You have learnt recently that a change in velocity of any object will cause it to accelerate. Acceleration is known as the rate of change of velocity with time. The changing direction, increasing speed, decreasing speed of an object are all factors of acceleration.
In general, high velocity doesn't cause harmful injuries. But what is dangerous is the high levels of acceleration or deceleration over a certain time interval. The common term "G-force" is a measure of acceleration and is not a measure of force eventhough the term 'force' is associated with its name.
Under normal conditions on Earth, a body (standing or sitting) will experience 1G, which is derived from multiplying the body's weight by 1. For example, in roller coasters, the only time you feel 1 G is in the loading station and on the chain lift. Everywhere else, the rider’s body will be subjected to varying G-force due to the dips and crests. Similarly, when a pilot in an airplane changes its orientation rapidly (tight turns, loops, etc.), the aircraft and pilot will undergo additional G-forces.
There are 2 kinds of G-force, positive and negative. They are caused by different conditions and will result in different effects on the human body. For pilots, they will experience positive G-force as the airplane pitches upwards (the nose pulls up) sharply. In roller coasters, the bodies will experience positive G-force at the roller coaster dips or troughs, pushing the bodies into their seats. Under this condition, the blood may rush away from the head to the feet, probably causing greyouts or blackouts to the riders at high G-force levels.
Click here to see a fighter pilot experience a blackout.
For negative G-force, pilots will experience it as they nose dive the airplanes (pitch downwards). Riders in a roller coaster will feel negative G-force at the crests of the tracks where they may experience momentary weightlessness due to the lack of contact force by the seat. Excessive negative G-force will cause the body to experience red-out where excessive blood is pumped to the brain, causing distorted vision. It is normally preceded with a pink-out.
Click here to see boy experience state of weightlessness on an amusement park ride.
Roller coasters give humans a thrill ride. They undergo G-forces of maximum up to 3 to 4Gs for brief periods of time. Pilots on the other hand are trained to undergo accelerations of 9Gs to pull quick maneuvers during flight for less than a second. If a force of 4 to 6Gs is held for more than a few seconds, the results could be devastating; such as blackouts to death. The devastating car crash of Princess Diana of Wales in 1997 was estimated to range somewhere between 70 - 100Gs. This accident was intense enough to pull the pulmonary artery from her heart.
Click here for another interesting video of a pilot experiencing more than 10Gs in his fighter jet.
You may want to explore further on the effects of G-force on the human body and what are some preventive measures that are taken to lessen the effects of G-force on the body especially for fighter pilots. It was said that even sneezing exerts a G-force of 2.5G on the human body. So how much G-force can a human tolerate?
Adapted from :
http://www.furball.warbirdsiii.com/krod/basic-physics.html, http://www.geocities.com/CapeCanaveral/6795/physics.html, http://hypertextbook.com/facts/2004/YuriyRafailov.shtml
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