Author Topic: The Physics of jumping  (Read 3000 times)

Offline onebigmonkey

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The Physics of jumping
« on: October 03, 2014, 12:32:38 PM »
Over at DIF, user oz93666 is trying to establish that the astronauts should be able to jump much higher than they are observed to do in the lunar footage.

He has taken his idea to a physicsforum:

https://www.physicsforums.com/threads/why-a-man-on-the-moon-can-jump-21-times-higher-than-on-the-earth.774140/

I am no physicist so find it difficult to prove the maths for either side of a debate, but there are many here who are.

So...erm...discuss :D

Is his logic sound? Are his assumptions reasonable? Here is his post in full:

Quote
Two men , one on the Earth, one on the moon, both bend their knees the same amount ready to do a 'standing' jump.
They both start , and accelerate upwards.
The earth man is pushing against the inertia of his mass and the downward force of his earth weight.
The moon man is pushing against the inertia of his mass and the downward force of his moon weight.

Clearly the moon man will have a higher velocity when his feet leave the ground than the earth man, and so will travel OVER six times higher.

How much higher will depend on how long the acceleration zone is.

When I do this I squat right down to the floor and can jump 30cm high. the acceleration zone is 90cm.

The energy the earth man expends against the extra gravity iis 0.9 x( 1 - 1/6)g

Equivalent to 0.9 x5/6 =75cm in earth gravity total jump =30 +75 =105cm

This means he can jump 6.3meters on the moon!! 21 times higher
 

Offline darren r

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Re: The Physics of jumping
« Reply #1 on: October 03, 2014, 12:42:29 PM »
I'm no physicist either, but has he taken into account all that mass the 'moon man' would necessarily be carrying? Or the fact that it might not be a particularly great idea for him to jump as high as he can in an environment where landing badly could be life-threatening?
" I went to the God D**n Moon!" Byng Gordon, 8th man on the Moon.

Offline JayUtah

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Re: The Physics of jumping
« Reply #2 on: October 03, 2014, 12:45:08 PM »
He's throwing out a hodge-podge of physics-related terms, and he's also missing several important factors in the comparison.

The factors he's missing are the much greater inertia of a space-suited astronaut (ca. 360 pounds-mass) and the frictional assistive devices in the space suit that work to keep joints bent once the astronaut has bent them.  The former limits the effective velocity he can attain and the latter limits how much force is applied to the underlying substrate and therefore the reactive force resulting in upward propulsion.

As for the math, I don't even know where to start telling him what's wrong with it.  His formulation is wrong literally (and no pun intended) at every step.
"Facts are stubborn things." --John Adams

Offline Dr.Acula

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Re: The Physics of jumping
« Reply #3 on: October 03, 2014, 12:48:31 PM »
Over at DIF, user oz93666 is trying to establish that the astronauts should be able to jump much higher than they are observed to do in the lunar footage.

He has taken his idea to a physicsforum:

https://www.physicsforums.com/threads/why-a-man-on-the-moon-can-jump-21-times-higher-than-on-the-earth.774140/

I am no physicist so find it difficult to prove the maths for either side of a debate, but there are many here who are.

So...erm...discuss :D

Is his logic sound? Are his assumptions reasonable? Here is his post in full:

Quote
Two men , one on the Earth, one on the moon, both bend their knees the same amount ready to do a 'standing' jump.
They both start , and accelerate upwards.
The earth man is pushing against the inertia of his mass and the downward force of his earth weight.
The moon man is pushing against the inertia of his mass and the downward force of his moon weight.

Clearly the moon man will have a higher velocity when his feet leave the ground than the earth man, and so will travel OVER six times higher.

How much higher will depend on how long the acceleration zone is.

When I do this I squat right down to the floor and can jump 30cm high. the acceleration zone is 90cm.

The energy the earth man expends against the extra gravity iis 0.9 x( 1 - 1/6)g

Equivalent to 0.9 x5/6 =75cm in earth gravity total jump =30 +75 =105cm

This means he can jump 6.3meters on the moon!! 21 times higher
 

I know this claim in many variations. My answer is as follows:
An astronaut has to accalerate his mass and the mass of his suit.
On Earth an astronaut (estimated weight about 90 kg) an his suit (let's take a round number: another 90 kg) have a mass of 180 kg. A jump with bended knees would be about 10 cm.
On Moon the weight is 1/6 (30 kg) due to the lower gravitation, but the mass (180 kg) is still the same. Youngs salute jump was about 50 to 60 cm high, which is almost 6 times higher.
Nice words aren't always true and true words aren't always nice - Laozi

Offline JayUtah

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Re: The Physics of jumping
« Reply #4 on: October 03, 2014, 12:51:36 PM »
By the way, this question comes up a lot and it's based fundamentally on a begged question.  We know by watching the Apollo 11 EVA that Armstrong achieved a vertical leap much taller (although certainly not at the theoretical upper limit) than would have been possible on Earth.  Most hoax claimants are unaware of this.  But the begged question is that the astronauts should have generally leaped higher on the Moon as a demonstration, and that the supposed lack of video evidence for this means it was not actually shot in diminished gravity.

