# Gravity: Will Low Velocity Bodies Revolve or Fall to Ground?

• batballbat
In summary: Earth. It is falling constantly but because of its tangential velocity it never gets any closer. So, to be clear, the Moon does not orbit the Earth. The Earth-Moon system orbits the common center of mass.In summary, an object with uniform motion at the surface of Earth would revolve around the Earth, but if its velocity is too low it would fall to the ground. The velocity can be determined by comparing it to the orbital velocity for that altitude. It is possible that the Moon was once a free-floating body and was captured by Earth's gravitational pull. The Moon must have been in motion to avoid falling to the center, and it is currently orbiting around the Earth

#### batballbat

a body with uniform motion at surface of Earth would revolve around the earth. is it true that if the velocity of the body is sufficiently low than it would not revolve and fall to ground? can we determine the velocity is it is so?
So is it true that the moon, on the grounds that moon was not a part of earth, was traveling in uniform motion before it came to Earth's field and started to revolve? And if had it not been in uniform motion, would collide with the earth?

batballbat said:
a body with uniform motion at surface of Earth would revolve around the earth. is it true that if the velocity of the body is sufficiently low than it would not revolve and fall to ground?

How can you say a bod with uniform motion at the surface of Earth would revolve around the earth? There is friction.. If you assume there is no friction then also it wouldn't be possible for Earth is not a perfect sphere. It has mountains and hills.

and what is difference between surface of Earth and ground in your question?? If it is already on the surface how can it fall to ground??

Are you talking about Orbits here? You mention "fall to the ground".
For an object to stay in a circular orbit, its speed must be exactly right so that the gravitational force is equal to the centripetal force needed for circular motion at the particular radius of that orbit.

to praksah phy sorry my question makes even someone interested in physics pose questions to a mathematical model. so please someone who is not deterred by thought experiments help me with this.

You need to describe your "thought experiment" in more detail if you want a proper response. So far you have not made it clear what you want.

batballbat said:
a body with uniform motion at surface of Earth would revolve around the earth.
If its velocity was orbital velocity, sure.

batballbat said:
is it true that if the velocity of the body is sufficiently low than it would not revolve and fall to ground?
Certainly.
batballbat said:
can we determine the velocity is it is so?
Certainly. Anything less than orbital velocity for that altitude.

batballbat said:
So is it true that the moon, on the grounds that moon was not a part of earth, was traveling in uniform motion before it came to Earth's field and started to revolve? And if had it not been in uniform motion, would collide with the earth?
Do you mean it was on a hyperbolic trajectory and got captured?

I think the question you are trying to ask is this:
Is it possible that the Moon was once a free floating body, on its own trajectory though the solar system and not gravitationally bound to Earth, but was captured when the Moon made a close fly-by of Earth?

The answer is: Mechanically speaking, it is certainly possible.

But there are other factors in considering that as a likely account of actual events. There are many facets of the Moon that must be considered in determining its origin - such as its composition when compared to Earth. They seem to be made out of similar materials - enough to suggest they were once the same body. However, there are competing theories.

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thanks dave
i just meant that the moon must have had been in motion because otherwise it would not revolve, rather fall to the centre. am i correct?

batballbat said:
thanks dave
i just meant that the moon must have had been in motion because otherwise it would not revolve, rather fall to the centre. am i correct?

Yes.

Sort of. Mostly.

Your question assumes that the Moon's motion is independent of the Earth Moon system. This is not necessarily so. There is at least one theory that suggests the Earth underwent a massive collision which knocked off a giant portion of it which eventually formed the Moon. So the formation of the Moon and its revolution about the Earth occurred as the same event. In this scenario it is nonsensical to talk about the Moon's motion independently of its origin.

However, even in this scenario, there was a body that has a respectable tangential velocity wrt Earth that it imparted to the Earth. So even in this case, angular velocity is required for orbit.

So: yes, insufficient angular velocity of a body at a distance from Earth would eventually fall to Earth (which much of the rubble from that aforementioned collision likely did).

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batballbat said:
thanks dave
i just meant that the moon must have had been in motion because otherwise it would not revolve, rather fall to the centre. am i correct?

Revolving (=spinning on its own axis) is not the issue. It is orbiting (translational motion around the Earth) that you are discussing. The two words have distinct meanings. But your ideas are correct. As I wrote in my earlier post, an object is in orbit when the force of gravitational attraction provides just the right centripetal force. If the object is not orbiting (has no tangential velocity) then there will always be an unbalanced force towards the Earth and it will fall.

