If I were in an elevator that was free falling, would it help at all if I perfectly timed a jump before it hit the ground floor? Would anything help the situation?
Asked by: Ron Cassford
In a state of free fall, both you and the elevator are weightless, and jointly
approaching the bottom of the shaft at the same high velocity. You are in a similar
position to an astronaut floating around in his capsule or spaceship, unable to stand or
get a secure footing on anything without recourse to magnetic boots and/or ropes and
handles to pull against.
Therefore, so far as jumping is concerned, it may be possible to
make some very rough inefectual attempt, but remember in a state of free fall, you do not
have the huge mass of the Earth to steady you and push against only, by comparison, the
minute mass of the elevator car. You may in your weightless struggles succeed in pushing
yourself away from the floor of the elevator and bumping on the roof but your original
velocity will be little, if at all, diminished, certainly not enough to save you, and
impact with the roof of the elevator will restore your velocity. Remember also that
astronaut has days, months, to push and pull himself around, you in the elevator have only
Disregarding any resistance due to friction or air pressure which would destroy the
concept of free fall, after little more than 4 seconds your velocity is about 40 metres
per second or say in the region of 90 miles per hour and increasing every instant. If you
are say a person weighing 80 kilograms then your momentum P=MV, that is P = (80Kg) times
( velocity 40 metres per second) ie 3200 kilogram meters per sec. This increases every
instant because of the accelleration due to Gravity which is 9.8 meters per second every
second. Your problem is somehow to get rid of this building momentum before you do so at
the bottom of the shaft. You certainly cannot reduce your mass of 80 Kg.The human body is
not capable, even under normal circumstances of jumping upwards at 40 meters per second or
90 miles per hour if you have it in mind to reduce your velocity to zero by moving at a
similar speed in the opposite direction. As indicated above jumping in the conventional
sense of the word is not possible. A sudden jump a split second before you and the
elevator impact the ground, were this posible, would be no good, for you would similarly
crush yourself by the abrupt onset of the required force in the opposite direction, which
is similar to what happens if you hit the bottom of the shaft.
What you need is space, in which you can gradually reduce your velocity, and an elevator
which has no roof. Retro rockets strapped to you waist could work provided their
counterthrust in the opposite direction to your fall did not have too rapid an onset..
Your downward plunge would be gradually arrested whilst the lift fell away from you ,
Similarly a parachute, if it could be made to deploy and work within the confines of the
lift shaft could save you, but remember things are happening very quickly and time and
adequate space for arresting devices are not on your side.
In point of fact being in the elevator is practically irrelevant, for the position is
almost exactly the same as if you had jumped into the shaft, you can readily imagine the
hoplessness of the situation and the maximum distance you could fall and survive, a few
meters perhaps. Thus lifts in free fall and at high velocity are to be avoided, as death
is almost certain.
Answered by: Don Bird, Berkshire UK
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'