Fasten your seat belt: This science can be a thrill

If you've ever ridden a roller coaster, you know how it feels to creep up a towering hill of steel, your train slowly, slowly squeaking its way to the steep drop ahead. You hold your breath and then, you're off - flying into a whirlwind of motion as you plummet and rise again into loop-the-loops and 90-degree bends.

But what you may not know is that, amid all the fun, you have experienced some basic laws of physics. Some of the most exhilarating aspects of an amusement-park ride - whether it's a roller coaster, a pendulum, or even a carousel - are the physical forces at play as the ride changes speed and direction.

Scientists describe the strength of these forces by comparing them to gravity. They use a phrase called "G-force," or G for short. G-forces measure the force you feel within Earth's gravitational field.

In zero G, you feel as though you have no weight at all. During a free-fall ride or a steep drop on a roller coaster, you may begin to experience this sensation.

At 1 G, you feel as you would standing on the ground. At higher G's, you feel increasingly squashed in your seat. Some coasters go all the way up to 6 G's - but only for a short time, since the higher the G-force, the more pressure is exerted on you.

When a roller coaster drops quickly, it is at the bottom of that drop that the G-force is highest. Sharp turns, acceleration, and quick slowdowns can cause G-forces to fluctuate.

Some roller coasters can move faster than 100 m.p.h. Did you know that a coaster can reach such speeds without even having an engine? Cars are pulled up the first hill, but then the coaster completes the ride on its own. This happens when potential energy (think of a compressed spring) converts to kinetic energy (actual moving energy) as gravity pulls the cars down the hill and the coaster's own inertia carries it to the end of the ride.

Inertia is the tendency of something to want to continue moving in the direction it is being sent. As a result, energy needed to complete the ride is there after the coaster ascends the first hill, according to Arthur Schmidt, a lecturer with the department of physics and astronomy at Northwestern University in Evanston, Ill.

Forces are similar on a pendulum ride. When riders are suspended in the air at the top of the arch, they can feel their weight pressing them against the restraining bars as gravity pulls them down.

"If your speed is too slow, you have to be strapped in - or else you would fall out of the pendulum," says physics teacher Robin McGlohn at Menlo School in Menlo Park, Calif.

But if the pendulum's speed is fast enough, G-forces accelerate you and the ride faster than gravity can pull you out. You stay in your seat, even without a restraining bar.

Centripetal force also comes into play on the pendulum ride. This is a force caused by circular motion, according to Mikell Lynne Hedley, a roller coaster expert at the University of Toledo.

Centripetal force pushes the swing around a center point, back and forth, until the energy is spent and the swing stops.

Carousels rely on some of the same laws of motion as other rides.

The spinning motion gently pulls riders outward due to inertia. Remember inertia is the tendency of something to want to keep moving in the direction it is going.

Think of an adult twirling you around and around. If that person were to let go, you'd appear to fly outward. Actually, you would just follow a straight line in the same direction you were going at the point at which you were let go.

When you are held by your hands, your feet still try to escape along a straight line as your body turns in a circle.

On a carousel, as the horses rotate, the ones farther out move at a faster linear speed than the horses near the center. This is because they cover more distance on the outskirts of the ride. It's a lot like racing around a track: If you are on the outside path, you must run faster to keep up with whoever is on the inside track. If the horses were racing in a straight line, the outer horses would win the race.

Sir Isaac Newton (1642-1727) was a mathematician who discovered many laws about motion and gravity. His third law of motion comes into play on the bumper cars. This law of interaction says that if one thing exerts force on a second thing, the second thing exerts a force equal in magnitude and opposite in direction on the first thing. It helps explain why you feel a jolt when you collide with another bumper car.

Galileo (1564-1642), an astronomer, mathematician, and physicist, first introduced the concept of free fall, which we experience on many rides as gravity pulls us toward the ground.

Galileo's experiments led to the finding that all objects free-fall at the same rate, regardless of their mass.

On the free-fall ride, riders are suspended in the air for a few minutes and then dropped. Just as Galileo and Newton explained in their theories of free fall, the least massive and most massive riders fall at the same rate of acceleration.

Moreover, the car they're riding in falls with the riders at the same rate of acceleration. There is no need for the car to exert any force on the riders in order for them to keep up with the ride. And that's why, Mr. Schmidt says, you feel weightless as you drop.

1. Mindbender, Galaxyland Amusement Park, Edmonton, Alberta. This ride reaches more than 5 G's and is the world's largest indoor triple-loop roller coaster.

2. Kingda Ka, Six Flags, Jackson, N.J.. Climbing 456 feet and reaching 128 miles per hour, Kingda Ka is the world's tallest and fastest roller coaster.

3. Mr. Six's Pandemonium, Six Flags, Agawam, Mass. Though it reaches the lower 3 G's, cars spin as they move up and down through hairpins and spirals, making this one of the most dizzying rides.

4. SheiKra, Busch Gardens, Tampa Bay, Fla. North America's first diving coaster climbs 200 feet before diving at 70 mph and at a 90-degree angle - twice. It also is the tallest dive coaster in the world.

5. Hades, Mt. Olympus Theme Park, Wisconsin Dells, Wis. Hades is a wooden roller coaster that has the world's longest underground tunnel and a 140-foot drop. It also features a 90-degree banked turn underground in total darkness.

Sources: Mikell Lynne Hedley, University of Toledo; Northwestern University, Evanston, Ill.; www.learner.org/exhibits/parkphysics, www.ultimaterollercoaster.com They reach awesome heights, travel at high speeds, and fall straight down. Some run almost entirely underground. Here are a few of North America's record-breaking roller coasters.