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Article 1 - Section 1

Spin keeps an object steady and stops it from simply falling into the water. The stone must have a certain speed or it will hit the water and sink. Flat, round stones are best for throwing. Their surface area creates a bounce when they meet the water. The best angle between a spinning stone and the water is about 20 degrees. This allows the stone to make the greatest number of skips.

• The Perfect Combination for Skipping Rocks

• The Best Angle for Skipping

Article 1 - Section 2

People have been skipping stones for thousands of years. The ancient Greek writer Homer mentioned stone skipping in a book. So did William Shakespeare, who wrote plays in the 1500s. Some Alaskans skip rocks on ice, while some Bedouin Arabs skip rocks on smooth sand. Scientists have used tools to study stone skipping. The tools record the motion using video. They show that stones usually skip a certain way. For example, the distance between each skip is about four-fifths of the last skip. In other words, the skips get a little bit shorter as they go along. But it is still possible for a skip to be longer than the last one. This may happen because the surface of the water is uneven.

• Famous People Who Skipped Stones

• Stone Skipping Through the Centuries

Article 1 - Section 3

A stone's travel through the air is considered ballistic. This means it is driven by its own moving force and the force of gravity. However, the way a stone interacts with the surface of water is different. Each time the stone skips the surface of the water it is sent upward. Its speed is reduced. Each bounce that follows slows it down. Eventually the stone breaks through the water's surface rather than skipping over it. Other things can also affect how many skips happen and how quickly the stone splashes down. For example, the height that the stone is thrown from is important. So is the angle that it's thrown from.

When the stone hits the water, the water pushes it back up. This force is equal to the pressure of the water multiplied by the area of the stone's surface. For the stone to bounce, it has to be moving at a certain speed. If it is going too slowly, it will sink.

• What Determines the Number of Skips?

• Does Speed Matter?

Article 2 - Section 1

Why don’t I fall out when a roller coaster turns upside down? Inertia is what keeps you from falling out. Inertia is a resistance against a change in direction. It keeps you pressed against the bottom of the car with a force stronger than gravity.

Have you ever wondered how roller coasters stay on their tracks and why people can hang upside down in them? It is all a matter of different forces and different kinds of energy acting together. Energy is the ability to do work. It is a kind of power.

• Inertia Makes it Work

• Forces and Energy Act as a Team

Article 2 - Section 2

A roller coaster does not have an engine. A cable pulls it up the first hill it climbs. As the coaster goes higher and higher, it builds up stored energy. Stored energy is also called potential energy. This energy will be used to go down the hill as the train is pulled by gravity.

Then, at the bottom of the hill, all of that stored energy is released as kinetic energy. Kinetic energy is the energy that builds up when a body or object is moving. It is what gets the train to go up the next hill. This type of energy pushes the coaster to the top of the next hill. Then the process repeats all over again. So, as the train travels up and down hills, its motion is constantly switching between potential and kinetic energy.

Most roller coasters are either wooden or steel. Wooden tracks are not as bendable as steel tracks. For that reason, they usually do not have complicated shapes, such as loops that flip passengers upside down. After steel tracks were introduced in 1959, more complicated and adventurous coasters became possible. Roller coaster wheels are designed to prevent the cars from flipping off the track. They secure the train to the track while it travels through fancy loops and twists.

• Potential and Kinetic Energy

• A Cable Helps it Climb

Article 2 - Section 3

Most roller coasters are either wooden or steel. Wooden tracks are not as bendable as steel tracks. For that reason, they usually do not have complicated shapes, such as loops that flip passengers upside down. After steel tracks were introduced in 1959, more complicated and adventurous coasters became possible. Roller coaster wheels are designed to prevent the cars from flipping off the track. They secure the train to the track while it travels through fancy loops and twists.

• It’s All About the Wheels

• Wood vs. Steel Tracks

Article 2 - Section 4

When you go upside down on a roller coaster, inertia keeps you from falling out. It pushes back when there is a change in direction. It is what presses your body to the outside of the loop as the train spins around.

Gravity keeps pulling you toward the earth when you go upside down, but inertia pushes you against the floor of the roller coaster car. This pushing force is stronger than gravity. The loop cannot be a perfect circle, though. If it was, the pushing force would be too strong for safety and comfort. For that reason, roller coaster loops are elliptical. They are shaped like stretched-out circles.

• Not a Perfect Circle

• A Force Stronger Than Gravity

Article 2 - Section 5

The earliest version of the roller coaster was a Russian sled ride from the 1400s. It was called Russian Mountains. La Marcus Thompson built the first American roller coaster. Known as the Switchback Railway, it opened at Coney Island in Brooklyn, New York, in 1884. One of the first high-speed coasters was the Drop-The-Dip. It opened at Coney Island in 1907, and it was the first roller coaster to use seat belts. In 1975 Knott's Berry Farm in Buena Park, California, introduced the Corkscrew. It was the first coaster to turn passengers completely upside down. The world's tallest and fastest steel roller coaster is the Kingda Ka. It is located at Six Flags Great Adventure in Jackson Township, New Jersey. Kingda Ka is 456 feet tall. It travels at a speed of 128 miles per hour.

• Ground Breaking Coasters

• The Fastest Coaster