Can two stationary masses repel each other through gravitational forces alone?

  • Context: Undergrad 
  • Thread starter Thread starter Bob not Alice
  • Start date Start date
  • Tags Tags
    Neutral Repel
Click For Summary

Discussion Overview

The discussion revolves around the possibility of two stationary masses repelling each other through gravitational forces alone, with a focus on theoretical scenarios involving mechanical devices and the splitting of one mass. The scope includes conceptual exploration and theoretical reasoning.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • Bob proposes that two stationary masses can end up further apart through gravitational attraction if the smaller mass splits into two during the process, allowing for the use of mechanical devices like springs.
  • Another participant suggests that many possible solutions exist, indicating a range of interpretations regarding the problem.
  • A participant questions the initial premise by pointing out that the wording implies only two objects, which may not allow for splitting, and asks for clarification on the definition of distance measurement between the masses.
  • Bob clarifies that he is not considering solutions involving permanent mass loss or direct contact between the masses, emphasizing that the only force should be their mutual gravitational attraction.
  • Bob introduces the idea that the smaller mass could end up spinning, potentially leading to an orbital relationship between the two masses, while noting that the ability to split the mass is crucial for achieving the final state.
  • Another participant asserts that their ideas do not involve the masses touching and sees no limitations if arbitrary rearrangements of the smaller mass's components are allowed.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of the proposed scenarios, with no consensus reached on the possibility of achieving the stated outcomes solely through gravitational forces and mechanical devices.

Contextual Notes

There are unresolved assumptions regarding the definitions of distance and the implications of splitting the smaller mass, as well as the conditions under which the gravitational interactions are considered.

Bob not Alice
Messages
6
Reaction score
0
Hi folks,

A system of two masses, stationary with respect two each other, is the starting point. For convenience I'll specify one mass as being much larger than the other. The only force acting between them is their mutual gravitational attraction. At time zero they are an arbitrary distance apart. Some time later they are further apart. Possible or not?

It's actually quite possible. Hint: the smaller mass splits into two during the manoeuvre. Springs, or similar mechanical devices, are allowed.

Just a bit of fun. ;)

Bob.
 
Last edited:
Physics news on Phys.org
Bob not Alice said:
Possible or not?
Sure, many possible solutions.
Let them attract each other, at the point where they nearly touch, activate a repulsive spring that splits the smaller mass into two pieces that rapidly move away from each other until they are far away. Find some way to avoid a collision of the spring with the larger mass. Wait. After a while, combine the two parts again.
You trade energy needed by the spring for kinetic energy and/or energy in the gravitational potential.

Alternatively: use a rocket. That is some sort of splitting as well but I guess it is not the intended solution as you lose the mass.

Alternatively: send bullets in a hyperbolic trajectory around the more massive object, catch them again. If you do this with sufficient mass, you get a net repulsion.

Alternatively: directly start splitting the smaller mass, position one half behind the larger mass and close to it, wait a bit, then combine the two smaller masses again. Probably needs some third piece or gyroscopes to avoid a collision.
 
  • Like
Likes   Reactions: Bob not Alice
Bob not Alice said:
A system of two masses, stationary with respect two each other, is the starting point. For convenience I'll specify one mass as being much larger than the other. The only force acting between them is their mutual gravitational attraction. At time zero they are an arbitrary distance apart. Some time later they are further apart. Possible or not?

It's actually quite possible. Hint: the smaller mass splits into two during the manoeuvre. Springs, or similar mechanical devices, are allowed.

The wording in your question implies that there are only ever two objects, meaning that they can't be split. Please clarify.
 
Oh, I have a trick solution. How is distance measured? Between their closest points? Between centers of mass? Between geometrical center for balls? For (1) and (3) here is another solution:
Have the center of mass of the larger object close to the point with the largest initial distance to the smaller mass, and have a small hollow tube in the larger mass where the smaller and lighter mass can pass through instead of impacting. Then just wait. The distance between the geometric centers will increase beyond its initial value after a while.
 
Hi mfb,

Good stuff. :oldsmile: The rocket concept isn't really what I was looking for as it involves a permanent mass loss to the system. Neither was the "bounce", hence my stipulation the "the only force acting between them is their mutual gravitational attraction". That was meant in reference to the two initial masses, which I may not have stated explicitly enough.
Your third solution is closest to my own which involved splitting the smaller mass into two equal masses following elliptical orbits.

With the same initial conditions as before I believe it should also be possible to end up with the two masses orbiting each other! Extra hint: The smaller mass has to end up spinning. I'll not say more so that anyone who wants to offer a solution can do so...

Not news for most here, but I think there is something to be taken away from this game. Without knowing you can temporarily split the smaller mass it's not possible to get from the initial state to the final state and if the only two observations one could make were the initial and final states then one would cry "fraud". And this is just in the world of classical physics. :oldlaugh:

Bob.
 
Which bounce? None of my ideas involve the masses touching each other in any way.

I don't see limitations if you allow arbitrary rearrangements of components of the smaller mass.
 

Similar threads

Undergrad Is this analogy between Magnetic Force and Gravitational Force accurate in helping to understand what contributes more to a stronger magnetic force?
  • · Replies 11 ·
Replies
11
Views
2K
High School How many electrons to make a stable gravitational object?
  • · Replies 5 ·
Replies
5
Views
2K
High School Gravitational & Electrostatic Force: Why They Attract/Repel
  • · Replies 3 ·
Replies
3
Views
2K
How can I make my homemade electromagnets repel each other?
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Will Two Identical Spherical Cavities in Free Space Attract or Repel Each Other?
  • · Replies 19 ·
Replies
19
Views
5K
Is it possible for opposite charges to repel each other?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Atomic Clock Speed graph between two stationary masses
  • · Replies 23 ·
Replies
23
Views
2K
Undergrad Can 2 spheres orbit each other say inside ISS?
  • · Replies 8 ·
Replies
8
Views
3K
Graduate Solve the positions of two masses gravitating toward each other
  • · Replies 13 ·
Replies
13
Views
2K
Balanced forces - Electrical and Gravitational forces
  • · Replies 8 ·
Replies
8
Views
2K