It’s not speed that’s the problem, but acceleration or deceleration. That’s what kills in a collision, for instance. Changes in speed are expressed in multiples of gravitational acceleration, or ‘G’. Most of us can withstand up to 4-6G. Fighter pilots can manage up to about 9G for a second or two. But sustained G-forces of even 6G would be fatal.
Astronauts endure around 3G on lift-off, one G of which is Earth’s own pull. This is marginally easier to bear because the crew sit in a laying down position, so the G-force acts from the back to the front of the body rather than top to bottom, where blood would be forced down from the head.
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