Terminal Velocity

Posted Jan. 3, 2007 – Terminal Velocity – It’s so catchy, they used it as a movie title. But “terminal velocity” is an important science concept that affects meteors, mankind, and the mass-extinctions that keep changing our planet. Plus it’s fun.

People who bewilderingly take up sky-diving learn that you don’t just keep falling faster and faster. By spreading arms and legs, a jumper stops picking up speed when she reaches 120 mph. It’s air, of course, that slows things down. Everything has its own terminal velocity: For raindrops it’s 23 miles an hour give or take. That’s the speed of falling rain, in case you ever wondered. (Actually it varies with the size of the drops. A light mist’s particles might fall at only a half inch per second, which is why clouds, made of tiny droplets, can just hang there).

There is no terminal velocity on airless bodies like Mercury or the Moon. On those places, meteoroids captured by gravity keep gaining speed, up to a maximum that happens to equal that world’s escape velocity, the speed necessary to wrench free from its gravity in a single upward blast. On Earth it’s 25,000 mph. In other words, if there were no air and you FELL to Earth from a great distance, even beyond the moon, you’d hit the ground at that same escape velocity speed of 25,000 mph. Throw a coin up and then catch it. The speed you tossed it exactly matches the speed it travels when it lands back in your hand. Gravity is like that. Symmetrical.

At first a falling person or meteor keeps gaining speed. After each second of falling, a plummetting stone or person goes another 22 miles an hour faster. Two seconds of dropping, achieved by falling from five-stories, causes a rock to hit the ground at 44 mph. The speed would just keep increasing, up to that maximum of 25,000 mph, if we had no atmosphere. We’ve already seen that air slows skydivers to 120 mph, a speed reached after falling 500 feet or 50 stories. That’s still fast, of course: The fatal human impact velocity ranges from 15 to 38 mph, so it’s hardly news that we humans can easily die in a fall. But that’s not true of all animals.

Some mammals like cats and squirrels have non-lethal terminal velocities. They can generally fall from any height and survive.
Here’s where we get back to astronomy. An incoming meteoroid can easily weigh a ton as it strikes our atmosphere; that was the estimated weight of the intruder that broke into dozens of fragments over a Chicago suburb on March 26, 2003. One piece invaded a teenager’s bedroom and broke a mirror. But it could have been much worse.

Meteoroids start out at a sizzling 7 to 44 miles per second relative to Earth. Fortunately, if the meteoroid weighs less than 8 tons — and nearly all of them do — air friction robs it of ALL its original speed. At a height of about 10 miles or 50,000 feet, it slows to just 2 or 3 miles per second, where it no longer glows. Nonetheless this 7,000 mph velocity, 3 to 6 times faster than a bullet, gives a one-pound meteor enough kinetic energy to easily destroy a jetliner. It hasn’t yet happened, but it could.

Continuing downward, now dark and unobservable, the meteoroid’s encounter with increasingly thick air slows it to a terminal velocity of about 240 mph. This is its final speed as it strikes the ground. That’s the speed at which nearly all meteorites land, plus or minus 20%. That’s still plenty fast – usually enough to pierce a roof and end up on the floor of some room. Buildings are penetrated every year or two in North America alone. Just since 2002, meteors have entered seven homes including two in the United States.

If the meteoroid weighs over 100,000 tons, our atmosphere won’t slow it down in the slightest: It slams into the ground at full cosmic velocity. This isn’t good, as the dinosaurs learned 65 million years ago. Yet even then, it’s not all doom and gloom. The
big ones shake up the biosphere, change the course of evolution, and create new bio-adventures. We mammals now rule the Earth solely because a single impactor had enough mass to make it immune to — terminal velocity.

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  1. Matt Russell
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    Thanks, that was very helpful and well written.