Gravitational force - questions.

[Sword7]

Hello folks,

I am learning some from my Astrodynamics books myself. I have some questions about gravitational force. I still am figuring out about mathematics for n-body alogrithms, etc. for gravity simulation because I am new to vector math. With my search through google, I had seen some gravity simulators but they are closed-source. Does anyone have any good open-source 2D/3D gravity/orbital simulation software for both Linux and Windows?

Also, for flying a plane at 600 mph, what is difference about gravitational force or micro-gravity difference? How does it detect G-force?

For flying a space plane into orbit or go moon/other planet from the runway, can it handle both aerodynamics and astrodynamics (orbital mechanics/determination) together?

For jumping and stand yourself on different each planets, etc.., what is gravity formula?

Thanks,
Tim

 

[tomek]

Gravity law is basically F=G*m1*m2/r^2. That is, the force exerted, say, on Moon by Earth, is Earth mass times Moon's mass times G (universal gravitational constant), all that divided by the square of the distance between them. To get acceleration of, say, the Moon towards Earth, you divide the result of above formula by Moon's mass, which basically cancels Moon's mass out the equation. And that's about it. Universal gravitational constant equals to 6.67300*10^-11, its dimensions are (m^3)/(kg*s^2).

http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html

 

[simonpro]

Also, for flying a plane at 600 mph, what is difference about gravitational force or micro-gravity difference? How does it detect G-force?

Speed does not affect the force of gravity (at least for speeds we are commonly used to). So a plane flying at 600mph feels the same gravitational force that it does at 10mph or 2000mph.
Height makes a difference, though. At the height of the ISS (something like 380km) gravity is 10% less than it is at the surface of the earth.
Microgravity can only be achieved by a vertical deacceleration equal to that of gravity. In an aircraft this is typically done by pointing the nose 45degrees above the hoziron and climbing until almost at stall speed before falling back towards the earth, at a negative pitch of about 42 degrees. If this path is followed the engines cna be used to cancel out the affects of air drag, and you experience microgravity.

 

[Sword7]

More questions...

Ok, thanks for some replies. I reviewed that web site and found about weight equations that using gravity formula (F = GMm/r^2). It can tell my weight on Earth or other planets by using my body mass. For jumping over the pole on different planets, what is that formula? How to determine pounds/kilogram from mass?

Also, I learned that a plane lost a few pounds (slightly less gravity) but same mass when climbed to 7 miles above ground. Oh, I remember that when I flew across the USA several miles above ground, I felt a slight different weight when I lifted a soda can.

For riding roller coastor, I felt varying G-force like riding NASA's centrigual ride. When I rode through a loop, I almost felt microgravity at top of loop, etc. What is formula for G-force for microgravity?

That's why I need to learn something for trying to develop space/gravity simulator myself.

Thanks again.

Tim

 

[tomek]

Erm... you're not talking about gravity here. Weight (not to confuse with mass) depends on the forces that act upon your body. Gravity is not the only force that may affect you. "Microgravity" is, in fact, experienced when gravity is the only force acting upon you, and it's a really bad term because what it stands for has nothing to do with diminishing gravity, it's about diminishing weight. "Freefall" is a better term.

The key difference between gravity and most other forces is that gravity pulls on every point of your body with about the same force (though if your body is few hundred meters long, it will become noticeable that gravity decreases with distance). Because of this, it's pretty impossible to feel gravity if it's the only force that acts upon you - it produces no noticeable effect on you. Most other forces, however, only act on you at the point of contact - and that produces the effect of "weight".

For example, if you just stand still, the floor exerts a force on your feet. That cancels out the effect gravity has on your feet, but not the rest of your body. The rest of your body is stopped by various parts of your skeleton pressing on each other from the bottom up, and soft tissues hanging on the bones or whatever they're hanging on. That's what you typically call "I feel gravity", but what you really feel is the reaction of the floor that counteracts gravity.

When you're flying in a plane and make a sharp turn, it is rapidly decelerated, and that deceleration is transferred to your body through the seat of your pants only, and that results in over-G (because your seat pushes you not only to counteract gravity, but to actually decelerate you as well).

