# Why are objects (such as planets) in motion and not static? [duplicate]

I am just curious, why are all the planets, rocks etc in motion? Would they fall into objects with larger gravity if they were not in motion? What is stopping the earth from falling into the sun or moon from falling into earth? Is motion the reason why air crafts can remain in air? what are the mechanics behind this? Or perhaps I am mixing up too many things?

• Thought experiment: All motion must be measured relative to a specified reference point. If there are a large number of possible speeds at which objects might be moving with respect to a given reference point, but only one speed (namely zero) at which they are not moving, what do you suppose the odds are of any given object being perfectly motionless? – Robert Harvey Apr 20 '16 at 2:07
• Have you had a course in freshman Physics and learned about centripetal acceleration? – Chet Miller Apr 20 '16 at 2:13
• @ChesterMiller: That explains orbits, but not overall motion unless your premise is that everything in the universe is in an orbit. – Robert Harvey Apr 20 '16 at 2:20
• @RobertHarvey Thanks, but I was trying to alert gweb to the fundamentals associated with why the earth doesn't fall into the sun or the moon fall into earth. – Chet Miller Apr 20 '16 at 2:29
• @RobertHarvey Thanks for your response. You are right, it depends on the frame of reference. The following may be a nonsensical question but none the less - is there a sort of a universal observer, an observer whose frame of reference contains the frame of reference of all possible observers? – gweb Apr 20 '16 at 2:47

For orbit, try thinking of it this way...

First simplify from three dimensional space to a 2 dimensional plane, say a sheet of paper.

Also for simplification, we will consider the case of one large object and one small object. We will consider the large object to be 1) not moving and 2) so much larger than the small object that the large object is essentially unaffected by the gravity of the smaller object.

The small object is already in motion, traveling along in a straight line, but not directly at the large object, just right past it.

Now lets introduce gravity. What will this do to the motion of the small object?

It will accelerate the small object in the direction of the large object.

If the object was traveling towards or away from the large object, it's existing velocity would be affected. But, since the small object has velocity parallel to the large object, that velocity is not affected by this acceleration, instead it is pulled from it's straight forward path in the direction of the large object while maintaining speed.

We see this as a curving of the path of the small object towards the large object.

Basically, it is the existing velocity of the object that keeps it from falling into the larger object and the gravity is what is keeping it from just flying off.

• " small object has velocity parallel to to the large object", you mean velocity as a vector? I need to find myself a fundamental physics book. Thanks anyways.. – gweb Apr 20 '16 at 10:53
• @gweb Yes, velocity is a vector, a speed in a specific direction. – Pevinsghost Apr 20 '16 at 12:46

The classic thought experiment (which I think is in Feynman's lectures) is to imagine a cannon on top of a hill shooting cannon balls. Ignore air friction for this.

Typically the ball curves in an ellipse (we often say parabola, but this is an approximation for a uniform gravitational field, though the two are basically identical for this case) and hits the earth at the bottom of the hill.

But you can imagine that if you shoot harder and harder the ball goes further and further.

Imagine now the other extreme. It you shot it really fast the ball would just zoom off into space, barely impacted by the earth.

From here if you reduce it, you'll see that the ball changes it's initial course by being pulled towards the earth and the going off.

Between these two extremes is a case where the ball goes around the earth further and further. At some point it would go all the way around and hit the cannon. Now shoot a bit faster still and you get into orbit.

The essential point being that all these curves are ellipses of different types and orbits are just objects falling in a particular way.

Why are most things we see in an orbit. Because if they weren't we probably wouldn't see them. Either they'd spiral into the central object or zip off away from it never to return!

Planets (take for example the solar system) revolve around a huge massive star under the influence of gravity exerted by the massive object on the planet if the planet is under the field of influence of the star. They are actually falling towards the star, but each time missing it from colliding. Consider the moon. It is constantly falling to earth under Earth's gravity, but missing each time due to curvature of the earth. This is how an orbit is formed.

Aeroplanes do not work in this principle. They are not under free fall. Space shuttles even don't make a vertical free fall adventure at times of zero gravity training. Aeroplanes make use of their power from the engine to increase their height from the ground by doing work against the component of gravity.

Consider the International Space Station. It's under free fall. It has a definite prescribed orbit upon which it moves so that it will not fall down to earth and keep on orbiting. For introductory ideas on this you should start with Newton's cannonball thought experiment. Google for it.

• Thanks for your response, but I dont exactly follow "They are actually falling towards the star, but each time missing it from colliding. Consider the moon. It is constantly falling to earth under Earth's gravity, but missing each time due to curvature of the earth".. need to look up orbit formation. – gweb Apr 20 '16 at 2:50
• Try playing around with my gravity simulator – M. Enns Apr 20 '16 at 2:59
• @M.Enns, Thanks, that is really cool stuff! I don't get why sometimes the smaller objects spiral into the bigger object and why at other times they orbit. Clearly that is something I need to figure out, but the visualization helps! – gweb Apr 20 '16 at 3:09
• I tried to explain using Newton's concept as you have tagged your question under Newtonian mechanics. If you need to know why small objects spiral around bigger ones, you should better invoke spacetime warping and general relativity, which could tell you what's gravity. – UKH Apr 20 '16 at 10:47
• @Unnikrishnan.. Oops.. Dont know enough physics to know what to invoke. I though newton physics would cover this, guess I was wrong. Thanks anyways. Spacetime warping sounds crazy – gweb Apr 20 '16 at 10:49

While the motion of airplanes is very different from the motion of moons, planets etc. motion is the reason aircraft stay aloft (with the exception of lighter than air cart - hot air balloons etc). To counter gravitational pull plane's wings generate lift by their motion through the air. If an airplane moves through the air too slowly it will stall and fall to earth.

• Thanks, so my intuition was not completely off. What determines "too slow" or what would be involved in making such a computation? – gweb Apr 20 '16 at 2:51
• Area of the wings, density of the air, weight of the aircraft are some factors. – M. Enns Apr 20 '16 at 3:06