What is a 'g'

How do you measure g's?
- question from Alejandro Echeverria
Let's first define what a "g" is. We start by looking at Newton's second law of motion that says the force (F) acting on an object is equal to its mass (m) multiplied by its acceleration (a), or

You may not realize it, but each of us is being accelerated at all times. The force we experience as a result of this acceleration is our weight, or the force that gravity exerts on our mass. If we drop a ball from some height, it will fall towards the Earth's surface at some constant rate of acceleration (if we neglect air resistance). This rate of acceleration is independent of mass, and we call the value of the acceleration due to Earth's gravity "g." Substituting into the above equation, we therefore obtain the following relationship for the weight (W) of any object being acted upon by Earth's gravitational force.

The value of g (near the Earth's surface) is a constant measured as 32.2 ft/s² or 9.81 m/s². Under normal conditions, this is the acceleration we are accustomed to experiencing in our daily lives, so it is often referred to as 1g or 1G. When we ride a roller coaster, drive over hills at high speed, or experience turbulence on an airplane, we are subjected to additional accelerations that add to or subtract from normal 1g conditions. For example, think of a time you rode a roller coaster and felt your stomach "jump." Or perhaps you've driven over a hill at high speed and were lifted up off your seat. Maybe you've seen pictures of astronaut trainees floating for brief periods aboard the C-135 "Vomit Comet" or the Russian Il-76 equivalent. In each of these cases, you experience an acceleration that partially or completely cancels the acceleration due to gravity. You actually lose weight, or even become temporarily weightless in the case of the C-135, because the gravitational acceleration acting on you is reduced by some acceleration a1.

In this case, you are experiencing less than 1g conditions. If the value of a1 is smaller than the value of g, you will actually experience negative g's or negative weight.

Similarly, when you reach the bottom of a steep descent on a roller coaster or when you travel up a high-speed eleavator, you may feel like some invisible force is pushing down on you and making you heavier. You do indeed weigh more because the gravitational acceleration acting on you is increased by some acceleration a1:

You are now experiencing positive g's that produce more than 1g conditions.

You will notice that g-limits are provided for a number of aircraft in The Aircraft Museum, typically a positive and a negative value. These limits define the maximum and minimum g-loads that an aircraft structure can take before it begins to fail. For example, the Eurofighter Typhoon has g-limits of +9 and -3. This means that the Typhoon can withstand vertical accelerations up to 9 times normal 1g conditions in positive acceleration (meaning the pilot will feel like he weighs 9 times as much as he normally does) or -3 times normal 1g conditions in negative acceleration.

Having established that a "g" is an acceleration, it makes sense that the device used to measure g's is called an accelerometer. Although these devices come in many shapes, sizes, and flavors depending on the application, the principal of how an accelerometer works is not very complicated. The simplest example I can think of is to tie a piece of string to the hole at the bottom center of a protractor while tying a small weight to the other end of the string. Holding the protractor upside down, the string will hang at 90°. When you accelerate the protractor horizontally, the weight will move and displace the string by a constant angle. This angle can then be related back to the actual acceleration of the protractor by simple geometry.

There are many other devices that work similarly to this example, like springs or pendulums for example. All employ the same basic idea--if there is a relationship between the force and the displacement (either linear or angular), then for a measureable displacement, there is a measureable force, and hence a measureable acceleration.
- answer by Aaron Brown, 19 August 2001

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