Can any object with positive mechanical energy collide earth?

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Discussion Overview

The discussion revolves around the conditions under which an object with positive mechanical energy can collide with Earth, exploring the nature of orbits and the mathematical underpinnings of different energy states in gravitational fields. Participants engage in conceptual clarifications, technical explanations, and historical perspectives related to orbital mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Historical

Main Points Raised

  • Some participants propose that an object with positive mechanical energy moves along a hyperbolic path, while those with zero energy follow a parabolic path, questioning the implications of these paths for potential collisions with Earth.
  • Others argue that for an object to collide with Earth, it must be given sufficient kinetic energy to exceed the escape velocity, allowing its kinetic energy to be greater than its potential energy.
  • Several participants seek clarification on why the paths of objects are hyperbolic, parabolic, and elliptic for positive, zero, and negative energies, respectively, indicating a desire for deeper understanding of the mathematical derivations involved.
  • Historical references to Kepler's laws are made, with discussions on how Kepler derived his laws without the concept of energy, and the limitations of mathematical models in explaining the "why" behind physical phenomena.
  • Some participants express satisfaction with the mathematical explanations but acknowledge a need for further study to fully grasp the concepts.
  • There is a recognition that while mathematics describes how phenomena occur, it does not necessarily explain why they happen, leading to a series of "why" questions that remain unresolved.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement on various points, particularly regarding the implications of mechanical energy on orbital paths and the historical context of Kepler's work. The discussion remains unresolved on several conceptual questions, with multiple competing views presented.

Contextual Notes

Participants highlight the mathematical nature of conic sections as solutions to the equations of motion under gravitational forces, but some express uncertainty about the derivations and implications of these results. There is also a recognition of the limitations of current mathematical frameworks in addressing deeper philosophical questions about the nature of physical laws.

Who May Find This Useful

This discussion may be of interest to those studying orbital mechanics, gravitational physics, and the historical development of astronomical theories, as well as individuals curious about the philosophical implications of scientific explanations.

AbhiFromXtraZ
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Object with positive mechanical energy in an attractive force field moves along a hyperbolic path and whose energy is zero, moves along parabolic path...isn't it?...But why?...Can't any object with ve energy move along the line joining centre of force?...If its radial component of kinetic energy is greater than effective potential energy? Then? If I'm right then spatial object can collide earth...
Please ellaborate conceptually and help me out...
Thanks.
 
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Object with positive mechanical energy in an attractive force field moves along a hyperbolic path and whose energy is zero, moves along parabolic path...isn't it?
Right.
But why?
Because our universe just looks that way.
Can't any object with ve energy move along the line joining centre of force?
Those are "special cases" of parabolas and hyperbolas. Usually straight lines are not called in this way, but that is just a naming convention.
 
The sum of Potential and Kinetic energy (total energy), on impact can be greater than Zero. All that's necessary is that it was given a big enough kick, whilst way out in space, in the direction of Earth, so that it hits the Earth with more than the escape velocity for an object being launched from the Earth's surface. So its KE (always positive) will be greater than its Potential Energy (negative).
 
Couldn't get it clearly. Can you please explain why the paths of objects are hyperbolic, parabolic and elliptic for energies positive, zero and negative respectively?
 
AbhiFromXtraZ said:
Couldn't get it clearly. Can you please explain why the paths of objects are hyperbolic, parabolic and elliptic for energies positive, zero and negative respectively?

I assume you know the mathematical form of the different conic sections. The 'reason' they turn out to be the paths of ideal orbits is very mathematical and they are what you get by solving the equation of motion of a body under the influence of an inverse square law force. The conic sections are not arm waving shapes and can't really be derived in an arm waving way. The Maths does it perfectly in very few lines.
I got a lot of google hits when I searched with terms like orbit, gravity, hyperbola etc.
This one may be interesting.
 
Thanks a lot...actually I already read the mathematical explanation...To totally get it I have to study more...
Anyway thanks to all for responding...
 
Well...finally after a google search for ''how did kepler derive his first law'', I could find how kepler did it... Although I couldn't understand totally...but I'm satisfied because he didn't use the Energy condition...By Brahe's observation data and his trial and error, he could successfully do it...
Here is the website...
http://math.berkeley.edu/~robin/Kepler/
 
Dear old Kepler didn't use the Energy method because no one was aware of the Laws of Gravitation at the time. He was just fitting the simplest mathematical model to the observations - really smart. I am left breathless by the early astronomers did with their very basic observation techniques (+ lots of total dedication).
 
  • #10
Ya...absolutely...but one thing is that Kepler just explained the nature of orbits. He didn't explain why the nature is so. Now a days we have lots of mathematics for that. But these mathematics could not satisfy me...Hope they will...
 
  • #11
AbhiFromXtraZ said:
Ya...absolutely...but one thing is that Kepler just explained the nature of orbits. He didn't explain why the nature is so. Now a days we have lots of mathematics for that. But these mathematics could not satisfy me...Hope they will...

Our math does a wonderful job of showing HOW many phenomena work, but it will never show WHY they work. The answer to a "why" question is always another "why" question.

You can say that the mathematical laws of motion show why planets have elliptical orbits but then you have to ask why to those laws work? You can get to general relativity and say that they work because of curved space-time, but you have to ask, why is space-time curved? The bottom question in the chain is always another "why" question.
 
  • #12
phinds said:
Our math does a wonderful job of showing HOW many phenomena work, but it will never show WHY they work. The answer to a "why" question is always another "why" question.

You can say that the mathematical laws of motion show why planets have elliptical orbits but then you have to ask why to those laws work? You can get to general relativity and say that they work because of curved space-time, but you have to ask, why is space-time curved? The bottom question in the chain is always another "why" question.


Totally agree with you...There is always a ''Why?''. But some times some answer to 'why' satisfies us and we don't ask further 'why'.
Although I have not read Relativity...so I think I have to wait until I gather sufficient knowledge...
 

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