What keeps spacecraft in orbit? Momentum or inertia

[Fez]

So I asked a similar question a few days back, but now I have a new one.  I know gravity is pulling spacecraft back to the thing it's orbiting, but what's keeping it from actually crashing into the surface?  So far i have narrowed it down to 2 possible reasons: Inertia or Momentum.  I know inertia is just a property of matter that makes it want to go in a straight line with the same speed.  And momentum (I think) wants the object to keep going as well, because it's harder to stop something with more momentum.  Inertia would work to keep the spacecraft from falling because they want to go out and away in a straight line, but gravity pulls it in, so there's a tug-of-war.  At the same time, I would think momentum does the same thing, it wants to go out and away.  So which is it, inertia or momentum? Or both?

[mikegarrison]

Momentum and inertia are the same thing. F=MA, and if the net F is zero then the net A is also zero. Both F and A are vectors, so if there no F in the direction of interest then there will be no A either.

[Fez]

I had heard that momentum is mass times velocity, and inertia is just a property of mass that keeps it going in the same direction at the same speed.  But so then is it both that keep things in orbit?

 

[mikegarrison]

1 minute ago, Fez said:

I had heard that momentum is mass times velocity, and inertia is just a property of mass that keeps it going in the same direction at the same speed.  But so then is it both that keep things in orbit?

 

F=MA.

Like I said, in this context you are talking about the same thing. More generally, inertia describes how hard it is to change your momentum.

[Fez]

So then inertia/momentum keeps it from crashing into the surface?

 

[Guest]

I think the answer to the question is momentum. It is the velocity of the object that keeps it in orbit. Inertia is the resistance to change motion, so inertia would measure the difficulty in changing an established orbit.

[Fez]

Thank you

[Bill Phil]

It's technically neither.....

It's the centripetal force brought about by the velocity of the orbiting object. It produces a force away from the primary body, countering gravity.

Inertia is resistance to change in state of motion.

If you look at this equation:

a=v^2/r

Where "a" is acceleration, "v" is velocity, and "r" is radius. Using the Gravitational constant, you can solve for the acceleration at a certain distance from the primary using the gravitational constant, and thus you can solver for the velocity required at that distance to enter a circular orbit. A variant of that is the Vis-Viva Equation.

[mikegarrison]

4 minutes ago, Bill Phil said:

It's technically neither.....

It's the centripetal force brought about by the velocity of the orbiting object. It produces a force away from the primary body, countering gravity.

Inertia is resistance to change in state of motion.

If you look at this equation:

a=v^2/r

Where "a" is acceleration, "v" is velocity, and "r" is radius. Using the Gravitational constant, you can solve for the acceleration at a certain distance from the primary using the gravitational constant, and thus you can solver for the velocity required at that distance to enter a circular orbit. A variant of that is the Vis-Viva Equation.

The key here is that forces and accelerations and velocities (and therefore momentums) are vectors. What keeps the object moving is the fact that there are no forces acting in the direction of motion. What turns it in a circle around the planet is that there is a force acting in the direction perpendicular to the direction of motion.

(Of course, this is somewhat complicated when the orbit is not circular, but the small deviations balance out. And in real life there are drag forces acting to slow things down, which is why orbits decay.)

[Alias72]

This is a historical film that answers your question. (Beware this is 1950's Disney so expect stereotypes.) Your question is answered at the beginning of part 2.

https://www.youtube.com/watch?v=beofFQ_QuiA

[Bill Phil]

41 minutes ago, mikegarrison said:

The key here is that forces and accelerations and velocities (and therefore momentums) are vectors. What keeps the object moving is the fact that there are no forces acting in the direction of motion. What turns it in a circle around the planet is that there is a force acting in the direction perpendicular to the direction of motion.

(Of course, this is somewhat complicated when the orbit is not circular, but the small deviations balance out. And in real life there are drag forces acting to slow things down, which is why orbits decay.)

What matters is the velocities, which are a component of momentums, but not the entire picture. As long as the velocity is the right value and the position is set, then the orbit is set. The momentum doesn't quite change it, except for the rocket needed to launch it.

For example, the momentum of a cubesat isn't the same as the momentum of the ISS, even if they're in the exact same orbit.