The present idea (I believe) is that two planets collided and the Earth - Moon pair were the result. Our particular Moon is very big, relative to the Earth's mass - compared with many of the moons around other planets.

sophiecentaur said:
Revolving (=spinning on its own axis) is not the issue. It is orbiting (translational motion around the Earth) that you are discussing. The two words have distinct meanings.
I do believe the Earth revolves around the Sun and the Moon revolves around the Earth.

http://oxforddictionaries.com/definition/revolve [Broken]
to revolve about/around - to move in a circular orbit around (as in) the Earth revolves around the sun

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One thing worth pointing out is that two bodies traveling through space cannot, independently, suddenly get into orbit round each other. In the same way that a spaceship needs to slow down in order to land on a planet, there needs to be some other influence at work (like a collision) for two objects to change from a hyperbolic (glancing) interaction to an orbital situation. It is necessary for kinetic energy to be lost at a critical time in the trajectories or they will just depart in the same way that they arrived.

DaveC426913 said:
I do believe the Earth revolves around the Sun and the Moon revolves around the Earth.

http://oxforddictionaries.com/definition/revolve [Broken]
to revolve about/around - to move in a circular orbit around (as in) the Earth revolves around the sun

That's one way of looking at it but the Moon does, actually, orbit around the Sun, too.
There are orbits (horseshoe orbits) for satellites around planets that share nearly the same orbital distance as their planet but they spend some time at the opposite side of their Star from the planet. They then 'come back' to their planet (catch up) and hare off past it to the other extreme point of their weird orbit. They then 'come back and the process repeats. I don't know what the local observers would say about that? I wonder what Galileo or Keppler would have made of it.

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sophiecentaur said:
One thing worth pointing out is that two bodies traveling through space cannot, independently, suddenly get into orbit round each other. In the same way that a spaceship needs to slow down in order to land on a planet, there needs to be some other influence at work (like a collision) for two objects to change from a hyperbolic (glancing) interaction to an orbital situation. It is necessary for kinetic energy to be lost at a critical time in the trajectories or they will just depart in the same way that they arrived.

Yes, I was thinking about going into this when I mentioned the hyperbolic trajectory, but I thought it premature. I figured the OP needs to sort out his scenario at a basic level first.

sophiecentaur said:
That's one way of looking at it but the Moon does, actually, orbit around the Sun, too.
The point being that the OP was not incorrect in his use of terminology.

I see where you're coming from but I would suggest there is an essential difference between 'revolve around' and 'revolve', which was the term he used. The dictionary extract uses 'around'. There are so many ways of getting hold of the wrong end of sticks that I often feel duty bound to resolve these things. We could often be half way through a thread where there is actual confusion in this respect.

btw what are sig lines??

sophiecentaur said:
btw what are sig lines??
Yours is
There are no 'really's in Science - they are all models. (Really)

Mine is

DaveC426913 said:
Yours is
There are no 'really's in Science - they are all models. (Really)

Mine is

Right!

## 1. What is gravity?

Gravity is the force that attracts objects with mass towards each other. It is a fundamental force in the universe that is responsible for keeping planets in orbit and objects on Earth from floating away into space.

## 2. How does gravity affect low velocity bodies?

Low velocity bodies, or objects with a relatively low speed, will experience the same gravitational force as any other object. This means that they will either revolve around a larger body, such as a planet, or fall to the ground depending on their initial velocity and the strength of the gravitational force.

## 3. What determines whether a low velocity body will revolve or fall to the ground?

The main factor that determines whether a low velocity body will revolve or fall to the ground is the strength of the gravitational force. If the gravitational force is strong enough to overcome the initial velocity of the object, it will fall to the ground. If the gravitational force is not strong enough, the object will continue to revolve around the larger body.

## 4. How does the mass of a low velocity body affect its motion?

The mass of a low velocity body does not directly affect its motion. However, the mass of the larger body it is revolving around will have an impact on its motion. Objects with larger masses will have a stronger gravitational force, which can affect the motion of low velocity bodies orbiting around them.

## 5. Can a low velocity body ever escape the gravitational pull of a larger body?

Yes, it is possible for a low velocity body to escape the gravitational pull of a larger body. This can happen if the object has enough initial velocity to overcome the gravitational force. This is known as achieving escape velocity and is necessary for objects to leave the orbit of a larger body, such as a rocket leaving Earth's orbit.