A roller-coaster ride may be the ultimate cheap thrill - but what does it do to your body? Guy Adams climbs aboard Britain's latest white-knuckle ride to find out

It takes a shade over 30 seconds to complete a journey on Infusion, a new ride that gives Blackpool Pleasure Beach the distinction of owning "the world's first suspended looping roller-coaster running entirely over water".

In that time, passengers are strapped into a 16-seat train, with their arms and legs dangling free. Then they're whisked up a steep hill and launched, at speeds of 50mph, through an elaborate series of loops, rolls and 360-degree twists.

Like anything that puts the "great" into the great British seaside, Infusion is all about fun. At last week's official launch, whoops and hollers filled the air, while trainloads of special guests struggled to maintain a grip on both their dignity and their half-digested English breakfasts.

The ride is expected to become one of Britain's top tourist attractions. According to its owners, it will attract a staggering six million visitors - something like one in 10 Britons - during its first 12 months in operation. Such figures don't just tell us about modern Britain's appetite for cheap thrills, though; the £8m construction of Infusion, and the boom in Britain's other theme parks, open a window on the intriguing world of white-knuckle technology.

The roller-coaster has been around since the 17th century, when slides were built in ice around St Petersburg. By the 1800s, copycat versions using wheeled cars and wooden tracks had spread to Europe.

In 1885, an American entrepreneur, LaMarcus Adna Thompson, patented the ride considered the first modern roller-coaster at a theme park at Coney Island, New York. Rickety, often dangerous imitations were soon common in the US.

All roller-coasters work by gravity. A powerless vehicle known as a "train" is hauled to the top of a hill, usually by an electric motor or winch. Then it is released down a track that contains twists, turns and inversions, giving passengers what is normally described as a "rush".

"I love the adrenalin I feel, the controlled danger, the speeds and dips," says Andy Hine, the chairman of the Roller-Coaster Club of Great Britain. "It turns me into an excited child."

In scientific terms, this "excitement" is the result of basic Newtonian physics. When Hine, who has an MBE for his services to the theme-park industry, travels down a roller-coaster, his body is subjected to varying degrees of gravitational forces, in a number of unfamiliar directions.

If one were to complete a ride sitting on a set of bathroom scales, the reading would fluctuate massively. Halfway round a typical loop-the-loop, for instance, a passenger can be four or five times their normal weight.

Scientists describe this relative weight in terms of G-force. One G is the normal level of gravity we experience standing still. On a ride, you can experience six times this amount (or 6G), making you six times as heavy.

A roller-coaster can also cause negative G, pushing a passenger upwards out of the seat. The body gets confused by these fluctuations and produces adrenalin, which increases heart rate and blood pressure. The result - a primeval thrill.

Infusion does this in spades. But it is by no means the biggest or fastest roller-coaster in the world. That honour previously belonged to its near-neighbour in Blackpool, the 250ft-high Big One, which was the tallest, fastest and steepest on the planet when it opened in 1994.

It lost its crown in 1996, to a Japanese rival. Today, the biggest ride is at Six Flags Great Adventure in New Jersey. It reaches up a staggering 450ft, and hits speeds of 120mph during a brief, almost vertical drop.

Good roller-coasters are not defined simply by sheer height or speed, though. Much of their appeal depends on oscillating passengers between positive and negative G-forces, including periods of weightlessness. "That's why roller-coaster fans put their arms up in the air," Hine says. "It helps to stretch out their innards so their stomachs can move more, and exaggerates that amazing feeling."

There is, of course, a limit to how much fun the body can take. At 9G or 10G, blood flow to the head can be severely restricted, to the extent that the majority of people would black out. Most roller-coasters are designed to produce no more than 6G.

On Infusion, you reach 5G a total of 11 times. According to Blackpool's director of engineering, Alex Payne, this makes for a peculiarly intense ride.