[Fez]

so gravity pulls it down, but its momentum wants to push it out in a straight-line, so there's a balance between the inward pull and outward push?

[Nothalogh]

41 minutes ago, Fez said:

so gravity pulls it down, but its momentum wants to push it out in a straight-line, so there's a balance between the inward pull and outward push?

While that's likely a simplistic explanation, that covers it

[mikegarrison]

59 minutes ago, Fez said:

so gravity pulls it down, but its momentum wants to push it out in a straight-line, so there's a balance between the inward pull and outward push?

There is no "outward push", so technically that's not correct.

[PB666]

2 hours ago, Fez said:

So I asked a similar question a few days back, but now I have a new one.  I know gravity is pulling spacecraft back to the thing it's orbiting, but what's keeping it from actually crashing into the surface?  So far i have narrowed it down to 2 possible reasons: Inertia or Momentum.  I know inertia is just a property of matter that makes it want to go in a straight line with the same speed.  And momentum (I think) wants the object to keep going as well, because it's harder to stop something with more momentum.  Inertia would work to keep the spacecraft from falling because they want to go out and away in a straight line, but gravity pulls it in, so there's a tug-of-war.  At the same time, I would think momentum does the same thing, it wants to go out and away.  So which is it, inertia or momentum? Or both?

energy = mass times c ^ 2. The combination of e and m bend space time. Sounds wierd but this bending creates orbits, gravity is an observation. Its the same think as color being created by the eyes and brain, they are otherwise just hv. 

So to less precisely answer the question an orbit is a inertial reference frame, that precisely means that, discounting drag, i dont need to add kinetic energy of remove kinetic energy to keep the object in its orbit. 

Imagine that i am playing basketball on the moon, i shoot, the ball appears to be moving, accelerating toward the moon, this is how gravity appears to us. Instead the moons surface is accelerating toward the basketball. If i were to move all the moons soil from its path, it would  be in orbit. The electrons of the moons soil are pushing the soil in the direction of the ball, the ball then hits the electrons and the ball is then in a non-inertial frame.

Centripetal force is also a faux force, its actually the force of a rotor holding an object as the rotor spins. 

This is were newton primarily screwed up. 

[StrandedonEarth]

Momentum (mass x velocity) =  impulse (force x time). Inertia (an object at in motion tends to remain in motion, an object at rest tends to remain at rest) is basically the same as momentum. What keeps an object in orbit is the horizontal velocity, which makes it miss the object it is constantly falling towards. Orbiting is the simply the art of falling but always missing the ground.

[Hysterrics]

2 hours ago, Otis said:

I think the answer to the question is momentum. It is the velocity of the object that keeps it in orbit. Inertia is the resistance to change motion, so inertia would measure the difficulty in changing an established orbit.

Inertia keeps it in orbit. The law states that an object in motion stays in motion unless acted upon by an outside force. Since there is no outside force(to a certain extent), the object in orbit will stay in orbit.

Edited January 26, 2016 by Sequinox

[Yourself]

It is neither momentum nor inertia because the mass of the orbiting object has almost no impact on the behavior of the orbit (at least in the limit where the orbiting object is much less massive than the body it's orbiting).  If you have more mass (and therefore both more momentum and more inertia) gravity just pulls proportionally harder.  The net effect is that mass is irrelevant.

What keeps an object from crashing into the surface is really very simple: the surface is curved more than the orbital path.  To put it another way, the object in orbit falls like anything else does, it just misses the the central body.

We don't need to talk about forces or bring any math into this really, we can make this explanation much more intuitive.  Let's say you shoot a ball horizontally out of a cannon.  The faster you shoot it, the farther the ball will get before it hits the ground, but it'll always take roughly the same amount of time to hit the ground.  This is because the force of gravity doesn't care how big or how fast the object is going (only how far away it is), it accelerates the ball just the same.

So, since we understand that the faster we shoot the ball, the farther it travels before hitting the ground, let's remember that the ground curves downward.  We're not standing on a big, flat plane, we're on a big ball.  So it should be easy to imagine that if we just shoot our cannonball fast enough, that eventually it won't hit the ground anymore, because the ground is curving downwards as fast as the cannonball.

