Hi. I’ve read a few articles trying to understand gravity or what we know about it anyway. I might just be looking in the wrong places for my answer but the articles always only mention that the Sun is pulling our planets in and that without it, we would just float away. I’m just trying to figure out what this “float away” force is. Or maybe a better way of phrasing the question is, what is the other force contributing to orbit besides the gravity from our Sun.
Thanks
It’s not a “float away” force; it’s a lack of force. Without any significant forces acting on the planets, they’d just keep on going in whatever direction they’re going in. I don’t know that “away” is a good description for this motion, though, because away from what? Without a star, any location in the Galaxy is just as good (or as bad) as any other.
You mean if the sun just suddenly ceased to exist? Imagine spinning a weight at the end of a rope over your head, and letting go.
Gravity is the only force you have to worry about. The important thing to realize is that no force is required for an object to “float away”. On Earth, our common experience is that it takes some force to keep an object moving; but that is because there is air resistance and other types of friction that exert a force to slow things down. In the vacuum of space, there is essentially no friction. If something is moving, it will stay that way unless some force acts on it.
So, for the Earth in space, it is easy to understand what would happen without the Sun; the Earth would travel in a straight line at constant velocity (ignoring the gravitational force from the Milky Way and other massive objects).
With the Sun, it is a bit more difficult. We have to explain why the Earth orbits the Sun. It is because the Sun is constantly pulling on the Earth (actually, they are both pulling on each other with the same force). The force keeps the Earth from drifting away, but since the Earth is moving fast, it just keeps moving while constantly being pulled back toward the Sun. The result is orbital motion. It is similar to swinging a weight around your head on the end of a rope. The rope provides a force to keep the weight from flying away, and it goes around in a circle.
Double Ninja!
That makes sense. Is it certain though that no force such as gravitational pull from stars, perhaps much larger than our sun, would act on our earth immediately after the Sun suddenly disappeared?
You mention that the Earth also pulls on the Sun. Wouldn’t that suggest that the two should get closer? You also mention that the Earth is moving so fast but not fast enough to escape the Sun’s pull. Why doesn’t the Earth lose speed from the pull of the Sun, or does it?
Gravity causes everything in the universe to pull on everything else. But the force of the pulling decreases exponentially with distance. (inverse square law.) The sun is so ridiculously big compared to the earth, and so ridiculously close compared to any other star, that its gravity is the only thing that really matters. If the sun were to suddenly disappear, the earth would go shooting off in the manner of the weight on a string. The gravitational effects of nearby starts would be so tiny they would make no difference, until and unless the earth happened to get close to them on its new journey.
In a perfect orbit, with zero friction, the two bodies would in fact not get closer to each other. Think of two spheres of the exact same size and mass circling each other. Now imagine one sphere gets a lot more massive. It moves less (because it’s heavier) but it still moves a little as the small sphere orbits it. So the sun and earth really orbit each other, even though the sun does not move very much with respect to the earth.
In real life, orbits do decay, due to friction from the small amount of matter in space, tidal locks, and so on. But this happens very slowly, at least compared to our puny lives.
Considering the immense distance to the stars, by the time (not “immediately”) their gravity traveled that distance to earth, it would be infinitesimal.
Imagine how it would be, if gravity from all the kazillions of stars - and galaxies - immediately began pulling, undiminished, on the earth, from all directions. Possibly, they would cancel each other out, but more likely the earth would be torn apart . . . or would have, long ago.
We don’t have the speed to leave the galaxy… We would end up in an orbit around the super massive black hole at the center of our galaxy. Other stars are inconsequential
I used to know this, but what imparts the momentum to the planets in the first place? Why do they all orbit in the same direction? (i.e., counterclockwise in the usual view of the solar system.) Also, the planets’ orbits are all elliptical with the sun at one of the foci. If you took the the second foci for each of the other planets, would they all be co-linear, and all on the same side of the sun? If so, well, why is that?
Sorry I’m very new to this subject but trying to learn. When you say orbits do decay, does that mean that the Earth would give into the Sun’s pull eventually, though very slowly?
That makes sense, though you do mention that it’s possible that they would cancel each other out but I don’t see it being more unlikely than the Earth getting torn apart since the pulls from the kazillion stars are so weak because of the absurd distances.
I’m just not understanding where Earth is getting it’s outward momentum away from the Sun from. Was the momentum created when the Earth was created? The Earth is traveling at speeds fighting the pull of the Sun but where did it get this speed from?
BIG Bang
In space, objects drift along essentially forever requiring no new force. Imaging sliding on some ice that was perfectly smooth, so you never stop.
