Energy required to the Moons orbit

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    Energy Moons Orbit
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Discussion Overview

The discussion centers on the hypothetical scenario of calculating the energy required to maintain the Moon's orbit without the influence of gravity. Participants explore the implications of such a scenario, including the nature of forces and energy in orbital mechanics.

Discussion Character

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

Main Points Raised

  • One participant questions the feasibility of deriving an equation to calculate the energy required for the Moon's orbit without gravity, suggesting that the universe's structure relies on gravity.
  • Another participant proposes that keeping an object in circular motion does not require energy if the distance remains constant, citing the work-energy principle.
  • There is a discussion about the force required to maintain the Moon's orbit, with references to tension in a hypothetical string connecting the Earth and Moon.
  • One participant mentions that power is zero in circular orbits since the force and velocity are perpendicular, implying no change in energy.
  • Another participant suggests using Newton's law of gravitation to determine the force needed to keep the Moon in orbit, offering a formula for calculation.
  • Participants discuss the units of force and how to express the required force in different measurement systems.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of energy to maintain the Moon's orbit without gravity, with some arguing that no energy is required while others suggest that a force must still be applied. The discussion remains unresolved regarding the implications of these viewpoints.

Contextual Notes

The discussion involves assumptions about the nature of forces and energy in a gravity-free scenario, which may not align with conventional physics principles. The calculations and hypothetical scenarios presented depend on simplifications and ideal conditions.

Who May Find This Useful

This discussion may be of interest to those exploring theoretical physics, orbital mechanics, and the implications of gravity on celestial bodies.

Bjarne
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How much energy would it require (per second or per orbit) to keep the moon in orbit, if gravity did not exist?

Pretend the gravity from Earth did not exist, and the moons still should orbit like it does.

Does it exist a equation to calculate that?
 
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I don't think such an equation will ever exist seems as our universe (that is the visible universe) we live in is dependent on gravity to keep everything in the cosmos in order. But I guess there is a chance that my assumption is wrong and their maybe some genius who may determine a formula and thus answer your question :)
 
Bjarne said:
How much energy would it require (per second or per orbit) to keep the moon in orbit, if gravity did not exist?

Pretend the gravity from Earth did not exist, and the moons still should orbit like it does.

Does it exist a equation to calculate that?
w=f*d
d=0, therefore w=0

Keeping an object moving in a circular path, whether via an orbit or just tying a string to it and pulling on it, does not require energy.
 
russ_watters said:
w=f*d
d=0, therefore w=0

Keeping an object moving in a circular path, whether via an orbit or just tying a string to it and pulling on it, does not require energy.

In that case you will have energy "on that string" pulling outwards
How much energy on that string per second ?'
Would the object lose speed ?
 
Bjarne said:
In that case you will have energy "on that string" pulling outwards
How much energy on that string per second ?'
Would the object lose speed ?
No, you won't have energy lost on the string. The force is perpendicular to the direction of motion.
 
As I mentioned in your other post, P=F dot v

F and v are perpendicular for circular orbits, and thus there is no power! No power means no change in energy since P=dE/dt=0

The only thing that matters with energy is Change in energy. You can assign any arbitrary energy for the system, but if it doesn't change, then it doesn't matter.
 
Bjarne said:
How much energy would it require (per second or per orbit) to keep the moon in orbit, if gravity did not exist?

Pretend the gravity from Earth did not exist, and the moons still should orbit like it does.

Does it exist a equation to calculate that?

Would you like to know the force required to keep the moon in orbit?

What units of force do you like to use?

In metric, the unit of force is usually the Newton.

In other systems force can be expressed in pounds-of-force, or tons-of-force, or some other way. If you would like to know the force required to keep the moon in orbit, would you like to know it in terms of tons-force?

There certainly is a formula! You just have to say what units of force you prefer.
 
If gravity was going to be turned off, and you tied the two bodies together with a long string, then the string would have to be 240 thousand miles long. (Simplifying the orbit to be circular)

And the force I am talking about would be the tension on the string that holds the two together.

It doesn't take any energy, because energy is work, and nobody has to do any work to keep the two bodies whirling around, attached together by a piece of ideal string. But there is this force, this tension in the string.

Try putting this into the google search window:

G*(mass of earth)*(mass of moon)/(240000 miles)^2

type it in, just like that, and press "return"

it will tell you 2 x 1020 Newtons.

That is how much force. If you want it in pounds you can, instead, type in

G*(mass of earth)*(mass of moon)/(240000 miles)^2 in pounds

and then divide the answer by 2000 to get it in tons, because there are 2000 pounds in a ton.
 
Last edited:
For the force, you can just use Newton's law of gravitation.


[edit: you beat me by a few seconds...]
 
  • #10
Great minds, Russ :biggrin:
 

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