G-Force Primer

"Every body perseveres in its state of being at rest or of moving uniformly straight ahead, except insofar as it is compelled to change its state by forces impressed."  Isaac Newton's First Law of Motion

Thrill rides are machines specifically designed to compel our human bodies to change state, over and over again, by impressing forces upon them. Each curve, drop, loop, launch, or brake alters the rider's state of being, triggering inertial resistance -- the feeling that your body is headed in one direction while the ride is pulling you somewhere else. The effect of acceleration on the body is produced by the resultant inertial force.

Acceleration represents the change in the speed and/or direction of a moving object.  Linear acceleration is a change in speed without a change in direction. Radial acceleration is a change in direction without a change of speed. Angular acceleration is a simulataneous change of speed and direction.

A 3-axis coordinate system is used to represent the direction of acceleration. The amusement ride industry uses "g"s as the unit of measure for acceleration, with 1g representing the acceleration of normal earth gravity.

As a roller coaster accelerates downward (-Gz), the effects of gravity are partially or totally neutralized and the rider feels a brief moment of weightlessness.  As the coaster bottoms out, the rider is pushed down into the seat, multiplying the effect of gravity (+Gz).  Curves create lateral accelerations (Gy), which push the rider's body sideways.  An increase in forward speed pushes the body into the seat back (-Gx ); braking pushes the body forward, away from the seat back (+Gx).

If the both the ride and the rider's body are well-designed and well-maintained, and if the rider's body is sufficiently supported and restrained by the containment system, then the physics end of the safety equation is satisfied. Flaws in a critical component (e.g., the ride track, braking system, rider's back/brain/cardiovascular system), or poor fit between rider and restraint, can be the trigger point for serious injuries on dynamic amusement rides.

Anyone who's spent time at amusement parks or carnivals knows that there is wide variability in the quality and quantity of motion produced by different amusement rides. Some of the newer steel coasters offer a smooth, sweeping, exhilarating swooooosh of a ride, sending your body through seemingly impossible elevations and angles. Older wooden coasters can rattle your teeth and bruise your bum. Computer-controlled rides can simulate the jerky thrills of off-road jeep racing. Human centrifuges create the acceleration of a space launch for millions of armchair astronauts. Spinning carnival rides can literally pin you to the wall or, alternatively, send smaller riders sliding across the seat through a combination of rapid changes in direction and unfitted restraints.

Each thrill ride design offers a different experience to patrons, and each new experience reshuffles the biodynamic equation. Amusement rides are designed for the masses. That means they're generally safe for most people, but it also means their generic design may put some vulnerable individuals at higher risk for accidents or injury on some rides.

Human tolerance to acceleration depends on the magnitude, direction, rate of onset, and duration of the acceleration stress, as well as differences in individual physiology and psychological reaction (i.e., the fear factor).

The response of the body to acceleration varies with length of exposure. Acceleration pulses of 0.2 seconds or less are considered to be "impacts" (e.g. slapping someone's back, crashing a car). The human body can accept a relatively high magnitude impact acceleration without harm as long as its duration is extremely short.

Accelerations lasting longer than 0.2 seconds are referred to as "sustained" or "prolonged" accelerations. It is sustained accelerations that create the exhilaration of a thrill ride experience. Body fluid and organ shifts become more important when studying the physiological effects of exposure to sustained accelerations. The overall effects of +Gz, and, to a lesser extent, +Gx, are very well documented in healthy populations and result in markedly increased heart workload and oxygen demand (ref: The Space Review).

Other aspects of thrill ride motion can impact a rider's comfort and may, in some cases, increase the risk of injury. These include jerk, vibration, and rotational acceleration.

An increase in heart rate has been one of the generally observed responses to +Gz in military testing. This response varied by individual, psychological stress, and the amount of muscular straining being performed by the experimental subject. The amount of absolute increase in heart rate is affected by the maximum G level reached and the rate of onset. Cardiovascular systems noted under G stress include loss of vision, loss of consciousness with accompanying seizures, convulsions, amnesia and confusion, cardiac dysrhythmias (tachycardia and bradycardia), heart blocks, and a stress cardiomyopathy.

Back, neck, and limb problems are the most frequently reported musculoskeletal problems. There are reported cases of intervertebral disk ruptures under high +Gz and many complaints of sore necks after centrifuge rides and flights in high-G aircraft. Permanent injury is rare enough to warrant a case report. (ref: USAF Flight Surgeon's Guide)

A number of factors are known to decrease tolerance to +Gz acceleration. The following information is based on Appendix D of the Australian standard for "Amusement Rides and Devices - Part 1: Design and Construction" (AS 3533.1-1997). Reprint permission granted by SAI Global Ltd. The full text of the standard may be purchased online at http://www.sai-global.com.

• Heat stress - An increase in core temperature and dehydration from heat stress reduces the +Gz acceleration required to produce blackout.
• Hyperventilation - Hyperventilation due to fear or anxiety can reduce the +Gz tolerance threshold by 0.6g.
• Hypoxia - A reduction in blood oxygen level compounds the reduction in cerebral/retinal blood flow, reducing tolerance threshold to acceleration.
• Alcohol - Alcohol reduces the acceleration effect threshold by 0.1g to 0.4g.
• Hypoglycemia - A reduction in blood glucose level reduces the threshold for loss of consciousness by 0.5g.
• Infections, such as colds, gastroenteritis - Infections and minor illnesses associated with dehydration or fever markedly reduce tolerance to acceleration stress.
• Age - Persons over 60 years old should restrict exposure to acceleration.

Parents should realize that children may be more vulnerable to the physical and psychological effects of acceleration than adults. Research on the effects of acceleration in humans has been restricted to healthy adults; there are no studies to guide ride designers in determining safe levels of acceleration for children. Fear can compound and escalate the effects of acceleration. Some high acceleration rides marketed to children may be too intense for younger children or even sensitive older children.

• Fast Cars and Wild Rides: Whiplash, Headbanging, and Sqeezeplays
• Acceleration Research Page

Saferparks owes special thanks to the sources quoted in this article for helping the public understand some of the complex issues associated with thrill ride dynamics.