1. Feb 11, 2009

### Newton V

1. The problem statement, all variables and given/known data
Everyone know earth orbit the su, however the orbit is not a full circular, but an oval...
however then why won't earth get stick into the sun? (when it get the close to sun?)
where did other force that pulled earth back? which planet is our savor????

2. Relevant equations
I know earth is a moving object, and it moves in the straight line, however gravity pulled and formed oval track.

however what caused earth to keep moving in this straight line?
no idea?

3. The attempt at a solution
This is not a really right or wrong problem...

I ONLY WISH TO HEAR FROM EVERYONE, PELEASE TELL ME WHAT YOU THINK IS HAPPENING TO EARTH...

2. Feb 11, 2009

### Andrew Mason

This was the very kind of question that your namesake asked himself 350 years ago that him to develop the principles that laid the foundation for physical science. To understand the answer, you need to study physics and, in particular, Newton's laws of motion and Newton's Law of Universal Gravitation.

AM

3. Feb 11, 2009

### Dick

The earth is moving around the sun in an ellipse, not an oval. It's certainly not moving in a straight line. It's trajectory is influenced by the force of the sun's gravity. Newton told us all about this. CALM DOWN.

4. Feb 12, 2009

### Newton V

I know!!!

But earth must has its own force, so it is moving!!! or we will get stink in the sun! where this force came from???

5. Feb 12, 2009

### mgb_phys

The earth doesn't fall into the sun because it is moving.
With no other forces the Earth would go off into space in a straight line, because of the attraction from the sun we 'fall' below the straight line - an orbit is just falling at the rate where you end up back at the same point.

A common picture of this is to imagine a cannon fired from the top of a mountain. 'D' is fired with just the right speed to come back to the mountain

The speed isn't a coincidence, each planet is at the correct distance for it's speed to put it in orbit because it is made up of all the bits of rock and dust that were at that distance when the solar system formed.

6. Feb 12, 2009

### D H

Staff Emeritus

What you are missing are Newton's laws of motion. Newton's first law says that, absent some force acting on an object, the object will move in a straight line path with a constant velocity. No force is needed to keep some object moving. It is the other way around: A force is needed to make an object change from this straight line motion.

This is where Newton's second law comes into play. A force makes an object change its velocity. The change in velocity per unit time, the acceleration, is proportional to the force acting on the object and inversely proportional to the object's mass. Newton's third law says forces come in pairs. If object A exerts a force on object B, object B will exert an equal-but-opposite force on object B.

Newton's laws of motion don't say what forces are. It only says what forces do. Newton's law of gravitation mathematically describes one particular force: gravity. Combining Newton's law of gravitation with his laws of motion leads to Kepler's laws. The orbits of the planets orbit the sun are ellipses (not circles), with the Sun at one of the foci of the ellipse.

Look at it this way: Circles are a special kind of ellipse, an ellipse with zero eccentricity. To have a perfectly circular orbit a planet must have a very specific velocity that is a function of the orbital radius and the Sun's mass. Now ask: What would happen if a planet has a velocity close to, but not exactly equal to, this circular orbital velocity? The motion should be (and is) close to, but not exactly equal to, the circular orbit. Newton's laws are nicely behaved functions. There is nothing in them to indicate that a tiny change will result in a huge change in behavior.

7. Feb 12, 2009

### Newton V

where did earth get the energy from????

8. Feb 12, 2009

### Newton V

It must has its own force, so it won't get stick into sun!

Last edited: Feb 12, 2009
9. Feb 12, 2009

### dantose

If you are asking what got the earth moving around the sun to begin with, it was a function of random paths of the initial collapsing dust cloud. Those particles with degenerate orbits tended to hit objects going with the flow and either being absorbed into the mass of a larger object or being slowed by successive collisions until it's orbit dropped into the sun. Those particles left after that whole process eventually condensed to form planets. These planets of course had the same orbit as the contributing particles.

So basically, the planet's velocity comes from the prior particles velocity, which comes from random velocities in a collapsing dust cloud.

The force involved is the centrifugal force, a pseudo force which counters the acceleration due to gravity.

10. Feb 12, 2009

### D H

Staff Emeritus

This is basically correct. The Earth's orbital energy and angular momentum are pretty much the same now as they were when the Earth formed.

This is not correct. Pseudo-forces are not "real". In particular, centrifugal force is not what gave the Earth that initial motion.

11. Feb 12, 2009

### Newton V

so where did earth get this much energy to begin with??? god made it? I have no clue... it can be anything, anyone get any slightest ideas?

12. Feb 12, 2009

### Delphi51

As someone said, the primordial dust must have been swirling around before it formed into a star and planets. There are some good pictures of this happening in the last Discover magazine. Any bits that were not moving fast enough DID get sucked into the sun. The bits that were moving just right formed the planets.

There is some energy loss going on due to tides, solar winds, etc. The planets will eventually fall into the sun as you say. But probably long after the sun blows up or expands into a red giant or something.

Clearly the life span of humanity is limited in a number of ways - we'll have to get our act together and solve some problems if we are to survive!

13. Feb 12, 2009

### Newton V

so how much force will be needed to pull earth out the orbit?

I wish to see some calculations, that can show me how they find out about this force?

14. Feb 12, 2009

### Delphi51

Do you mean how much force is required to keep the Earth in orbit - pull it out of a straight line?

That would be the centripetal force required to hold mass m in circular orbit at radius r.
It is F = m*v^2/r, directed toward the center of the circle (where the sun is).
Look up the mass of the earth, radius of Earth's orbit and calculate the velocity from the circumference divided by 1 year (in seconds). You can find that force yourself!!

