eli5 how does jumping before an elevator hits the ground not save your life?
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Imagine that the elevator is falling at 50mph. You can jump in the opposite direction at about 5mph. You’re falling at the same rate as the elevator. Your jump reduces your falling speed to 45mph when the elevator hits the bottom of the shaft. You’re moving slower, but not slow enough to avoid injury and/or death.
I assume you’re talking about an elevator plummetting down a shaft.
Let’s say the elevator is falling at 50 mph and a half second before it hits the bottom you jump. Your speed relative to the elevator is at first negative (it must be for your feet to leave the ground) but you quickly slow and fall, momentarily you’re the same speed as the elevator at the peak of your jump (still relative to the moving elevator).
After the peak of your jump you accelerate towards the floor of the elevator so you’re moving faster than the elevator (maybe at 10mph relative to the elevator). At that moment the elevator hits the ground and stops but continue to be going 60mph and you hit the floor at that speed.
Had you just stood there you would have had less impact speed.
Velocity is relative. When you are falling in an elevator lets you are essentially in free fall. This means depending on the height you can reach terminal velocity of 120 mph. Realistically the elevator is going to have some mechanical resistance on the way down as well as having a different terminal velocity based on its air resistance, but that doesn’t matter for this explanation. You are falling a 120 Mph. When you jump you are pushing away from the elevator at 4 mph max. This means that you are now going to hit the ground 116 mph. From your perspective the elevator you are moving away from the elevator, but from the grounds perspective you are both hurdling towards them at basically the same speed. In order to save yourself by jumping you would have to jump with enough force to completely nullify **all** of the speed that you have gained while failing.
Depending on how far you’ve fallen (you + elevator), you will have a lot of momentum built up.
The amount of force that you can generate from one jump will lessen the downward momentum, But not negate it entirely, unless the fall was very short (about as high as you’d normally be able to jump).
The amount of damage you take from the fall isn’t based on your momentum relative to the elevator, it’s a based on the momentum of you plus the elevator relative to the ground.
To make it more obvious than the other answers, let’s rotate everything sideways.
Instead of falling down a shaft at 50mph, imagine you’re in the back of a truck driving 50mph. Straight towards a wall of rock. At the last second before impact, you jump away from the rock wall.
What do you think will happen?
And that’s assuming you time things right, because if you jump late you’ll never jump, and if you jump early you’ll be accelerating towards the wall faster than the vehicle at the point of impact due to gravitational acceleration.
If your jump was powerful enough to make a difference, your legs would be powerful enough to withstand the impact even without jumping.
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For the same reason that you can’t save your life in a crashing airplane by doing a little jump in the aisle just before impact, or rolling off the roof of a tall building with a skateboard under your feet and then jumping up off of the board just before ground. As others have said, the speed that you jump up does get “taken away” from your downward crash speed (if you time it just right) but that amount of energy reduction is insignificant vs what still remains.
Everyone is talking about speed. Speed is what kills you. Everything falls at the same rate.
A fatal fall would be about 30 feet. Everything that falls 30 feet ends up at the same speed.
You can jump up, at best, 4 feet. Everything that falls 4 feet ends up at the same, non-fatal speed.
So, at best, if you jump up at the last moment, you can reduce the speed from that of a 30 foot fall to a 26 foot fall. That’s not enough tp make a difference. 26 feet is still fatal.
Extremely simple: the entire elevator will have enough speed and momentum to hit the ground with enough force that the roof of the elevator bounces off of the ground! So whether or not you “jump” right at the moment of impact with the ground, you would be pancaked into the ground.
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