# Jumping from a chair while falling from a building

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1. May 15, 2018

### Koalala

A few years back when I was quite young I stumbled upon this.I'm asking whether this is plausible. I'm guessing it isn't, since the chair hasn't absorbed any of the falling force, and you are still travelling at the same speed of descent, just wanted to discuss.

2. May 15, 2018

### Staff: Mentor

Keeping it simple and neglecting some effects due to air resistance, both you and the chair are in free fall, so you wouldn't be able to "jump off" the chair. Since you are heavier than the chair, you would just send it faster to the ground.

Also, what hurts/kills is the deceleration, the fact that you go from travelling fast to not moving at all that is the problem. That's why Superman catching a falling Lois Lane by flying in the opposite direction is not a good idea.

3. May 15, 2018

### PeroK

That's pretty much it. You are falling at high speed. The best you could do is reduce your speed slightly by pushing the chair away.

4. May 15, 2018

### Koalala

Thanks,hope the creator sees this

5. May 15, 2018

### sophiecentaur

Hi and welcome. This is a very Tom and Jerry situation and it sort of feels right, intuitively. But it isn't, I'm afraid.
There are many arguments against this one but my favourite would be that your legs are not powerful enough (cannot supply enough Energy) for you to jump up more than a metre or so. So how would they cope with all the Kinetic Energy your body has acquired whilst falling 20m or more?
If this system were to be workable then no one would need to use a parachute - just carry a chair as they jump out of a plane.

After a fall of 20m, your body would be travelling at around 20m/s. The Kinetic Energy would be 20000J for a person with a mass of 100kg. (That could be me). If you had 1m to bring your self to rest with your legs (very long legs!) then this would involve a constant force of 20kN (= a lift of 2 tonnes)
Feel free to check my arithmetic; I have been known to drop the occasional zero along the line!

6. May 15, 2018

### Staff: Mentor

I hope the 0 took a chair with it

7. May 15, 2018

### jbriggs444

There is a small grain of truth in the scenario. If you are falling and seated in a chair and if you get to choose when to jump off the chair, you get the maximum reduction in impact energy by jumping off at the last possible moment. [Unfortunately, the reduction is not enough to help much].

The later you jump off, the greater the velocity with which the chair is propelled downward. Jumping at the last moment maximizes the kinetic energy that the chair carries away. One can alternately argue that jumping off early reduces your velocity early, leaving you in the air for longer and giving gravity more time to act. Both arguments describe the same effect.

8. May 15, 2018

### Staff: Mentor

For what reason? You know these aren't meant to be true, right? That's why it's called "trolling".

Anyway, to me the simplest way to view this is from an energy perspective:

Most people can only jump about 1m up and the building in the picture is 6 floors or about 20m. So the best you could hope for is to dissipate 1/20=5% of the impact energy (assumes no air resistance and that you can actually execute the jump).

[Edit: looks like that analysis is wrong for not considering the energy in the moving frame. Don't have time to redo right now...]

9. May 15, 2018

### jbriggs444

Let's accept that 1 meter figure. [It is probably very generous for what could be accomplished jumping from an unsupported chair].

A 1 meter jump height corresponds to a velocity $v=\sqrt{2gh}$ which is about 4.5 meters per second.

A 20 meter fall corresponds to a velocity $v=\sqrt{2gh}$ which is about 20 meters per second.

If you jump off at the last minute, that's a resulting impact velocity of about 15.5 meters per second. Which corresponds to a fall from $h=\frac{v^2}{2g}$ which is about 12 meters. Call it a four story equivalent drop instead of six.

It is an example of the Oberth effect. [As @russ_watters surely knows]

Last edited: May 15, 2018
10. May 17, 2018 at 10:57 AM

This kind of reminds me of a debate I had with my High School GF's family about parachutes. I do not recall how we got on the topic, but they were all convinced that when a chute opens - the person "shoots up" - because that is the way it looks to the camera that has not had the chute open yet - ugh, drove me crazy.

Relationship did not last.

11. May 18, 2018 at 7:38 AM

### Koalala

Not to be rude but probably for the best. Someone once gave a present to his gf but said it was from their cat.She and her parents were guinuinly upset that the cat got her a present and not him

12. May 18, 2018 at 7:41 AM

### jbriggs444

Jumping out of a sixth floor window onto a cat present would be adding insult to injury. The relevant physics question would be whether a cat dropped at the last moment would still land on its feet.