It would be interesting to also calculate the force of the sun's gravity on the Earth.
It is F = G*M*m/r^2, where M is the mass of the sun, m the mass of the Earth.
G is "big G" the gravitational constant - it's in the back of the book!

15. Feb 12, 2009

### Newton V

However i think they just used that formula to find the mass of earth....

16. Feb 12, 2009

### Delphi51

Yes, the pair of formulas are used with data about the moon to find the Earth's mass.
I can see you are a skeptic digging into the fabric of science, and I hope you are finding it interesting. Many fail to question things, and that is the essence of science. If it doesn't get questioned, it isn't science.

We don't KNOW that the formulas are true, but they have been "questioned" zillions of times by calculating different things. The slightest discrepancies have led to the discovery of new planets and of course we know they aren't quite right when there are large masses and high speeds - relativity must be used there. After all that, most people feel comfortable using them and letting a few skeptics carry on questioning.

17. Feb 12, 2009

### Newton V

I am a 9th grader... sure i am dumb...

18. Feb 12, 2009

### Delphi51

Most impressive, young Newton!
Careful, don't think too little of yourself. A 9th grader who knows the formulas for centripetal force and gravitational force can figure out those answers! All the better if you need a little help - you can strike up a potentially useful conversation with a teacher or older student.

I once taught a grade 9 student calculus in a few lunchtimes - one of the great experiences of my career.

Very nice having you on PF!

19. Feb 13, 2009

### dantose

You misunderstand, I'm not talking about centripital force as a "source" of anything. I'm just saying that's what is keeping the sun's gravity from pulling us in the sun.

Well, the originating gas cloud had some net rotation. This wasn't so much due to anything but rather just that it didn't happen to be perfectly balanced by chance. As the gas cloud colapses, rotation speed must increase to maintain angular momentum. Pretty much the same thing as an ice skater pulling in her arms while spinning.

20. Feb 13, 2009

### D H

Staff Emeritus

In your first post you talked about centrifugal force. I'll assume you meant centrifugal force (a fictitious force directed outward) rather than centripetal fprce (a real force directed inward) here.

To be blunt, you are wrong. You don't need centrifugal force to explain why the Earth doesn't fall into the Sun. Moreover, centrifugal force does not explain why the Earth does not fall into the Sun. Centrifugal force is a fictitious force. It arises solely from viewing things from the perspective of a rotating frame. The Earth is more or less stationary in a Sun-centered frame rotating at one revolution per sidereal year. The Earth would be stationary in this frame if the Earth's orbit was circular. In this rotating frame, centrifugal force does counterbalance gravity.

What about Jupiter? In this frame, Jupiter still orbits the Sun, but now in the opposite direction that it orbits when seen from an inertial frame. To explain why Jupiter doesn't fall into the Sun from a centrifugal force point of view you need a frame rotating at one revolution per 11.85920 sidereal years. Now Jupiter is more-or-less standing still, but the Earth is now orbiting the Sun once every 1.09208781 sidereal years. So, why isn't the Earth falling into the Sun?

The natural frame for explaining what is happening is a non-rotating, non-accelerating frame: an inertial frame. In such a frame, the Earth orbits the Sun with a period of sidereal year, Jupiter orbits the Sun with a period of 11.85920 sidereal years, and there is no centrifugal force acting on either the Earth or Jupiter. So, why don't the Earth and Jupiter fall into the Sun?

The answer is momentum. The Earth's acceleration toward the Sun is about 6 millimeters/second2. The Earth's orbital velocity around the Sun is about 30 kilometers/second. Over one second, the Earth's orbital velocity changes by a tiny, tiny bit, and meanwhile the Earth has moved about 30 kilometers. The Earth is always falling toward the Sun. Because the Earth is also moving at 30 kilometers/second, the direction toward the Sun is always changing.

21. Feb 13, 2009

### Newton V

oh, so that is why...

Last edited: Feb 14, 2009
22. Feb 13, 2009

### Delphi51

Hi Newton,
The moon and the Earth must be considered as a single complex "object" whose center of mass orbits the sun. The moon does cause a bit of wobbling in that orbit, but the center of mass of moon+sun goes smoothly around the orbit.
It seems that the clusters of galaxies are getting further apart, but not things in our solar system. There are indeed destructive forces in the universe, but they don't seem to be having much affect here. I think our immediate worries are more to do with what we are doing to ourselves and our environment.

23. Feb 14, 2009

### malawi_glenn

The sun has greater mass than the moon, so the force is bigger from the sun even though it is more far away.'

The sun is also decreasing its mass due to its energy emission, so that makes the average radial distance from the sun to the earth INCREASE.

Maybe pick up an introductory textbook on astronomy? I started reading my dad's while I was in 5th grade :P

24. Feb 14, 2009

### D H

Staff Emeritus

Not really. The acceleration of the Earth and Sun toward each other is, per Newton's law of gravitation,

$$a=\frac{G (M_{earth}+M_{sun})}{r^2}\approx\frac{G M_{sun}}{r^2}$$

The Earth's orbit is nearly circular and will stay that way for a long, long time. It would take a truly immense amount of energy to make the Earth's orbit change. The Sun's mass is nearly constant. (The Sun is very slowly losing mass by the solar wind and by giving off sunlight. The effect of that mass loss is to make the Earth slowly move away from the Sun: millimeters per century slowly.)

The Sun's gravitational pull on the Earth is a lot more than the Moon's, by a factor of 179 or so. The Moon causes greater tides than does the Sun, but that is because tidal forces are proportional to the inverse cube of distance compared to the gravitational force's inverse square relationship.

I know you are only in the ninth grade, but ninth graders can and do write better than this. Leaning to write better will help you think better.

25. Feb 14, 2009