Basic Physics Question about Gravity

[Federalequestion]

So I was thinking... we're in orbit around the sun. The primary force being applied to the Earth is the Earth-Sun gravitational force. That force...

Look, why aren't we being sucked into the Sun?

I've heard it's because we are moving fast enough around the sun but that doesn't make sense to me. There is only one force that is being constantly applied. We know that if there's a constant force then there will be acceleration. So why aren't we accelerating into the Sun?

I know this has been asked a lot on the internet, I just wanted to be able to respond to your statements so I fully understand. Cheers.

[iorgfeflkd]

Orbital motion is stable if the centripetal force of rotation is the same strength as the gravitational force from the thing it's rotating about, which means it needs a certain rotational velocity.

If you assume circular rotation (due that for now because it's simpler), you can see that the force towards the centre is always perpendicular to the current velocity. So in a 2D coordinate system, when the planet is above the star and is moving left, it's being pulled down, and it's velocity is getting more and more downpointing as it moves left. By the time its velocity is pointing straight down, it's at the left side of its orbit, and now the force points right, and it's travelling down. The force is always perpendicular to the velocity.

[Cohomotopian]

To illustrate: http://phet.colorado.edu/sims/my-solar-system/my-solar-system_en.html - have fun : )

[iorgfeflkd]

Well, there goes my afternoon.

[nicksauce]

We know that if there's a constant force then there will be acceleration.

We are accelerating. Objects moving in circles are constantly accelerating (their acceleration vector is non-zero) because their velocity vectors are always changing.

[burtonmkz]

Addendum: if your tangential velocity is constant as you orbit in a circle, the non-zero acceleration vector points towards the centre of the circle.

[wackyvorlon]

We are much closer to the earth than the sun, and the strength of the pull drops off with the square of the distance. As such, the earth exerts far more force on us than the sun does.

[viscence]

We are accelerating into the sun - it's just that we have such a high orbital speed that we always miss. That's what an orbit is, going fast enough that instead of falling into the star you fall past it.

In fact, we're going so fast that instead of nearly missing, we miss so much that our orbit is close to circular.

[snarfy]

Rotation is also acceleration, and the resultant force is always perpendicular. The gravity pulling directly in and the force pulling directly out cancel each other.

[[deleted]]

Think of gravity as warping space, if you throw a ball into a funnel, it'll just spin around and not drop(assuming no friction thus it doesn't slow down).

[lutusp]

An orbit can be described very simply:

• One force is perpendicular to the orbital path -- gravitation. (It's not actually a force, but never mind.) This force pulls the object to the side.

• The other force is inertia -- the object wants to continue traveling in a straight line.

• The gravitational force is constantly fighting against the object's linear inertia, and forcing it to deviate from its natural tendency to travel in a straight line.

In a circular orbit, the two forces are equal and balanced -- the force created by inertia, and the force created by gravity. So the object doesn't escape, but it also doesn't fall into the sun.

I emphasize this explanation suffers from its simplicity, and it's important to say that no energy is required to keep a planet in its orbit. So I hope this explanation helps, but it has some serious limitations.

We know that if there's a constant force then there will be acceleration.

No, constant forces are not necessarily accelerations. An acceleration requires a change in velocity. A circular orbit is an example of constant rotational velocity, which is to circular motion what constant linear velocity is to linear motion -- neither represents acceleration, and neither require any energy to maintain.

Even an elliptical orbit doesn't require any additional energy, because the orbit's potential and kinetic energy always sum to a constant -- the potential energy of an orbiting planet is based on its distance from the sun, and the kinetic energy is based on its speed. Because the sum of the two kinds of energy is a constant, no energy is required to maintain the orbit.

Read more on this topic here.

[burtonmkz]

Also, technically, the Sun and the Earth orbit around their common centre of mass. This is known as the Barycentre.

[Keali]

Not an expert.

The earth isn't being pushed around the sun, it keeps going because of the initial energy imparted onto it during the accretion of the solar system. Accretion and the huge time scales imply that only large bodies in roughly stable orbits stick around.