Imagine that an object slid past the sun. The object is affected by the sun’s gravity, but it’s existing velocity doesn’t just disappear. The result is that the object may follow a curved path as it passes the sun, before continuing it’s friction-free sliding (though in a new direction).
Sometimes an object’s path may be curved so much that it forms a stable orbit. The object is perpetually “trying” to slide past the sun but instead is getting pulled around it.
The earth is not a captured object; it formed from the same cloud of material as the sun. I’m just using captured objects as a way to explain orbits.
The Earth doesn’t have outward momentum. It’s always moving tangentially to the direction of the sun (well, in a slight ellipse, but drawn to scale you’d hardly notice that). Try doing an image search on newton orbit and you’ll get an illustration from the first proper explanation of gravity, showing how a cannonball on an airless Earth would move if you added distance until it “missed” the Earth and went into orbit. Now suddenly remove the Earth and the cannonball keeps going in the straight tangential line it was aligned with at the moment of Earth’s disappearance.
One important issue you should grasp here, and might already have even if I read otherwise from your comments, is how things behave when the force making them move in a circle disappear. Twirl a ball on a string in a circle. We’ll consider the case where the circle is vertical, because even though we get the possible confusion of gravity we get a convenient reference direction.
As you swing the ball it’s kept moving in a circle by the force through the string. Now let go of the string as the ball passes the very top of the movement. What direction does the ball now move?
If your answer included any upward component, you need to rethink what you assume about circular motion. It’s easy to get confused and think that as the ball is exerting an outward force it is now going to go outwards, but if you examine the situation carefully, you’ll find that this outward force is the opposite to the force that is exactly enough to change the direction of movement into a circular motion without changing the speed. (Ignoring the confusing involvement of gravity when the movement is vertical.)
Remove the force of the string (and ignore gravity) and the ball will move in a straight line as per Newton’s first law. The direction will be the direction it was already moving, tangential to the string.
It all dates back to how the solar system formed. It started as a big shapeless nebula that gradually contracted due to gravity. As it got smaller, it started to spin faster, due to conservation of angular momentum (see a great demonstration of it here). As it spun faster, it bulged out along the sides, so it formed into a disk. Most of the gas material was pulled into the center to form the Sun, the rest of the stuff in the outer disk clumped together to form the planets. This tells us how the planets got moving in the first place, and why they all move in the same direction and in the same plane.
The planets’ foci don’t line up in any particular way, and the orbits actually precess very gradually. The magnitude of the precession effect depends on how strongly they are pulled by the Sun, so Mercury precesses the most.
Vector arithmetic.
What the Earth has is momentum to keep going in a straight line. That’s what momentum is: An expression of an object’s tendency to move in an inertial path unless it’s being acted on by a force. In this case, the force is utterly irrelevant to what we’re discussing here, because gravity isn’t a force, but the curvature of space-time induced by mass.
The Earth has that momentum because it coalesced out of smaller particles which had that momentum, going back to the accretion disk which also formed the Sun and Jupiter and most of the various other debris in our solar system. Basically, this disk was spinning around a central point somewhat more massive than the rest of it, and various parts of it chanced to get more massive than other parts; those various parts became the Earth, Jupiter, Mars, the Mars-sized object which slammed into Earth hard enough to fragment it sufficiently to create our over-sized Moon, etc., and the central part became the Sun.
Finally, the part you asked about: Going around in circles is what happens when you have a constant acceleration towards a fixed point which is large enough it doesn’t cause a slingshot effect (which makes you leave the solar system) or a spiralling-in (which makes you part of the Sun); it’s a balancing act, and it ultimately comes from the fact the various dust and gas molecules weren’t all at rest relative to each other, combined with the winnowing effect of the spiral-vs-slingshot dichotomy I just mentioned. We only see the stuff which stuck around and survived.
The closest star to our Solar System is Proxima Centauri, about 4.3 light years away. It’s also much smaller than our sun (about 12% as massive). It’s part of a 3-star system with Alpha Centauri A and B, each of which is basically the same mass as our Sun. It does provide some gravitational force, but it’s VERY VERY tiny.
Now, in the event that our Sun disappears and the Earth goes shooting out of orbit, there’s a chance that, eventually, it could end up getting close enough to another star to end up in orbit around that star. But that chance is very, very small, and it wouldn’t happen for a very long time. Life on Earth would have ceased long before that.
Nitpick: An inverse square decrease is not the same thing as an exponential decrease. The weak and strong forces have a (roughly) exponential decrease, which is why they have such short effective ranges compared to gravity.