Confusing speed of man in moon question

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    Confusing Moon Speed
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Discussion Overview

The discussion revolves around a hypothetical scenario involving a man with superpowers jumping from the Moon to Earth in 10 seconds. Participants explore the implications of gravity, speed, and the physics involved in such a jump, considering both theoretical and practical aspects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the speed can be calculated simply using distance over time (d/t) or if other factors need to be considered due to the jump from the Moon.
  • Another participant suggests that gravity's effect is minimal at high speeds over a short duration, proposing that the man could be moving at approximately 15% of the speed of light.
  • It is mentioned that gravity would add about 14 m/s over the 10 seconds, and that the specific landing location on Earth significantly affects the calculations.
  • A participant provides an estimate of the man's speed, suggesting he could be traveling around 40 million meters per second, with a discussion on the negligible impact of gravity at that distance from both the Earth and the Moon.
  • Concerns are raised about the energy and force required for the man to accelerate to such speeds, with calculations suggesting an enormous force and energy output that would likely cause significant damage to the Moon's surface.
  • One participant clarifies that the 14 m/s figure is an upper limit for velocity gained from free fall under Earth's gravity, indicating that it may not apply directly to the scenario discussed.

Areas of Agreement / Disagreement

Participants express differing views on the calculations and implications of the scenario, with no consensus reached on the validity of the assumptions or the methods used to derive the speed and effects of gravity.

Contextual Notes

Participants acknowledge various assumptions, such as the initial conditions of the jump, the effects of gravity over the distance, and the significance of the landing location on Earth. There are unresolved mathematical steps and dependencies on definitions that impact the discussion.

thedudereturns
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Consider a man having superpower,weighing 70 kg jumping from moon to reach Earth in 10sec..what's the speed at which he travels..considering gravity etc...
 
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thedudereturns said:
from moon to reach Earth in 10sec..what's the speed at which he travels..considering gravity etc...
Gravity won't do much at that speed over this short period.
 
A.T. said:
Gravity won't do much at that speed over this short period.
So is it simple d/t or do we have to consider anything else ..he is jumping...to Earth from moon
 
thedudereturns said:
So is it simple d/t or do we have to consider anything else ..he is jumping...to Earth from moon
Ignoring all of the impossibilities, yes. It's simple d/t. He's moving at something like 15% of the speed of light.
 
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and over 10s gravity adds about 14m/s. You get more error by not specifying which bit of the Earth he lands on.
 
CWatters said:
and over 10s gravity adds about 14m/s. You get more error by not specifying which bit of the Earth he lands on.
Pardon me ..didn't quite understood what you said...please do explain
 
thedudereturns said:
Pardon me ..didn't quite understood what you said...please do explain
Understand*
 
thedudereturns said:
Pardon me ..didn't quite understood what you said...please do explain
The man is traveling in the neighborhood of 40 million meters per second. Most of that time will be spent far from the Earth and far from the moon where the acceleration from gravity is low. @CWatters has done some work and obtained an estimate of 14 meters/sec2 for average acceleration over the ten second interval.

As a crude estimate, that acceleration would amount to about a one part per million discrepancy in velocity. Maybe three parts per million if you are careless in the estimate. Three parts per million of the 400,000 km distance to the moon is about 1.2 km. There are mountains on the Earth taller than that. So the error from neglecting the acceleration is smaller than the error in neglecting to specify a landing spot.

[Note that the radius of the Earth is 6000 km. It's not just how high a mountain you land on. Whether you land dead center or nearer the horizon is even more important].
 
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CWatters said:
You get more error by not specifying which bit of the Earth he lands on.
Which is nothing compared to the variation in the distance between the Earth and the Moon.
 
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  • #10
jbriggs444 said:
Note that the radius of the Earth is 6000 km. It's not just how high a mountain you land on. Whether you land dead center or nearer the horizon is even more important

The location of the moon in its' orbit makes even more difference - about 45,000km between apogee and perigee...
 
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  • #11
So this superman accelerates from 0 to 4e7m/s in the distance of extending his legs and body. Say his center of mass moves 1m. The acceleration lasts 50ns. Let's say his mass is 100kg. That requires a force of 8e16N imparting an energy of 8e16J. That assumes the moon outweighs him by a very large margin (it does) and that the moons surface can take the massive impact (it can't!)

Rather than accelerating toward the Earth most of superman's energy will go into putting a very large crater in the moon. How big? Let's picture an asteroid colliding at the much more usual collision speed of 10 m/s. For the collision to have the same energy the asteroid would have to weigh 1.6e15 kg which is the mass of an asteroid about 20km in diameter. Good thing superman attempted this jump from the moon. If he tried it on Earth we'd all go the way of the dinosaurs!
 
  • #12
This reminds me of "The man in the moon came down too soon and asked the way to Norwich ...".
 
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  • #13
The 14m/s figure I mentioned isn't really valid. It's the velocity you reach if you fall, starting from rest and accelerating at 9.8m/s/s, for 10s. So it's the most that gravity could add (I think).
 
  • #14
PeroK said:
This reminds me of "The man in the moon came down too soon and asked the way to Norwich ...".
You turn right off the M11 motorway and drive about 1 hr in NE direction if you started in London
It doesn't rhyme though.
 
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