The force exerted upon your body by the floor (or whatever you're sitting/standing on) is equal to the force that would accelerate you at about 9.8m/s^2 if gravity wasn't acting upon you. If you were in elevator accelerating at 9.8m/s^2 upwards, you would experience 2Gs. If you were in elevator accelerating at 13m/s^2 downwards, it would actually run away from under your legs, and if it had ceiling, you could actually stand upside-down on it, feeling about the same weight you would on Mars. These all of course assume that elevator is moving in a uniform 1G gravity field, which never really happens in real life, but it serves to illustrate the point.

 

[Sword7]

G-force

Well, I now understand it. I searched for more information and found some information at wikiepdia web site. There are two forces - gravity force and normal force. When I rode roller coastor, I rode through a loop. At top of loop, normal force cancels some gravity effects. At top of loop, that's why I felt lighter.... Same way with riding elevator. Some time ago, I went to WTC and took elevator to the top floor. When it accelerated, I felt heavier. When it reached top floor, I felt much lighter.

When spacecraft launched off ground, it will experience a few G-forces. When it reached its orbit, G-force should reach zero. When it slows to deorbit for reentry, when/where would it start feeling gravity?

When it reaches orbit, both normal force and gravity force are same - results as zero gravity or 0 G-force?

Thanks again,
Tim

 

[thomasantony]

You can see an orbit in two ways. IF it is a prefectly circular orbit, you can consider that the centrifugal force, resulting from the circular motion of the craft around the earth completely cancels out the gravity.

or mv^2/r = GmM/(r^2)

But basically, a craft is in orbit when its horizontal speed is so high, that it keeps missing the ground on falling. It misses the ground because of the curvature of the earth.

~
Thomas

 

[tomek]

When spacecraft launched off ground, it will experience a few G-forces. When it reached its orbit, G-force should reach zero. When it slows to deorbit for reentry, when/where would it start feeling gravity?

Again, you never really "feel gravity". What you feel is weight, when something pushes you with constant force - to keep you from being accelerated by gravity, or to accelerate you while in orbit, or for any other reason. You can't really feel the difference between the reasons you are pushed. It all makes you feel weight in same fashion (though exact weight you will feel depends on how much force is pushing. And your mass of course. Same force pushing higher mass will result in lower weight).

When it reaches orbit, both normal force and gravity force are same - results as zero gravity or 0 G-force?

What do you mean by "normal force"? In orbit, when engines are shut down, gravity is the only force acting on you. It does have about the same effect on your trajectory as the normal force on the rollercoaster. But the important difference is that normal force is exerted by rollercoaster rails onto your car, and by your car onto your body. Gravity, in orbit, acts on both your body and spaceship, so spaceship doesn't push on the body to keep it "on track". Which resulting in zero weight (not gravity) and zero-G environment.

 

[ijuin]

What do you mean by "normal force"? In orbit, when engines are shut down, gravity is the only force acting on you.

A "normal force" in physics refers to the force that an object pressed against a surface feels exerted upon it by the surface. For example, an object lying on a flat, level area feels the normal force pushing it upward in resistance to gravity (if there were no force resisting gravity, then it would fall THROUGH the ground). On a slanted surface, the force would be perpendicular to the surface.

http://en.wikipedia.org/wiki/Normal_force

 

[tomek]

Well, yes, except he wrote "when it reaches orbit, both normal force and gravity force are same". So I wondered what does he mean by a "normal force", because I don't see any normal forces, in a conventional sense, that would act on an orbiting spacecraft.

 

[simonpro]

He probably means centrifugal force, which is a misnomer anyway.

 

[Sword7]

He probably means centrifugal force, which is a misnomer anyway.

Yes, I got it. I checked web site about 'normal force'. Oh, I see. I now mean centrifugal force. Thanks again.

Tim

 

[Scrooge McDuck]

Hello,

I'm not sure, but I think you still don't have a clear idea about the concept of an orbit, gravity, and 'weightlessness'. So let me give a basic explanation.

Let's consider you're on a planet without an atmosphere.. When you are standing on a tower, and drop a box, it will fall straight down, until it hits the ground.
If this planet was earth, the box would accelerate during its drop with the acceleration of 9.81 m/s/s.
Imagine you where inside this box. You would feel 'weightless', because the box and everything inside experiences the same acceleration (so it's in freefall). For you the experience is that you would float relative to the walls of the box.


