If there is a fire in a building and I am on 10th floor and I can't exit the building.Is it safe to jump out of the window with a chair and when chair is close to the ground I just jump of the chair and I will be safe. Is this safe or not? Would I live or not
$\begingroup$
$\endgroup$
2
-
$\begingroup$ discovery.com/tv-shows/mythbusters/mythbusters-database/… $\endgroup$– GenlyAiCommented Jun 25, 2019 at 1:45
-
$\begingroup$ I don't agree that the cited question is a duplicate, because an elevator has a lot more mass compared to a person than a chair. In an elevator, the force applied by a jumping person would do little to change the elevator's velocity, so the jumper would lessen their downward velocity by whatever force they applied against the elevator. However, a chair has so little mass compared to a person, that someone jumping from a falling chair would do little to change their velocity. $\endgroup$– BillDOeCommented Jun 25, 2019 at 21:01
Add a comment
|
3 Answers
$\begingroup$
$\endgroup$
3
It's safe, if and only if you are able to jump off this chair with such vigour, that you compensate for all the momentum you have acquired during your falling, $p \approx 70 \ \text{kg} \cdot 10 \ \text{m/s/s} \cdot 2 \ \text{s} = 1400 \ \text{kg m/s}$. The chair needs to carry your and its own momentum downwards when you jump off it. Unfortunately that is a lot of momentum. And no human could push so hard off this chair (you'd have to accelerate the chair downwards probably with some 100's of km/h terminal velocity after full extension of your legs, because of the mass difference. Your legs would break). So no, it's not safe.
A more interesting question could be: What mass of chair should you pick if you had different chairs available for maximum survival probability with increasing height of the jump (the chairs all being of the common "vanishes on impact" variety of course).
-
$\begingroup$ The jump from the chair ought to be as violent as the collision with the floor. He would be as dead. $\endgroup$– SwikeCommented Jun 25, 2019 at 0:14
-
1$\begingroup$ Nice perspective. But one could at least distribute the momentum transfer on two events, thus halving the peak force on the landing. $\endgroup$– Georg E.Commented Jun 25, 2019 at 11:51
-
$\begingroup$ True, that's actually quite interesting. I guess if you did a multiphase eyection at the right instants you could collide with the floor with less energy than the one required to kill you $\endgroup$– SwikeCommented Jun 25, 2019 at 12:52
$\begingroup$
$\endgroup$
2
In principle well, yes, you will make your chances a bit better: that would be like an ejection seat in military planes. You will separate from the barycenter and get an acceleration upwards, if that is sufficient will depend on the mass of the chair and on the spring force you can exerce. However that will be significantly lower than needed to counteract the speed acquired, unfortunately ..
$\begingroup$
$\endgroup$
2
No, this is impossible. First of all, assuming 3 meters (10 ft) per floor and you're jumping from just above the ninth floor, you and the chair will be travelling about 24 m/s (78 ft/s) or about 85 Km/h (53 mph). When you try to jump, you'd have to jump at a rate of 24 m/s (78 ft/s), and even if you could, the chair would have to have sufficient mass to counter yours, which would make it extremely difficult to lug to the window. If you could jump with that velocity, you would surpass the record high jump by about twelve times.
Edit: Say you weigh about 70 Kg (154 lbs) and were strong enough to lug a chair of equal weight to the window. When it came time to jump (you'd already have to be squatting) and you could manage to propel yourself at the aforementioned 24 m/s, since the chair has as much mass as you, whatever acceleration you imparted to yourself would also be imparted to the chair, and since the chair has the same mass as you, that force would be divided equally between you and the chair. Applying enough force to propel yourself vertically at 24 m/s would instead give an additional velocity of 12 m/s to the chair and 12 m/s to you. Your speed away from the chair would still be 24 m/s but relative to the ground, only 12 m/s. I'd say a collision with the ground at a velocity of 12 m/s would still be fatal.
