| MadSci Network: Physics |
"g"-forces are really a measure of acceleration, which is the rate of change of velocity of an object. If a car accelerates from zero to sixty miles/hour in 6 seconds, it has an acceleration of 4.3 meters per second per second. That is, every second, its speed is 4.3 meters per second greater.
To convert this into "g-force", we compare this acceleration to the acceleration produced by gravity, which is 9.8 meters per second per second. The car is accelerating at about 0.4 times gravity, or 0.4 G's.
The "g" can be thought of as a unit of acceleration whose value is 9.8 m/s^2. In many cases (such as the one above), gravity has nothing to do with the situation. In others, it does. For example, the strength of gravity at the cloudtops of Jupiter is 2.5 G's, because objects would fall at 24.5 m/s^2 there.
So the "g" is simply a unit of measurement, but it's a convenient one for describing accelerations in terms people can identify with. Here's a handy table, reproduced from Physics, by O'hanian, 1989:
| Protons in Fermilab accelerator | 9e12 G |
| Ultracentrifuge | 3e5 G (300,000 G) |
| Baseball struck by bat | 3000 G |
| Soccer ball struck by foot | 300 G |
| Automobile crash (100 km/h into wall) | 100 G |
| Parachutist during opening of parachute | 33 G |
| Gravity on surface of Sun | 27 G |
| Explosive seat ejection from aircraft | 15 G |
| F16 aircraft pulling out of dive | 8 G |
| Loss of consciousness in man ("blackout") | 7 G |
| Gravity on surface of Earth | 1 G |
| Braking of automobile | 0.8 G |
| Gravity on surface of Moon | .17 G |
Extra bonus factoid: if you lock your knees when you hit the ground after jumping off a 1-meter platform, the acceleration is about 50 G. So don't.
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