Okay, next step; imagine we're standing on the ground this time, and let's have someone to throw the straight box straight up. As soon as the person releases the box, the box is in freefall. The box, and everything inside it, is in freefall, until it hits the ground.


Next step. Let's go back to the tower again. We've dropped the box straigt down, but this time we give it a little vertical velocity. We throw it with a little force towards the horizon. It'll still accelerate towards the center of the planet, now it will follow a small curve. Remember, as long as it's in true free fall, the only force acting on the box, is the force of gravity pointed towards the center of our planet.
I've tried to create an example of this situation with a simulator, imagine the grey sphere is the planet, the blue shape is the tower, and the yellow ball is the 'box' we are talking about:

It will fall down like this:

Now if we throw it harder, it will fall down further, like this:

Ok, time for the next step.
We throw even harder this time. It will not hit the planet anymore, but it will just continue to keep falling all the way 'around the planet', until it is exactly at the original starting point:

From there, it'll of course repeat the same orbit all over and over again.
I hope this basic explanation helped you little, and otherwise it could help others
With this applet you can experiment with it. I once new a better one, but I can't find it now..


regards,
mcduck

 

[tomek]

[offtopic]Hehe, I think I even recognize the simulator you used for this [/offtopic]

 

[n0mad23]

Isn't this also Newton's famous "thought experiment," with a tower instead of a very tall mountain with a cannon on top?

Still no idea what gravity really is, but it's so incredibly difficult to escape....

alchemists.

 

[mikey451]

By the way, a spacecraft on orbit is affected by gravity about 90% of that experienced at sea level, so it's not really the fact that it's in space that causes microgravity. Rather, it's a balance of accelerations that does it - the acceleration of the vehicle caused by its motion, V*V/R, and that due to gravity, G/R^2. Those accelerations are in balance only in the orbital plane, by the way, at a single radius. Go outward or inward from that, and the forces change (this is why, when you saw pictures of solar panels being released on orbit during EVA, all the astronauts had to do was let go of them). This is why it's properly called microgravity in low earth orbit, rather than zero gravity.

 

[Scrooge McDuck]

[offtopic]Hehe, I think I even recognize the simulator you used for this [/offtopic]

[even more offtopic]

It is (MSC.) Working Model 2D 2005 (in short WM. It's a slightly more advanced version of the educational 'Interactive Physics').
You can 'play' endlessly with Working Model, it's a great tool. I learned (experienced) most of the orbital mechanics using (older versions of) this simulator before I discovered Orbiter years ago. I've made some pretty advanced (manned) capsules/spacecraft with WM, with Lin/Rot RCS control for docking, etc. With the collision detection, it's lovely to watch the 'humans' float around in the capsule...
If we're talking about same simulator, then maybe we could share some wm2d files, or could post some screenshots if you like

By the way, the 3D version is called Visual Nastran 4D, which is also nice.

[/even more off topic]

 

[tomek]

[further into offtopic]I've got an Interactive Physics CD of with a textbook, which I got from some garage sale I believe. Mine is a demo version, and I don't think I can actually save my own scenarios, but it's a pretty cool thing nevertheless, and I was rather embarrassed to actually learn a couple of rather elementary things from it (like if you apply certain force to an edge of a stick, it will gain just as much linear velocity as if you applied same force at the center, or anywhere else for that matter - I always kind of thought you'd get angular velocity "at the expense" of linear as you apply force further from center).

So anyways, even though I can't save any files of my own, I should be able to open your scenarios once I dig the textbook with the CD from my bookshelf. Or maybe I'll even consider buying a full version... So you're definitely more than welcome to post your vm2ds.

PS Let's just transfer this conversation to a different thread.[/offtopic]

 

[yashrudroju]

We know that, we are now standing, walking, running etc.., and other works on the earth because of the gravitational force. The gravitational force is also called as “Force of Gravity”. This Force of ...

Answer Link: http://howwhywhat.net/gravitational-force-exist-sun-planets/