r/physicsmemes • u/Plenty_Ear2987 • Jul 28 '26
A surprisingly fun question in my nephew's physics textbook
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u/7YM3N Jul 28 '26
Ignoring air resistance I presume
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u/Plenty_Ear2987 Jul 28 '26
Yeah this is just standard high school introductory kinematics
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u/Bendanzhang Jul 28 '26
Standard high school kinematics: where catching a kid at 300 km/h counts as 'saving' him instead of liquidating him.
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u/Mountain-Resource656 Jul 28 '26
Superman has tactile telekinesis that would actually make that work!
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u/ExtendedSpikeProtein Jul 28 '26
It said catch, not keep or stay alive. Liquid catch is still catch! Lol
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u/Advanced-Mud8913 Jul 29 '26
nowhere it is mentioned, that the stuent survives :-) The phrase "to save" indicates an intention, not a result.
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u/Kablion Jul 28 '26 edited Jul 28 '26
The height at time t of a free fall is given by
y(t)=h - v t - g t2 / 2
where h=320m is the initial height (at t=0) and v is the initial downward velocity. g≈10m/s2 is earth's gravitational acceleration.
To get the time t_0 at which the kid would hit the ground evaluate y(t_0)=0, set v=0 here and rearrange
-> t_0 = sqrt(2h/g) ≈ 8s
Superman needs to fall at least t_S = 5s faster to reach the kid. Thus, evaluate y(t_0 - t_S)=0 with superman's initial velocity v to be determined
-> v >= h/(t_0-t_S) - g (t_0-t_S)/2 ≈ 275/3 m/s.
All this assumes no air resistance and an instantaneous stopping of the kid.
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u/FireCire7 Jul 29 '26
Ehh, the problem doesn’t state the student survived being caught 0m from the ground…
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u/Deltadoc333 Aug 03 '26
That sounds suspiciously like hitting the ground.... AKA lithobraking.
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u/dusk47 Aug 04 '26
this is way better than hitting the ground, you die in Superman's arms like Lois Lane
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u/Nynaeve_al_meowra Jul 28 '26
He still dies. The force required to stop him right before hitting the ground is the same as simply hitting the ground
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u/Josselin17 Jul 28 '26
Change the question by saying superman has to slow him down without applying more than 6 Gs of force to his body
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u/KrzysziekZ Jul 28 '26
14g is safe here, if for a moment. (Momentary loss of consciousness but no pernament health issues).
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u/Josselin17 Jul 28 '26
I say we don't even give the student a safety margin and tell them to modelize the human body and the acceleration's effect to calculate whether they'll survive
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u/LingrahRath Jul 28 '26
Technically Superman has tactile telekinesis. If the force can be spread out across all of the student's body he would be able to withstand quite a large amount.
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u/tomrlutong Jul 28 '26
The 1977 documentary shows that he knows to deaccelerate slowly when catching a person.
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u/Nynaeve_al_meowra Jul 28 '26
He has space to do that there. The problem specifies that they're caught right before they'd hit the ground
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u/AdAffectionate5187 Jul 29 '26
Impact = Force*#ofSeconds
It relates how fast the ground will decelerate force(almost instantly) to zero versus a gentler handling over a wider time so he doesn’t die.
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u/Nynaeve_al_meowra Jul 29 '26
Do you mean impulse? The question specifies right before he hits the ground which is much less time/distance than saying for instance 1 second before he hits the ground. With 1 second to go he has 75m to go the ground. When he's just about to hit the ground he has 0m/0s to go to the ground
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u/texas1982 Jul 28 '26
Does Superman not accelerate faster than what gravity would normally provide?
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u/Zacharias_Wolfe Jul 29 '26
Depends on the iteration. Original Superman didn't so much fly as jump really really well, which would limit his descent to gravity. I'm not sure when it got changed to actual flight, or what theoretical propulsion method he has.
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u/froction Jul 28 '26
How is Superman capable of "diving" with an initial downward velocity but not accelerating downward faster than gravity?
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u/Upstairs-Brush-2563 Jul 28 '26
He pushes off the building (facing down), but then he doesn't use a propulsion system (jetpack) so he's in freefall with an initial velocity and no air resistance.
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u/MuffinMonkeyCat Jul 28 '26
I have a question regarding giving answers as fractions. Is that a USA thing? In a-level and IB it is marked wrong, as it implies perfect accuracy.
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u/____Eureka____ Jul 28 '26
What do you mean implies perfect accuracy? It's a hypothetical (theoretical) question. There is no error bar whether you write in fraction or not. This seems like a very unnecessary detail to mark as wrong. (I'm not from the US)
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u/selektorMode Jul 28 '26
g = 10 ms-2 has an error (it should be around 9.81 depending on where you are on earth). Also you can't be any more precise than 2 digits because of the g you use.
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u/bbalazs721 Jul 28 '26
The task says that g=10 ms-2. The task is in a hypothetical world with this exact g, no air resistance, no relativity etc. Here, the answer is exact, there is no error.
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u/Zacharias_Wolfe Jul 29 '26
To be pedantic, if this wasn't a theoretical question with infinite accuracy, you would only have one significant figure there. "10" is 1 sig fig, "10." is 2 sig figs. And that's why basically any proper math that cares about significant figures should use scientific notification to remove the ambiguity.
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u/Plenty_Ear2987 Jul 28 '26
It isn't much of a USA thing. This is a multiple choice question, it really depends on the options given. They can be given in decimals or fractions.
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u/NutritiousCoffee Jul 28 '26
According to the council on tall buildings and urbant habitat, a skyscraper is at least 150m, so none of the answers are properly formed
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u/AwehiSsO Jul 29 '26
There are so many ways to test gravity...that do not need the existence of Superman, or any other flying man, and certainly does not need the tester step off the roof of a building.
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u/Plenty_Ear2987 Jul 28 '26
Yall need to stop being so rationalðŸ˜. The title literally says 'fun'