Gravity: What Forces Keep an Object in Orbit?

  • Context: High School 
  • Thread starter Thread starter pr1de
  • Start date Start date
  • Tags Tags
    Gravity
Click For Summary

Discussion Overview

The discussion revolves around the forces that maintain an object's orbit, particularly focusing on the Moon and hypothetical scenarios involving objects close to Earth. Participants explore concepts related to gravitational interactions, orbital mechanics, and the effects of atmospheric drag.

Discussion Character

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

Main Points Raised

  • One participant describes an orbit as a fast object falling endlessly due to Earth's curvature and questions what prevents it from slowing down.
  • Another participant suggests that the absence of wind resistance in space allows the Moon to maintain its velocity, although it does lose energy due to gravitational interactions with Earth.
  • A different viewpoint states that no force is needed to prevent slowing down, as the absence of drag is sufficient for maintaining speed in orbit.
  • Participants discuss whether an object could orbit at a very low altitude, such as 6 feet above Earth, if it were thrown with sufficient speed, with some arguing that wind resistance would still be a factor at that height.
  • There is a mention of the Moon's gradual movement away from Earth and its effect on orbital dynamics, with one participant noting that the Moon's orbit is not perfectly circular and varies over time.
  • Questions arise about the sensation of weightlessness in orbit compared to skydiving, with participants noting that atmospheric resistance affects the experience of free fall on Earth.
  • One participant emphasizes that orbits can vary for complex reasons and that the Moon's orbit is not fixed but stable over long periods.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the mechanics of orbits, the effects of atmospheric drag, and the nature of weightlessness. The discussion remains unresolved on several points, particularly regarding the implications of low-altitude orbits and the nuances of gravitational interactions.

Contextual Notes

Some statements made by participants include assumptions about ideal conditions, such as the absence of atmosphere, which may not hold true in practical scenarios. Additionally, the complexity of gravitational interactions and orbital mechanics is acknowledged but not fully explored.

Who May Find This Useful

This discussion may be of interest to those studying physics, particularly in the areas of orbital mechanics, gravitational theory, and the effects of atmospheric conditions on motion.

pr1de
Messages
9
Reaction score
0
Hello, I'm new here and am very glad I found these forums, I've been seeking answers for some questions that have been bothering me for some time.

Ok, first of all I want to ask about orbits. I know what an orbit is ( a very fast object virtually falling endlessly , never touching the ground because of the Earth's curvature , like Newton's canon demonstrates). My question is though, what forces keep the certain object from not slowing down? Like the Moon, what keeps its velocity so accurate that allows the moon to complete an orbit in exactly the same amount of time as the previous one?

That will be all for now, because I'd like to keep you guys concentrated on one question, to get the best answers possible for them. I'll keep asking as we move on :D
 
Physics news on Phys.org
Because there is no wind resistance in space. The moon does however slowly lose energy (velocity/distance from the earth) from the gravitational interaction between it and the Earth (but this is very small).
 
pr1de said:
My question is though, what forces keep the certain object from not slowing down?
You don't need a force to stop it slowing down - you just need there to be no force to slow it down.

In practice drag from tiny bits of the outer atmosphere does slow down satelites and cause them to change orbit and eventually re-enter.
At the distance of the moon there is nothign to cause drag. There are other effects to do with tides but these actually cause the moon to speed up - the moon is moving away from us at a measurable rate
 
My 2nd question is..

Does orbit depend on distance from the planet? Say, if i would have had superman powers and could throw a rock from just 6 feet above the Earth with enough speed so that the rock can "fall" into orbit, considering there's no atmosphere to create drag, could the rock orbit the planet at just 6 feet of height, given the speed of the rock is high enough?
 
considering there's no atmosphere to create drag, could the rock orbit the planet at just 6 feet of height, given the speed of the rock is high enough?

Six feet above the surface of the Earth there definitely IS wind resistance.


Also mgb_phys, I didn't know it was speeding up! Snap... I guess it's stealing the energy from own own rotation?
 
James Leighe said:
Also mgb_phys, I didn't know it was speeding up! Snap... I guess it's stealing the energy from own own rotation?
Yes the days are getting longer
 
James Leighe said:
Six feet above the surface of the Earth there definitely IS wind resistance.


Also mgb_phys, I didn't know it was speeding up! Snap... I guess it's stealing the energy from own own rotation?

noo, i said imagine there's no atmosphere, like on the moon.
 
pr1de said:
noo, i said imagine there's no atmosphere, like on the moon.

Oh, then yeah... that's fine.
 
so it can orbit?
 