"Our bodies are used to feeling a load coming straight down on us, so we can cope with a lot of positive G. But upwards, things get difficult above minus 1G," Payne says.

At extremes, hardened riders can experience nausea. This is caused by one of two things: either their balance systems are confused, or their bodies are producing too much adrenalin. "Being sick is often due to the balance in your inner ear going," Payne says. "Adrenalin can also play a part: some people who are involved in a traumatic incident are fine at the time and then, after they've calmed down, that's when they throw up."

So roller-coaster science involves careful calculation of the forces a ride will put its passengers through. The engineers, meanwhile, must create circuits that trains can successfully, and safely, navigate without running out of speed.

This is harder than it sounds. A roller-coaster carriage is at the mercy of gravity and friction, influenced by variations such as passenger weight and wind resistance. To build Infusion, the British roller-coaster engineer Dr John Roberts used a computer program to make sure that trains would have sufficient energy to do five upside-down loops without grinding to a halt.

"Potential energy is equal to mass times height," Roberts says. "When the train goes down the track, it gives up potential energy and turns it into kinetic energy. The further the train falls, the higher its speed. If a ride goes back up a hill, it reconverts this velocity back into potential energy."

All the while, of course, a roller-coaster is losing energy through the friction of its wheels on the tracks, and wind resistance. Roberts must calculate energy losses to establish whether trains can complete the course. "If you get the geometry wrong, the roller-coaster will just stop," he says. "You build in contingency plans for that, and put in a ladder for those bits of track so that people can escape, but you don't want it to occur, obviously."

Once the ride's circuit has been designed, engineers are called in to make sure the construction is strong enough to cope with the physical demands of carrying heavy trains every day of their working lives. "Changes in G-force give your body the impression that it's been to hell and back, but they do create problems," Roberts says. "At 5G, you really do weigh five times as much. So a 15-ton train becomes a 75-ton train, with obvious structural implications."

So Infusion has more than 100 tons of steel in its foundations, and almost a 1.5 million pounds of the stuff in its overall design. That includes eight tons of nuts and bolts and 96 steel columns.

On the ground, meanwhile, I was being strapped up to a heart-rate and blood pressure monitor and belted into one of Infusion's carriages. Up and off we went. My pulse rate almost doubled on the initial incline, then settled back at about 50 per cent higher than normal during the ride itself. My blood pressure increased by about 20 per cent.

"I love all roller-coasters, and this is a cracker," Hine said as we came to a breathless halt. "There's so much bombarding you that when you finish, your body knows it has been through something huge."

As I stepped, panting, from the train, with flushed cheeks and a stomach like cold porridge, both the scientist and the Blackpool day-tripper in me had to agree.

Britain's top five roller-coasters

As chosen by Andy Hine MBE, chairman of the Roller-Coaster Club of Great Britain.

1. - Mega Fobia, at Oakwood in Pembrokeshire

This is an old-fashioned wooden ride that doesn't take you upside down but instead has a series of dips that gives you a huge amount of "air time" - the expression we use to describe zero G.

2. - Nemesis, at Alton Towers in Staffordshire

The first suspended roller-coaster outside the US; on this, your feet dangle beneath the train. It runs through a big hole in the ground, and is one of the most intense rides in the world: the G-forces literally take your breath away.

3. - The Big One, at Blackpool Pleasure Beach

You won't get a better view from the top of any other ride. On a clear day, you can see as far as Morecambe and Southport. The 235ft drop is the highest in Britain, and going down it feels like you're jumping off Blackpool Tower.

4. - Stealth, at Thorpe Park in Surrey

A launch coaster, powered by electromagnets that fire the train from a stationary position to 80mph in two seconds around a single vertical loop. It's the fastest ride in the UK.

5. - G-Force, at Drayton Manor in Staffordshire

The only ride I know where the lift hill is situated in a loop. A chain pulls your train slowly upside down, before releasing it. The seatbelt only covers your knees, not your shoulders, increasing the fear factor.