Why doesn't the moon fall on earth?

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Discussion Overview

The discussion revolves around the question of why the moon does not fall into the Earth, exploring concepts of gravitational forces, orbital mechanics, and the nature of elliptical orbits. Participants also draw parallels with the Earth's orbit around the sun.

Discussion Character

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

Main Points Raised

  • One participant questions why the moon's elliptical orbit does not lead to a spiral fall into the Earth, especially when it is closest to the planet.
  • Another participant explains that while gravitational force is stronger when the moon is closer, its high sideways velocity compensates for this force, allowing it to maintain its orbit.
  • A participant raises a follow-up question about why speed increases as an object approaches the center of its elliptical orbit.
  • Conservation of energy is mentioned as a way to explain the increase in speed when an object is closer to a massive body, as gravitational potential energy decreases and kinetic energy increases.
  • One participant likens the moon's motion to jumping off a diving board with enough forward velocity to miss the ground, illustrating the concept of an elliptical orbit.
  • Another participant notes that the moon is gradually moving away from the Earth, suggesting that it is always "falling" towards the Earth in a sense.
  • A historical reference is made to the launch of Sputnik, connecting the question to past discussions about orbits.
  • One participant discusses the perspective of falling from a great height in space, emphasizing how the vast distances make it difficult to "hit" the Earth despite its gravitational pull.
  • A participant points out that both the Earth and the moon exert gravitational forces on each other, questioning why the focus is solely on the moon's fall towards the Earth.
  • Another participant raises a philosophical question about the concept of "up" in relation to gravity and outer space.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics of orbits and gravitational interactions, with no consensus reached on the implications of these concepts. The discussion includes both agreement on certain principles and differing interpretations of the moon's motion and gravitational dynamics.

Contextual Notes

Participants discuss the complexities of gravitational interactions and orbital mechanics without resolving all underlying assumptions or mathematical details. The discussion reflects a range of perspectives on the nature of orbits and gravitational forces.

ankeshk
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I am curious as to why the moon doesn't fall into the earth. Did a few searches and found:

https://www.physicsforums.com/archive/t-58667_Why_doesnt_the_moon_just_fall_to_the_earth?.html

But that doesn't explain one thing.

Moons orbit is not an exact circle. Its "almost" circular. So when the moon is closest towards earth, shouldn't Earth's gravitational force on moon be stronger resulting in a spiral down-fall and eventally moon crashing into the earth?

And a similar question: why doesn't the Earth fall into the sun? Earths orbit is even more oblique. So shouldn't the sun attract Earth with greater force when it is nearer on days of equinoxes and eventually lead to a spiral down fall too?
 
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When the moon is closest to the Earth, it's also traveling "sideways" at its fastest rate. This is generally true for any elliptical orbit. An extreme case is that of a comet, such as Halley's comet. When it is close to the Sun, it travels at such a high speed that it takes less than a year to go a fair portion of the distance around, but when it's far away it takes another 75 years to return again. Why? Because it slows down as it gets further from the sun.

You're right that the attractive force of gravity is greater when the moon is closer to the Earth, but the extra speed compensates for the extra pull, and the moon still "misses" the Earth as it falls around it.
 
Ah, that makes sense.

And then the question arises: why does the speed of an object increase when its closer in its elliptical orbit?
 
Conservation of energy is one way to explain it easily. As the planet comes close to the sun it's gravitational potential energy is lowest so it's kinetic energy is highest.
 
ankeshk said:
Ah, that makes sense.

And then the question arises: why does the speed of an object increase when its closer in its elliptical orbit?
Simply put, when an the moon is moving towards the earth, it speeds up because of the gravitational pull that is pulling it closer to the earth. Its exactly the same as when you jump off a diving board and accelerate as you fall. Imagine, though, if you jumped off a really tall diving board with enough forward velocity that you'd miss the Earth when you fell toward it. That's an eliptical orbit.
 
the spped that it travels counter acts the gravity on it. So it doesn't come flying to the earth.
:D
 
ankeshk said:
why the moon doesn't fall into the earth

In a way it does, all the time.

Even if each year it is a little bit more distant.
 
I was asked the same question on October 4, 1957, when I told my soccer coach that the Russians had just launched Sputnik, the world's first artificial satellite.

Bob S
 
When you're on Earth and you fall down from something, you will always hit the ground because that's all there is "down".

However, it's not the same when you get into astronomical distances. Imagine you you go way up into space, as far as the moon is, and you look "down" towards the earth. This is what you see:

[PLAIN]http://reconstruction.eserver.org/072/images/Uddin_AS08-14-2383.jpg

In fact you see even more sky and less earth, as that picture is cropped. So now the Earth is just a small part of the sky. If you "fall" towards the earth, it's not so easy to hit anymore. Chances are you will end up missing it even though it's pulling you towards it. If you do miss it, you'll still a have a huge speed due to the Earth's gravity, and you'll end up orbiting the thing.
 
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  • #10
Let us not forget that both the Earth and the moon are pulling on each other. Why are we not asking about the Earth falling onto the moon?

Furthermore, it's been decades since we landed on the moon. Why do we still consider outer space to be the in the direction of "up"? Is "up" intrinsically tied in with our definition of gravity?

====8<------------------
 

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