  • #10
So long as you don't stand in its path! :biggrin:

(or there aren't any hills, etc.)
 
  • #11
My third question is.. if orbiting is just a continuous free fall without any resistance , wouldn't you feel the same weightlessness when skydiving out of the plane? if yes, the how are you able to distinguish up and down while skydiving and not in space where every way you turn you feel the same?
 
  • #12
pr1de said:
My 2nd question is..

Does orbit depend on distance from the planet? Say, if i would have had superman powers and could throw a rock from just 6 feet above the Earth with enough speed so that the rock can "fall" into orbit, considering there's no atmosphere to create drag, could the rock orbit the planet at just 6 feet of height, given the speed of the rock is high enough?

Yes. Suicide method :

Go to the moon. Fire a bullet horizontally at 1679 m/s, and wait for 108 min in the same spot. :wink:

pr1de said:
My third question is.. if orbiting is just a continuous free fall without any resistance , wouldn't you feel the same weightlessness when skydiving out of the plane?
Initially yes, but then you feel resistance. You are not in free fall in the atmosphere.
 
  • #13
pr1de said:
if orbiting is just a continuous free fall without any resistance , wouldn't you feel the same weightlessness when skydiving out of the plane?
Yes. that's how you can train astronauts in a falling plane

if yes, the how are you able to distinguish up and down while skydiving and not in space where every way you turn you feel the same?
Only by wind resistance - if you skydived inside a sealed box you couldn't tell
 
  • #14
Hi pr1,
excellent...this is a great forum..just take answers as "tentative" and "mostly correct" most of the time, especially mine!

( a very fast object virtually falling endlessly , never touching the ground because of the Earth's curvature , like Newton's canon demonstrates).

does not have to be fast...for example, higher orbits require less speed..


My question is though, what forces keep the certain object from not slowing down
?

very complicated question..one I could not answer in a few sentences...but looking into gravitation and studying its various aspects will begin to give you insights for further understanding...Wikipedia is a free place to start...

In another discussion posted within the last year I posted an interesting fact that I had come across: the moon moves closer to the Earth by a small amount every year... it might have been a cm every ten years, but don't quote me on that..in any case orbits do vary for complex reasons...

Like the Moon, what keeps its velocity so accurate that allows the moon to complete an orbit in exactly the same amount of time as the previous one?

it's velocity is not really "accurate" but varies from Earth perspective as it passes in a non perfect, non circular orbit...I guess mostly elliptical...anyway the moon does not really orbit the earth, they both orbit their combined center of mass...hence the roughly elliptical orbit and the moon does NOT orbit "in exactly the same amount of time..." all we can say is that it's orbit IS fixed enough that it has been in place for many millions of years and likely will remain up there for many more millions of years...

If you want to get some insights here is a good place:

http://en.wikipedia.org/wiki/Orbit_of_the_Moon
 
Last edited:
  • #15
When skydiving on earth, you reach terminal velocity.
 
  • #16
pr1de said:
My 2nd question is..

Does orbit depend on distance from the planet? Say, if i would have had superman powers and could throw a rock from just 6 feet above the Earth with enough speed so that the rock can "fall" into orbit, considering there's no atmosphere to create drag, could the rock orbit the planet at just 6 feet of height, given the speed of the rock is high enough?

Yes. period squared is proportional to radius cubed. As for the rock, yes, that is possible.
 

Similar threads

High School Time to fall from 200 miles
  • · Replies 8 ·
Replies
8
Views
2K
High School Why Is an Object's Weight Equal to the Force of Gravity It Experiences?
  • · Replies 51 ·
2
Replies
51
Views
5K
Undergrad Remark on centrifugal force in Heino Falcke's black hole book
  • · Replies 22 ·
Replies
22
Views
3K
Graduate Orbital mechanics: is ballistic capture possible without acceleration?
  • · Replies 7 ·
Replies
7
Views
2K
Undergrad Stability of circular orbits in an arbitrary central force field
  • · Replies 3 ·
Replies
3
Views
2K
High School Gravity and speed of information for orbiting planets
  • · Replies 22 ·
Replies
22
Views
1K
High School Calculating the Effects of Velocity and Gravity on Orbital Motion
  • · Replies 6 ·
Replies
6
Views
2K
High School Does an orbiting mass exert force on what it is orbiting around?
  • · Replies 7 ·
Replies
7
Views
3K
High School A scenario of non-uniform circular motion
  • · Replies 4 ·
Replies
4
Views
942
Graduate How much mass with the centrifugal forces of earth to break gravity?
  • · Replies 32 ·
2
Replies
32
Views
4K