# What causes an object to orbit around another?

1. Aug 28, 2014

### Basimalasi

I watched a lot of videos and read a lot of articles regarding the orbit of the earth around the sun and I just did not get it ..if they're both attracted to each other by a force then why dont the earth just hit the sun like two magnets attracted to each other ..what causes that "Orthogonal" force to make the earth orbit around instead of just collide with the sun?

2. Aug 28, 2014

### DEvens

There is no orthogonal force. All of the force is radial. And that's important.

It's easiest to understand when you consider a nice circular orbit. In that case, the motion of the planet is at constant radius. The force is always 90 degrees to the motion. So what happens to the angular momentum of the planet? What is the total work done by gravity?

It's just a tiny bit harder to see when you think about non-circular orbits. But if you go look up "directrix" in your geometry text, you can fairly directly get the orbit in Newtonian gravity field. Armed with that, you can work out what happens to the angular momentum over an orbit.

3. Aug 28, 2014

### Basimalasi

ok it's a perfect circular motion.
what causes the force represented in by the red arrow

4. Aug 28, 2014

### A.T.

In uniform circular motion the force represented by the red arrow is zero.

5. Aug 28, 2014

### kuruman

Tie a rock at the end of a string and swing it around in a circle. The rock pulls (through the string) on your hand and your hand pulls (also through the string) on the rock. Nevertheless the two do not collide. That's because the rock has some speed relative to your hand and it tends to move in a straight line as the string pulls it toward the center of the circle. Got it?

Now replace the string with the invisible force of gravity and you will understand the Sun-Earth system.

6. Aug 28, 2014

### Basimalasi

okay Im really confused now ..why does it orbit then? you said something about angular momentum, can you explain more please..my physics is poor but Im trying my best to study it.

7. Aug 28, 2014

### Basimalasi

yes but you're using your hand to swing it in two directions when you "swing" it ..one is towards you and the other is to the left or to the right ..my understanding of gravity is that it pulls objects not "swings" it

8. Aug 28, 2014

### enorbet

There are at least two ways in which one body will orbit another. The first is "capture" in which a wandering body spirals in due to attraction of another mass, often larger, and as it is accelerated it is possible, even likely that a speed will be achieved that balances with the gravitational pull and a stable orbit evolves. Note that it is far more likely to have a course resulting in near miss than a head on collision. Once sufficient velocity is attained the "miss" becomes essentially permanent.

The case for our Earth is different since it was apparently not captured but formed from an accretion disc. Accretion discs tend to flatten and spin because of the interaction of many bodies. It is in this environment, multiple forces resulting in spin, that angular momentum begins and evolves into a stable orbit.

9. Aug 28, 2014

### A.T.

And that's enough, because in space you have no air resistance, so you don't need the tangential force (red arrow).

10. Aug 28, 2014

### A.T.

Look up Newtons Laws of motion.

11. Aug 28, 2014

### Staff: Mentor

I think the point you're missing is Newton's First Law. "Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it."

There is no force like the red arrow. As the previous poster told you, either the body in orbit was captured or it formed from a spinning disk. In either case, the motion existed before the body went into orbit, and no force is necessary to keep it in motion.

12. Aug 28, 2014

### Basimalasi

so you're saying that the spinning of the earth around itself is what causing angular momentum that will make the earth orbit ?

13. Aug 28, 2014

### Basimalasi

so the earth was already in motion before the sun captured it in its gravitational field and that eventually made the earth have this orbit?

14. Aug 28, 2014

### A.T.

Yes. Depending on the initial conditions, there are two possibilities for stuff in space bound by gravity: collide or orbit. Both is happening, but after a while the collisions form bigger mostly orbiting objects.

No. Watch this video, which explains the role of angular momentum:

Last edited: Aug 28, 2014
15. Aug 28, 2014

### Basimalasi

Thank you very much that was really helpful I get now the reason behind that orbiting around the sun...
but I couldnt understand why the solar system is flat lol now i have another question..why does the up and down x-z and y-z plane motions "tend to cancel each other", why cant that happen the particles that are moving in the x-y plane?

16. Aug 28, 2014

### A.T.

Inelastic collisions cancel motion, while preserving total momentum (linear and angular). The x-y plane in the video is defined to have non-zero angular momentum, and that non-zero value is preserved.

17. Aug 29, 2014

### enorbet

It is not the Earth rotating that makes the Earth orbit the Sun. It is the spin of the accretion disc from which the Earth was formed.

18. Aug 29, 2014

### D H

Staff Emeritus
That's not a force. It's momentum.

