Some roller coasters get to 3-4 g at the bottom of the initial drop, or going around a tight turn.Ĭars, planes, and roller coasters are useful comparisons because the acceleration lasts long enough to get a really good sense of what it feels like. This acceleration increases like the square of the speed, so it's probably the most common way to experience a substantial "g-force." A basic playground swing produces a maximum force at the bottom of its arc (where the swing is moving fastest) that can come close to 2 g (it would be exactly 2 g for an arc where the chains make a 60 degree angle with the vertical at the extreme, which is hard to do but not impossible). So you can have a substantial acceleration from motion at constant speed in a circle, just due to the change in direction. While we tend to use "velocity" and "speed" interchangeably, there's an important difference between them in physics terms, which is that velocity includes the direction of motion. You might get as high as twice that acceleration when you brake suddenly, or on an airplane during takeoff and landing.
While you're accelerating, you'll feel like there's a force pressing you back into your seat during that time, your weight feels about 7% bigger than normal, and directed at an angle of about 23 degree from the vertical. So, for example, if you floor the accelerator in your car when the light turns green, you might go from a standing start to 30 m/s (about 67mph) in 8 seconds, an acceleration of 3.75 m/s/s, or about 0.4 g.
The sensation you need to compare to is the sensation of your weight when you're standing still that feeling of weight can be temporarily increased by acceleration, and that's the "g-force" people talk about.Īcceleration is just a change in velocity over time, and if you're talking about simple straight-line motion, you can calculate the acceleration by just dividing the change in velocity by the change in time. This is the realization-that the effect of gravity is indistinguishable from any other acceleration- that led Einstein to General Relativity, which he completed one hundred years ago this week. While the standard of comparison is the acceleration of a falling object, somewhat paradoxically it doesn't make sense to talk about the feeling of falling, because while you're falling you actually feel weightless. The first step, of course, is to establish what we mean when we talk about acceleration as a means of talking about force. I got involved in discussion of g-forces this week in a completely different context, but having spent some time trying to sort it all out, I thought I'd devote this week's Football Physics post to some everyday (-ish) examples of different sorts of acceleration, to help put things in perspective. Damage from acceleration depends on the magnitude of the acceleration, the direction, the duration, and probably some individual variation in brain structure and so on. It doesn't help that the threshold for what acceleration causes damage is still fairly ill-defined- necessarily, because it involves messy biological systems, rather than the frictionless spheres beloved of physicists. This helps, sort of, but as more numbers get thrown around, it can get a little confusing- this tackle was five gees, that one twenty-seven, and so on. (There's a good discussion of this in a sports context in this paper from the Journal of Athletic Training).
So, people colloquially talk about a force of some number of "gee"s, meaning that the resulting acceleration was that many times the acceleration of a falling object. This constant acceleration provides a basis of comparison for thinking about forces- since what we're actually sensing when we experience a force is the acceleration, and everybody experiences a constant force of gravity, we can attempt to give a more intuitive sense of the forces involved in football collisions (or anything else involving significant acceleration) by expressing the resulting acceleration in terms of the constant acceleration of a falling object.
#G force calculator from distance over seconds free#
The repeated "per second" reflects the fact that this is a change in velocity, not position- after one second of free fall, a dropped object will be moving at 9.8 m/s (about the speed of a full-out sprint), but it will have covered a distance of 4.9 meters. The term comes from the fact that near the surface of the Earth, the force of gravity causes any dropped object to fall at a constant rate of 9.8 meters per second per second (32 feet per second per second if you prefer american units), traditionally given the symbol " g". It's all but impossible to talk about the issue of head injuries in the NFL without "g-forces" coming up.
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