The time take to two balls (gravitationally attract), to reach each other

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Discussion Overview

The discussion revolves around the problem of determining the time it takes for two gravitationally attracting balls to reach each other, given their masses, radii, and initial distance. The conversation explores theoretical approaches and mathematical methods to solve this problem, including calculus and numerical simulations.

Discussion Character

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

Main Points Raised

  • One participant poses the problem of calculating the time for two balls to attract each other under gravity, noting the complexity due to varying forces as distance changes.
  • Another participant suggests using calculus or a computer program to model the changing gravitational force and acceleration over time.
  • A participant expresses difficulty in using calculus to derive precise values for the distance between the spheres at specific times.
  • Concerns are raised about the interdependence of distance and acceleration, creating a catch-22 situation in solving the problem.
  • A participant references a previous thread that purportedly contains a solution using calculus.
  • There is confusion regarding the integration steps in the calculus approach, particularly in relation to a specific equation involving gravitational force.
  • One participant expresses a lack of understanding of the mathematical concepts involved and seeks resources for learning the basics.

Areas of Agreement / Disagreement

Participants generally agree on the complexity of the problem and the need for calculus or computational methods, but there is no consensus on the specific steps to solve it or the clarity of the mathematical processes involved.

Contextual Notes

Participants mention the need to consider the distance between the centers of the spheres and the fact that the distance will not reach zero as the spheres touch. There are unresolved mathematical steps and dependencies on understanding calculus.

Mad_Eye
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two balls with known mass, radios, and distance between each other, are attract, due to gravitational force. how much time will it take them to reach each other?

this is NOT homework... just something i thought about, and have no idea how to solve.

(pay attention the mass and radios of each ball is not necessarily identical)
 
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Have you done calculus?

If not you can solve this with a computer program or a spreadsheet.
the tricky thing is that the force depends on the distance and is constantly changing
So you could work out for each second what the separation is, then what the force is an then the accelration which would give the speed an the change in distance for that second.
You keep doing this until the separation is zero.

Remember the distance in the gravity equation is the distance between centres, while the processes ends when the spheres touch - the distance will never got to zero.
 
Mgb_phys,
How can we solve this using calculus and get precise values for distance between the two spheres at a particular instant?
I haven't mastered calculus but can follow the steps with some difficulty..
 
Hmm yeah I've tried to solve this problem but it's a catch-22
You need to know the distance to find the acceleration at time t,
But you need to know the acceleration to find the distance at time t.
 
sganesh88 said:
Mgb_phys,
How can we solve this using calculus and get precise values for distance between the two spheres at a particular instant?
I haven't mastered calculus but can follow the steps with some difficulty..

+1 exact same with me
 
Thanks DocZaius,

with that answer, I'm confused why arildno multiplies both sides of

[tex]\frac{d^2D}{dt^2} = \frac{-2G}{D^2}[/tex]

by [tex]\frac{dD}{dt}[/tex]

[tex]\Rightarrow \frac{d^2D}{dt^2}\frac{dD}{dt} = \frac{-2G}{D^2}\frac{dD}{dt}[/tex]

And also how you're supposed to integrate that o.O
 
wow. i didn't understand anything... where can i learn this stuff from the very basic?
 
Last edited:

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