I posted a question several weeks ago about surviving a fall if an elevator cable breaks. My students think jumping up at the moment of impact will increase the rate of survival. I want to build a small elevator with a spring to make an egg, Ken or even Barbie jump a few inches before impact. I need advice on how to build this or trigger the spring. I have a few ideas such as using a rat trap to hit the arm of a first class lever to launch an egg on the other arm. Some have suggested a solenoid as a trigger. I’m not sure how to do this.
Any help would be appreciated.
Thanks, Dave
it really should be much easier to explain that considering the best possible jump by a human is somewhere less than 3 feet, simple math will show that you could only offset the fall by a similar amount.
For instance if you fall 30 feet by jumping (with absolutly perfect timing, and assuming the jump itself were essentially instantaneous) and jump a full 3 vertical feet you would still impact as if you had fallen 27 feet.
obviously the effect would be even less as a percentage of impact offset at greater heights, 300 feet - 3 feet =impact equivalent of a 297 foot fall.
Obviously this doesnt take into account terminal velocity of a falling elevator where air in an enclosed column and friction from the sides would limit top speed, but obviously the effect simply could not be large enough to effectively change the outcome.
Welcome to the SDMB, drdr. This question comes up surprisingly often. Quint Essence has pretty much nailed it. For the definitive Straight Dope on the matter however, look no further than this
He is not looking for the answer to the question. He is looking for help to build a device to demonstrate the situation for his students.
Try this: Blindfold one of the kids and have him leap off a chair, at the moment of impact with the floor have him jump in the air. Did his jumping at impact lessen the jolt of hitting the floor?
Well, thats just silly. The student would have no material to jump off of as he would in an elevator. And as **PaperTowel** suggested, the OP is looking for a small scale design to replicate the scenario. Probably to prove that it doesnt work.
Not to mention with your idea the student would probably be visiting the ER in short order.
I see your point, so have the student stand on some kind of platform while the platform is suddenly dropped. I think most people can see that there will be no lessening of the impact with out even trying it.
Hey I understand the physics plus have used many different style to teach the concept! I need help in design. If you have taught 8th graders to them seeing is believing. This is for the concrete learner plus it would be a little fun.
If an elevator cable breaks, the inhabitants won’t go plummeting to the ground. There is an emergency brake built into the top of the elevator car (the car is normally stopped by a brake built into the rotating drum the cable is wrapped around, and is mounted near the top of the shaft). Powerful springs hold the brake shut, but the tension in the elevator cable holds the brake open. If the cable breaks, the emergency brake slams shut immediately; no electricity, switches, relays, etc. are involved. They are actually quite foolproof.
Also, despite what you may have seen in movies (or, more specifically, an episode of L.A. Law), it’s not possible to inadvertently fall into an elevator shaft when the doors open. The doors that you see when waiting for the elevator (the outer doors) have no independent opening mechanism. The only thing that opens them is the inner doors (what riders stare at); they hook the outer doors after the car reaches the floor, and the inner door motor does all the work. No car, no open. You may have seen the outer doors be open with no car when elevator mechanics are working on it, but that’s an different; in that case, the mechanics have unlocked the outer doors and forced them open. Finally, modern elevators generally have a “toebox”, a box that hangs underneath the elevator that prevents people from falling down the shaft if the car stops short of the floor, but is close enough to still enable the doors to open.
Art
I will answer your original question, since you don’t want to do the live experiment.
You can use a spring activated dart gun or a b-b gun if you need more power, just have the falling cage activate the trigger with levers,cords or some such.
Since this is a school enviroment you probably won’t be able to use anything that resembles a gun, so you will have to come up with a similar mechanism.
While unrelated to the two situations you’ve mentioned, elevators are not completely foolproof…
A good way
I propose that the best way to involve the concrete learners is to ask them to help design this experiment. You can break off into a discussion of what a “scale model” is used for if that serves, but make sure that the students understand that to accurately model this, they need to figure out:
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How to make Barbie’s mass roughly proportional to her shape. Let the kids know that Barbie (or Ken, depending on your gender) is going to be a scale model of YOU (use your height and mass to do the scaling). Do not give anyone the opportunity to say you are forcing Mattel’s ideals of the female figure on any of your students, yada yada. Do not let the Barbie discussion devolve into sociology or sex ed.
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How to make Barbie jump a 3-scale-feet height, repeatably. I like the mousetrap idea, personally, but I’m still a little puzzled about what you’d use for a trigger. Anything that uses the elevator’s downward motion to do the triggering will probably slow the descent of the elevator and skew the experiment.
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How to bring the simulated elevator (I’m imagining a shoebox with one side covered in Saran Wrap) to a realistic height. If you’re doing anything more than ten storeys, you’ll probably need to build this contraption in the gym where there’s a high ceiling.
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How to measure the difference in impact between a jumping and non-jumping Barbie. It’s crucial that you can measure a difference, and that the difference is small. You also want to be able to scale up the measurements when you’re done, to prove that a non-jumping Barbie gets (e.g.) 3 kN of force transferred to her femurs, while the jumping Barbie gets 2.998 kN.
If you use this as an exercise in experimental design you’ll be doing all of the kids a great favor. Make this a long-term class project, maybe even write a unit on it and submit it for a grant!
Another good way
You may have better luck (but get more flak from the school board) designing an experiment where you or a student are actually the test subject. If the kids think you’re biased (and would not jump reliably), ask for volunteers, get permission slips. I think a one-and-a-half meter drop onto something designed to measure impacts (a digital scale hooked up via RS-232 to a 386 on a lab cart) should serve your purpose. You could get four adults to hold a platform on which your test subject stands. The adults all release simultaneously, and the kid “rides” the platform right down into the impact point; after ten trials, you instruct the kid to try jumping on the way down. This experimental setup puts four adult “spotters” around the kid. You can give the kid a helmet, kneepads, wrist guards, and duct tape a pillow to his ass… whatever safety demands. I think most eighth graders would “jump” at the chance to be the test dummy.
If you can prove to them that the difference between the two impact forces is a constant (and not linear or quadratic) with variable drop height, then the measured difference is your answer to “how much does jumping help?”.
2 things that will make this a little difficult is that
- the spring or whatever needs to go off just BEFORE impact.
- The spring and item being pushed must be substantially lighter than the falling “box” or whatever, or the action of the spring will simply accelerate the box faster in a downward direction.