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and it floats down slowly. But the block
of lead wins out against gravity and falls at a rate much
closer to the theoretical acceleration (9.78 m/s/s) of
1G would suggest. You can demonstrate this effect by carrying
out the following experiment. *
(1) Remove your right shoe and sock.
(2) Drop a feather repeatedly till it falls on your foot
(feathers tend to go off - course, so it'll take a few
attempts to hit the target). Note that the speed at which
the feather hits your foot causes no pain and no damage.
From this, you can ascertain that (i) the weight of the
feather is very small (ii) air resistance is causing it
to fall very slowly.
(3) Now repeat the process, but substituting a 10kg block
of lead. As long as you can overcome the subconscious
impetus to not drop a 10kg block of lead on your foot,
you will note that air resistance does not affect it's
fall anywhere near as much. It will travel directly and
rapidly to the surface of your foot.
The subsequent pain, broken bones and
visit to Casualty (I believe that's ER for my American
audience) illustrates that our atmosphere has a lot less
effect on the fall of a 10kg block of lead.
The highest speed a falling object reaches
is termed "Terminal Velocity." After a certain
time and distance falling in free fall (in an atmosphere),
the speed of an object reaches a maximum and will not
increase any higher. This is decided by such factors as
shape, weight and surface area. It is the point at which
the pull of gravity is equalled by the drag of air resistance,
preventing any further acceleration.
A sheet of paper has a large surface area
compared to its weight, so it'll act like a sail as it
falls. Air resistance will slow it down. But drop a box
of five reams of paper on your foot, and the effect is
very different, even though the surface area exposed to
the passing air is the same. The result of this experiment
would be closer to that of the earlier one with the block
of lead.
So for living things, terminal velocity
really can be "terminal" velocity. For humans,
over much of a distance, it usually is. For cats, it usually
is not.
A person in free fall reaches terminal
velocity after about 3 seconds, at a speed of around 120
- 140 mph, in a distance of only around 200 feet (30 metres
per second, 60 metres). That means whether we fall out
of a 15 story building or an airliner at 30,000 feet,
it makes no difference, we still hit the ground at about
the same speed. Adopting a diving profile will increase
that speed, to about 200 mph. Fully spreading arms and
legs will reduce it a little.
The terminal velocity of a cat is about
60 mph, reached after about 70 feet or five stories. A
cat travelling at terminal velocity landing on its feet
has an excellent chance of survival, though possibly with
all four legs broken. In fact, the posture adopted by
a falling cat actually reduces its speed after it reaches
terminal velocity. The very act of presenting all four
legs to land on presents the most surface area, slowing
the fall. A human falling feet first will present less
surface area, increasing the speed of fall.
Statistics have shown that cats falling
15 stories have a higher survival rate (95%) than those
falling 4 stories (90%). This is probably because a cats
natural abilities allows it to prepare on the longer fall
by positioning itself feet first. On the shorter fall,
the cat is more likely to come down head first.
* The author accepts no liability for
injury caused to any person foolish enough to attempt
this experiment.
Lets
get rich, rich, rich together!
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