That's it.  That's all an orbit is.  You're moving so fast horizontally that in the time it takes gravity to pull you down, the ground has curved away under your feet and you just keep falling.

[Guest]

I wasn't confused before, but I sure as hell am now after reading this thread. I think I'll take a nap now and try to forget I ever read this.

[WestAir]

Isn't there geometry that shows a body in orbit is always moving in a straight & direct line towards the CoM?

[magnemoe]

9 hours ago, Yourself said:

It is neither momentum nor inertia because the mass of the orbiting object has almost no impact on the behavior of the orbit (at least in the limit where the orbiting object is much less massive than the body it's orbiting).  If you have more mass (and therefore both more momentum and more inertia) gravity just pulls proportionally harder.  The net effect is that mass is irrelevant.

What keeps an object from crashing into the surface is really very simple: the surface is curved more than the orbital path.  To put it another way, the object in orbit falls like anything else does, it just misses the the central body.

We don't need to talk about forces or bring any math into this really, we can make this explanation much more intuitive.  Let's say you shoot a ball horizontally out of a cannon.  The faster you shoot it, the farther the ball will get before it hits the ground, but it'll always take roughly the same amount of time to hit the ground.  This is because the force of gravity doesn't care how big or how fast the object is going (only how far away it is), it accelerates the ball just the same.

So, since we understand that the faster we shoot the ball, the farther it travels before hitting the ground, let's remember that the ground curves downward.  We're not standing on a big, flat plane, we're on a big ball.  So it should be easy to imagine that if we just shoot our cannonball fast enough, that eventually it won't hit the ground anymore, because the ground is curving downwards as fast as the cannonball.

That's it.  That's all an orbit is.  You're moving so fast horizontally that in the time it takes gravity to pull you down, the ground has curved away under your feet and you just keep falling.

This enplanes it well, think Newton used an cannon on an mountain to show the effect. Throwing an ball or shooting with an cannon and ignoring air resistance speed has an horizontal vector who stays constant and an vertical part who get an 9.8 m/s^2 downward acceleration.

it can bee seen well in KSP, go out toward minmus, reduce your orbital speed to zero, you are now falling towards Kerbin, add an small orbital speed and you miss it and is in orbit. 
Gilly is an fun place to play with this as its easy to get even up into escape speed using jetpack. 

[Bill Phil]

11 hours ago, Fez said:

so gravity pulls it down, but its momentum wants to push it out in a straight-line, so there's a balance between the inward pull and outward push?

Perhaps a more accurate explanation is that the radial velocity pushes it outward. Like a centrifuge, but it's connection to the center is gravity.

[GeneralVeers]

On 1/26/2016 at 5:24 PM, Fez said:

So I asked a similar question a few days back, but now I have a new one.  I know gravity is pulling spacecraft back to the thing it's orbiting, but what's keeping it from actually crashing into the surface?  So far i have narrowed it down to 2 possible reasons: Inertia or Momentum.

It's really neither. The force exerted by gravity depends on the mass of the object gravity is pulling on, effectively removing mass from the equation. It's velocity--how fast the object is traveling, and in what direction. The thing that keeps an object "in orbit" around a planet is the fact that the object is "falling" at the same rate the planet's surface is curving away from it. The velocity required to maintain an orbit is the same, regardless of the mass of the orbiting object. Barring contact with atmosphere or other matter, anyway.

[YNM]

It really depends on where you are - inertial frame or rotational frame ?

Inertial frame (say, on the surface of Earth that's stationary, poles maybe ?) : you see this object (a spaceship orbiting the planet you're on) speeding away from you (at least it wants to). But there's gravity, and it happens that the magnitude of gravity equals the magnitude of centripetal force needed to have a circular orbit (for elliptical, that's where the semi major axis is), so the object makes an orbit (circular path). And it's on the right altitude (read: radius) not to crash down.

Rotational frame (say, in the orbiting spaceship) : you happen to be above a planet. You don't fall onto it. For a while, you thought that it's stationary ; but then you realize that the planet pulls you by gravity. Of course, you don't move at all ; so some other force must be keeping you the way it is. That's where centrifugal force comes in (and this is precisely why centrifugal force is as legit as centripetal force).