In a previous article, we showed that interstellar travel had intractable energy problems, simply in achieving the needed high speeds, and the huge impact energies at these speeds.1 And as will be shown, there are other problems, involving what are popularly called “g-forces”.

Actually, the term “g-force” is misleading, because it refers to acceleration due to gravity. Under Newton’s Second Law, F = ma, or force = mass × acceleration. It is used because the weight force is proportional to mass, while acceleration is inversely proportional, so the acceleration of all objects due to gravity is equal. This explains Galileo’s apocryphal experiment of dropping a heavy ball and a light ball from the Leaning Tower of Pisa, and finding that they hit the ground at the same time (except for air resistance).

At the earth’s surface, the acceleration due to gravity is 9.80665 m/s², or 1 g, which will be rounded to 10 m/s² for the “back of the envelope” calculations in this article. Now “acceleration” means change in velocity, which means any change in speed or direction. At 1g, the speed changes by 10 m/s (22 mph) each second, hence 10 m/second-squared.

High g-forces are a big problem for astronauts, fighter pilots and racing drivers. How damaging they are depends on duration and direction. Short duration is obviously better— “Several Indy racing car drivers have withstood impacts in excess of 100 G without serious injuries.”2 But here, the high g-forces are just for a fraction of a second. Even much lower g-forces sustained for even one minute could be fatal.

Direction also matters. The most damaging are “downwards”, when blood rushes into the brain and eyes, where –2 to –3 g is the limit (the negative sign is because of the downwards direction). The least is “forwards” or “eyeballs in”, as in speeding up a car, or an astronaut lying on his back as the rocket shoots upwards….

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