When you push or pull on an object, you exert a force on it. This is something we're all familiar with... but what exactly is a force?
In physics, force is a fundamental concept, so it is something that must be defined and measured precisely. The measure of force varies depending on what system is being used.
The use of "G's" is a way to simplify the measurement of forces.
With
Coaster Dynamics,
the mks (meter-kilogram-second) system of measurement is used. This is also called the "Metric System". In the mks system, the unit of force is called a newton (named for Isaac Newton, who first defined the relationship between forces and motion in 1686). By definition, a newton of force will accelerate a one-kilogram mass at one m/s².
Another system of measurement sometimes used in science is the cgs (centimeter-gram-second) system, which is also called the "Gaussian System". In the cgs system, the unit of force is called a dyne -- which is the force that will accelerate a one-gram mass at one cm/s².
The newton and dyne are units most people cannot relate to easily -- even in countries that primarily use the metric system. In the United States, which still uses the "British Engineering System", the situation is even more confusing. Although many Americans think they understand, they are often mistaken.
In the British Engineering System, the unit of force is a pound-force -- which by definition is the force that gravity exerts on a one-pound mass on earth at sea level. Since earth's gravity is used as part of the definition of a pound-force, to be precisely correct, it must be specified at sea level. Alternatively, it is true that a pound-force will accelerate a pound-mass at the "unit acceleration" of 32.174 ft/s². Notice also that the term "pound" is used for both a force and a mass (they should be distinguished by specifying either "pound-force" or "pound-mass"). With such confusing terms, it's no wonder the metric system is the one most often used in science!
To simplify our lives... we can measure force in terms of a "G" -- which is a dimensionless unit.
A "G" is the force on an object divided by the force exerted on that object by gravity on earth at sea level. In other words, if you are standing on earth at sea level, and there are no other forces acting on you, the force due to your mass is 1.0 G. If the force on you were 2.0 G's, you would seem to weigh twice your normal weight.
Actually, if measured using a "Global Coordinate System" attached to earth with the positive direction pointing upward, away from the center of the earth (which is the typical case), the force exerted on you by gravity while standing on the ground would be -1.0 G (and according to Newton's 3'rd Law of Motion, the force exerted on you by the ground is +1.0 G).
To be precisely correct, you can impress your family and friends by saying that "a force of magnitude 1.0 G is defined as a force that causes a unit of acceleration when applied to a unit of mass" -- this is fundamentally correct regardless of the system of measurement being used.
For roller coaster enthusiasts, G's are an easy way to measure force in a loop. These forces are what press you into your seat when going through a tight loop. High G forces are fun for thrill seekers... but care must be taken to limit the forces to a safe level. As a point of reference, a typical "Top Gun" jet fighter pilot may begin to loss conciousness if subjected to a sustained force of 5 G's, or more.
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