# Circular motion of earth and gravity

• mineys
In summary, the conversation discusses the concept of inertia and how it relates to the Earth's rotation and the universe's movement around it. The question is posed about what would happen if gravity suddenly disappeared, and various explanations and theories are given about why objects would still "fly off" without the force of gravity. The conversation also touches on Mach's principle and how acceleration is absolute rather than relative. In the end, the correct answer is not definitive and can vary depending on different perspectives and theories.
mineys
Just to tease your minds, I propose this question. The Earth is rotating, right? Centripital motion is provided by gravity. Objects undergoing circular motion tend to travel in a tangental trajectory relative to the circle they are released from. If we say the Earth is still, and the universe is moving around it(using frames of reference), then how come if gravity suddenly went away, we would still fly off? This not only pertains to the Earth but any other object, where the example of gravity which i used can be replaced by any other center-seeking force, such as tension.

PS, I already know the answer, i just want to see who else knows it too.

But we can't just say the Earth is still, because that is an accelerating (non-inertial) frame of reference... on the other hand, if you physically stopped the Earth's rotation and then spun the rest of the entire universe around, "Mach's principle" (which is basically that you would still fly off except for centripetal forces) is as yet only conjecture.

we would still fly off?

nope

The universe as a whole cannot spin, spinning involves frames of reference and reference is with respect to the total mass of the universe

mineys said:
If we say the Earth is still, and the universe is moving around it(using frames of reference), then how come if gravity suddenly went away, we would still fly off? This not only pertains to the Earth but any other object, where the example of gravity which i used can be replaced by any other center-seeking force, such as tension.

PS, I already know the answer, i just want to see who else knows it too.
You cannot say that, since acceleration is not relative but absolute. You can measure it when you are accelerating.

when you say we would still fly off do you mean us as people, or the planet as a whole?

MeJennifer said:
You cannot say that, since acceleration is not relative but absolute. You can measure it when you are accelerating.

then how come if gravity suddenly went away, we would still fly off?

Fg = ma
a = 9.8ms-2
if gravity suddenly went away then a would be 0, thus force would also be 0. However if we flew off then a would be negative because we would accelerating away from earth. However in terms of graitational force there would be none as gravity is no longer in force.

Earth is rotating around the sun, which is subject to the sun gravitational force. Therefore Earth would continue in its orbit. I would also assume the moon would no longer rotate Earth and be under the pull of the sun.

I think I am wrong. I had a wild guess.

mineys said:
Just to tease your minds, I propose this question. The Earth is rotating, right? Centripital motion is provided by gravity. Objects undergoing circular motion tend to travel in a tangental trajectory relative to the circle they are released from. If we say the Earth is still, and the universe is moving around it(using frames of reference), then how come if gravity suddenly went away, we would still fly off? This not only pertains to the Earth but any other object, where the example of gravity which i used can be replaced by any other center-seeking force, such as tension.

PS, I already know the answer, i just want to see who else knows it too.
I suppose if you want to look at it like that you could say we don't fly off, but everything around us does.

general relativity does teach us that the universe as a whole cannot spin.

inertia is a function of the rest of the mass in the universe:

"If, in a material spatial system, there are masses with different velocities, which can enter into mutual relations with one another, these masses present to us forces. We can only decide how great these forces are when we know the velocities to which those masses are to be brought. Resting masses too are forces if all the masses do not rest. ... All masses and all velocities, and consequently all forces, are relative. There is no decision about relative and absolute which we can possibly meet, to which we are forced, or from which we can obtain any intellectual or other advantage. (Mach, The Science of Mechanics, ch.2, vi-3, Open Court, 1960, 279) "

In other words: interia here is related to mass there...

This is excellent. A lot of interesting responses, and a lot of correct ones too! very good...am I the only one who found this fun?

mineys said:
This is excellent. A lot of interesting responses, and a lot of correct ones too! very good...am I the only one who found this fun?
Yes you probably are.

Just for clarity, what is the "correct" answer?

Since the laws of physics are only assumed to be equivalent for all inertial frames, the same thing doesn't have to happen for the two cases you describe

## 1. What is the circular motion of the earth?

The circular motion of the earth refers to the path that the earth takes around the sun. This path is approximately circular, with the sun located at one of the focal points of the ellipse.

## 2. What causes the circular motion of the earth?

The circular motion of the earth is caused by the gravitational pull of the sun. The sun's mass creates a gravitational force that keeps the earth in orbit around it, following a circular path.

## 3. How does gravity affect the circular motion of the earth?

Gravity is the force that keeps the earth in its circular orbit around the sun. The force of gravity is directly proportional to the mass of the two objects and inversely proportional to the square of the distance between them. This means that the stronger the gravitational force, the closer the objects are and the larger their masses, the faster the circular motion will be.

## 4. What is the role of inertia in circular motion of the earth?

Inertia is the tendency of an object to resist changes in its state of motion. In the case of the circular motion of the earth, inertia keeps the earth moving in a constant direction and speed, even as it is pulled towards the sun by gravity. This results in a balanced force, allowing the earth to maintain its circular orbit.

## 5. How does the circular motion of the earth affect our daily lives?

The circular motion of the earth is responsible for the changing seasons, the length of our days and nights, and the tides in our oceans. It also plays a crucial role in the Earth's climate and weather patterns. Additionally, our calendar and timekeeping systems are based on the earth's orbit around the sun, making the circular motion of the earth essential for our daily lives.

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