longer be used after 2010, and from 2014 on would be replaced by the Orion, a new space vehicle that is designed to take humans to the Moon and beyond.
Babies are born with about 300 to 350 bones, but many of these fuse together between birth and maturity to produce an average adult total of 206. Bone counts vary according to the method used to count them, because a structure may be treated as either multiple bones or as a single bone with multiple parts. There are four major types of bones: long bones, short bones, flat bones, and irregular bones. The name of each type of bone reflects the shape of the bone. The shape of the bone also tells about its mechanical function. Bones that do not fall into any of these categories are sesamoid bones and accessory bones.
First built in 1960 by American physicist Theodore Maiman, lasers are machines that produce intense beams of high-energy light. Laser light is more powerful than ordinary light because all its rays have the same wavelength and move together in exactly the same direction, allowing them to be focused in a narrow beam with great precision.
Laser light beams vary in strength, depending on the materials and amount of energy used to make them. Lasers can melt, burn, or cut through a variety of different surfaces, from hard metal to the delicate human body, which is why they are often used in surgery today. Lasers can be used to make precise measurements, to reshape corneas to correct poor vision, to transmit telephone signals, to guide weapons, and to read supermarket bar codes.
The Soviet satellite Sputnik 1, which was launched into space on October 4, 1957, was the first spacecraft to go into orbit around Earth. It had no crew members or animals aboard, but instead contained machines that sent information back to Earth via radio.
The former Soviet Union’s (now Russia) launch of Sputnik prompted the United States to get its first satellite, Explorer 1, into orbit quickly, igniting the so-called space race. This was the two countries’ rivalry over being the “first” in many areas of space exploration. Explorer 1’s test run in December 1957 burned on the ground, but the satellite was successfully launched into orbit around Earth on January 31, 1958.
The Earth is not round, but slightly squashed; it’s diameter at the equator (the imaginary line on Earth’s surface that divides Earth into a Northern and a Southern Hemisphere) is about 24 miles (38 kilometers) greater than its diameter at the poles. Why? Because the planet is constantly spinning, forcing material out toward the equator. The Earth’s surface is both smooth and bumpy, with vast oceans, tall mountains, rolling plains, canyons, swamps, and deserts.
The tallest mountain on our planet, Mount Everest in the Himalayas, stands at an altitude of 29,108 feet (8,872 meters), while Africa’s Sahara, the largest desert on Earth, spans over 2.1 million square miles (500,000 square kilometers) of land.
The good thing about fission-generated nuclear energy is that very little fuel is needed to produce huge amounts of energy. (Two pounds of nuclear fuel could produce as much energy as 6.5 million pounds of coal, for instance!) The challenging part is that the process must be very carefully controlled. (In a nuclear reactor, control rods that absorb neutrons are moved in and out of the core to control the process.) If it isn’t controlled, the result could be a build up of pressure within the reactor. If this continues, radioactive gases might be released along with steam. It was a situation like this that happened at the Chernobyl plant in the Soviet Union in 1986, resulting in radioactive pollution that still exists today. An uncontrolled nuclear reaction can cause harmful radioactive materials (such as iodine isotopes that can cause thyroid cancer) to be released into the environment. This by-product of nuclear fission is a problem connected with nuclear power. Nuclear reactors are encased in thick layers of steel and concrete to keep radiation from escaping.
And because leftover nuclear fuel is highly radioactive, it must be carefully stored far away from people for decades or even centuries before it is safe again. Transporting and disposing of dangerous waste is another challenge presented by nuclear power; at present, used fuel is sealed in safety containers and buried deep underground. The nuclear process that we get our power from is called fission, where atomic nuclei that break apart produce great energy and heat. But nuclear power can also be created by a process called fusion, where atomic nuclei join together. Scientists are still working on creating a satisfactory fusion reactor. The Sun produces its great energy and heat through the nuclear fusion of its hydrogen gases.
The brain is the body’s command center; everything we do—eating, talking, walking, thinking, remembering, sleeping—is controlled and processed by the brain. As the most complex organ in the human body, the brain tells us what’s going on outside our bodies (whether we are cold or hot, for instance, or whether the person we see coming toward us is a friend or a stranger) as well as what’s going on inside our bodies (whether we have an infection or a broken bone, or whether we feel happy or sad).
The key to the body’s nervous system, the brain contains between 10 billion and 100 billion nerve cells, or neurons. Neurons combine to form the body’s nerves, thin cords that spread from head to toe and all parts in between. Neurons take in and send out electrical signals, called impulses, that control or respond to everything your body does and feels. The brain is like a very busy, high-speed post office, constantly receiving messages and sending them out all the time; it handles millions of nerve impulses every second.