Suppose you take a quick drive to the grocery store. You put the groceries you bought on the passenger's seat and head for home. On the way home, some fool cuts in front of you, slams on their brakes, and makes a sharp turn. You in turn have to slam on your brakes, hard.

What happens to you and those groceries?

The answer is that you pitch forward, and hopefully you are wearing your seatbelt. Presumably you didn't put a seatbelt around those groceries. They fly off the seat, hit the glove compartment, and land on the floor. No force made you pitch forward or made the groceries fly off the seat. That was just Newton's first law in action. A force is applied a moment later when your seatbelt locked, and when the groceries hit the glove compartment.

Introductory level students don't appear have a conceptual problem with Newton's laws when applied to straight-line motion. Misconceptions arise when the motion is not along a straight line. This is where I think Basimalasi is getting confused.

Basimalasi, that red line isn't a force. It's momentum, the exact same thing that made the groceries in my example fly off the seat. Momentum is a vector. Any change to that vector requires an external force. That can be a change in speed, in direction, or both.

In the case of uniform circular motion, the force is necessarily perpendicular to the momentum vector. In other words, it points toward the center of the circle. Always. That this is a perpetual source of confusion is why introductory physics educators spend so much time on uniform circular motion problems.

19. Aug 29, 2014

### sophiecentaur

Two isolated, massive bodies will collide if their paths intersect at the same point in space at the same time. Mostly, they will near miss and, after being diverted, mutually, they will carry on in two different directions from their original paths.
There is no mechanism for this spiralling situation to occur when there are just two bodies involved and there is no appreciable atmosphere extending from one to the other. Two bodies can only either collide, if their paths bring them to 'touching proximity' or follow a elliptical, circular or hyperbolic orbit around their centre of mass. This is what happens in a system of just two bodies.

When you look into space and see objects all in apparently stable orbits around stars and each other, you are seeing the result of processes that have taken place over aeons of time. The Solar system has settled down to its present form (which may well change, apparently) by the interaction of all the stuff in orbit round the Sun. The time for major collisions was way in the past but there are many relatively small bodies that are influenced by the major planets as they orbit the Sun and individual asteroids can be redirected by one planet so that they collide with another planet. The main planets of the Solar system appear to be in stable orbits but they could, given the right conditions, start to act in an unstable way. See this link on Orbital Resonance

If an object enters the Solar System. it can be 'grabbed' by the influence of all the planets etc. and end up in a Solar orbit or it can be deflected and 'flung out' again at a higher speed than it had when it entered the region of the Sun's influence.

20. Aug 30, 2014

### dean barry

If you look at the solar system and the amount of planets in stable orbit compared to the empty space, then stable orbit is actually a rare event, imagine you are in free space with two masses and you know that to get them orbiting around each other at a particular distance apart you need to get the velocity of each correct when you release them.

21. Aug 30, 2014

### A.T.

For two bodies a stable orbit is not a rare event at all. Unless they happen to collide they will be in stable eliptical orbits.

22. Aug 30, 2014

### sophiecentaur

One thing you need to realise that every astronomical situation has a history. If you find two bodies in an elliptical orbit then they must have arrived in that situation because of interaction with one or more other bodies, which left them that way, by slowing them down at a just the right time so that they are mutually captured. If the two bodies just 'happen to meet' then their total energy (PE +KE) will be too high for them to hang around with each other. They will follow hyperbolic orbits about their common CM and buzz of, never to see each other again.

23. Aug 31, 2014

### enorbet

How do we calculate such excessively long (to humans) comet periods such as for the West Comet which is as I understand it some 250,000 years? It would seem to me that even 100 years of good data is still such a small sample with which to provide reliable predictions. Or maybe I should have asked with what degree of inaccuracy (plus or minus 20%? 30%? ??) we can determine this?

Further it is my understanding that periods in the millions of years are at least possible. Thus I tend to agree with A.T. that stable capture orbits aren't rare, although I would imagine that "cast offs" are far more difficult to track.

24. Aug 31, 2014

### sophiecentaur

Stability doesn't really go with 'capturing'. The only 'capture', involving just two objects is a collision. Afaik, even where there is an atmosphere involved, a third body is necessary to provide the situation where two bodies can actually achieve a stable mutual orbit.

The point is that energy needs to be taken away from the approaching object, to bring about the situation where the Energy of the two objects is suitable for a stable orbit.

25. Sep 7, 2014

### mrspeedybob

Both objects will accelerate as they interact with each other. This should radiate some energy as gravity wave(s). If the objects are dense and approach each other closely they could orbit one another for a while until they radiate enough to fall into each other.