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I like little trivia books like these. Ltitle nuggets of information to drop in random convos.
If I'm in an elevator and it starts to fall to the bottom of the shaft, can I jump up at the last instant and cancel the
impact?
Ho hum. I don't know how many times this question has flashed into the minds of worrywarts in elevators, or how many times it has been
asked of every friendly neighborhood physicist. It is easy to answer in one word (No), but thinking about it does raise a whole bunch of fun
questions.
First, here's the quick answer: Your objective is to arrive at the bottom of the shaft like a feather, without any appreciable downward speed,
right? That means that you have to counteract the elevator's downward speed by jumping upward with an equal amount of speed. The elevator
(and you) might be falling at, say, 50 miles per hour (80 kilometers per hour). Can you jump upward with anywhere near that speed? The best
basketball players can jump at maybe 5 miles per hour (8 kilometers per hour). End of quick answer.
Let's consider the instant before your elevator's cable snaps. In the seventeenth century, long before elevators, Sir Isaac Newton (1642–
1727) realized that when a body exerts a force on another body, the second body exerts an equal and opposite force on the first body. Today,
that's known as Newton's Third Law of Motion. When you're standing on the elevator floor and gravity (force number one) is pulling you down
against the floor, the floor is pushing you back up with an equal force (force number two). That's why gravity doesn't win out and make you fall
down the shaft. It's the same with the elevator car itself; in this case it's the cable's upward pull that counteracts gravity's downward pull on the
car. So neither you nor the elevator falls down the shaft. You both move upward or downward at a speed that is controlled by a motor's slow
winding and unwinding of the cable from a big drum at the top of the shaft.
When the cable snaps, both the upward pull of the cable and the upward push of the floor are suddenly gone, so both you and the elevator
are free to succumb to gravity's will and you both begin to fall. For an instant you are left floating—feeling “weightless” because the customary
push of the floor on your feet is gone. But following that instant of blissful suspension, gravity has its way with you and you fall, along with the
elevator.
NITPICKER'S CORNER
About that moment of “weightlessness” when the elevator begins to fall: Obviously, you haven't really lost weight. Earth's gravity is still
pulling on you as it always has, and the strength of that pull is what we call weight. What you've lost is apparent weight. Your weight just isn't
apparent because you're not standing on a scale or a floor that feels your pressure and presses back upon your feet.
Of course, this whole question of falling elevators is hypothetical because elevator cables just don't snap. And even if they did, there are
spring-loaded safety devices that would keep the car from falling more than a couple of feet. But, as roller coasters prove, some people seem
to enjoy the contemplation of imminent disaster.
If you happen to be one of those roller coaster fans, that “floating” feeling you get as the car falls from
I'll try it. They finally opened the library by my house so it'll be easier to get books.