It begs the question of propriety.  Just because they theoretically could have done a certain thing doesn't mean it was wise to do it.  Apollo 11 astronauts, for example, operated under severe limits for mobility, flexibility, and locomotion because engineers advocated conservative operations until they could get the EVA suits back for a performance analysis.  By Apollo 17 the astronauts were happily leaping all over the place.  (The joke passed up to Harrison Schmidt was that the Houston Ballet wanted him to audition.)  Not 5-7 meters above the surface, as the ignorant claims here suggest, but clearly not in Earth gravity.
"Facts are stubborn things." --John Adams

Offline Bryanpoprobson

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Re: The Physics of jumping
« Reply #5 on: October 03, 2014, 01:31:11 PM »
[youtube]http://youtu.be/DgYSua-sTV8[/youtube]

Do that on the Earth..  ;)
"Wise men speak because they have something to say!" "Fools speak, because they have to say something!" (Plato)

Offline raven

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Re: The Physics of jumping
« Reply #6 on: October 03, 2014, 02:04:11 PM »
This is why you really don't want to try a really high jump in a spacesuit.
'That ain't very smart', indeed!
It also looks nothing like jumping on Earth, or wire work for that matter.

Offline Dr.Acula

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Re: The Physics of jumping
« Reply #7 on: October 03, 2014, 02:16:00 PM »
This is why you really don't want to try a really high jump in a spacesuit.
'That ain't very smart', indeed!
It also looks nothing like jumping on Earth, or wire work for that matter.

Game, Set and Match raven  ;D
Nice words aren't always true and true words aren't always nice - Laozi

Offline twik

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Re: The Physics of jumping
« Reply #8 on: October 03, 2014, 02:38:26 PM »
Quote
Clearly the moon man will have a higher velocity when his feet leave the ground than the earth man, and so will travel OVER six times higher.

I'm just not quite grasping this concept. He's under 1/6 gravity, so he should leap *higher* than 6 times an earth jump?

Offline Luke Pemberton

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Re: The Physics of jumping
« Reply #9 on: October 03, 2014, 02:42:07 PM »
I am a physicist, and his physics is wrong. Utterly and totally wrong. So wrong in fact, I have no idea where he started with his formulation. He's just throwing numbers around.
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former - Albert Einstein.

I can calculate the motion of heavenly bodies, but not the madness of people – Sir Isaac Newton.

Offline Andromeda

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Re: The Physics of jumping
« Reply #10 on: October 03, 2014, 02:43:16 PM »
Agreed.
"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' but 'That's funny...'" - Isaac Asimov.

Offline JayUtah

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Re: The Physics of jumping
« Reply #11 on: October 03, 2014, 03:24:54 PM »
I am a physicist, and his physics is wrong. Utterly and totally wrong. So wrong in fact, I have no idea where he started with his formulation. He's just throwing numbers around.

I know, right?  It's like the phrase goes:  "That's not right.  That's not even wrong."  I don't know how else to describe but it as a tossed salad of words from physics along with arithmetic that bears no relationship to any concept in Newtonian dynamics.
"Facts are stubborn things." --John Adams

Offline raven

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Re: The Physics of jumping
« Reply #12 on: October 03, 2014, 03:38:18 PM »
This is why you really don't want to try a really high jump in a spacesuit.
'That ain't very smart', indeed!
It also looks nothing like jumping on Earth, or wire work for that matter.

Game, Set and Match raven  ;D
Thank you. I admit, I don't know the math of physics, but I do know that trying to jump straight up with a heavy load on your back without forward support means whoops! And when that heavy  load is keeping you alive, quintuple  whoops, at least!

Offline Dr.Acula

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Re: The Physics of jumping
« Reply #13 on: October 03, 2014, 03:41:43 PM »
This is why you really don't want to try a really high jump in a spacesuit.
'That ain't very smart', indeed!
It also looks nothing like jumping on Earth, or wire work for that matter.

Game, Set and Match raven  ;D
Thank you. I admit, I don't know the math of physics, but I do know that trying to jump straight up with a heavy load on your back without forward support means whoops! And when that heavy  load is keeping you alive, quintuple  whoops, at least!

Similar to you I'm not a physicist. But I do know how some important details work, and I do know how to do the maths behind it (most of the times  ;D) And I think the most important difference between us (you, me and all the others) and the hoax believer is: We know, what to do, when there's something we don't understand. And this is simply asking experts in the special field.
Nice words aren't always true and true words aren't always nice - Laozi

Offline beedarko

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Re: The Physics of jumping
« Reply #14 on: October 03, 2014, 11:19:59 PM »
This is why you really don't want to try a really high jump in a spacesuit.
'That ain't very smart', indeed!

If you asked Charlie Duke, he would probably agree with you.

 :o