Gravitation Ltd. is a graphic, two dimensional orbital simulation. Users can enter custom designed solar systems or run previously created ones. Systems can be views at varying zoom levels, recorded and played back at high speed, and edited via the graphic solar system editor.
#A Gravity Primer
Gravity is the force of attraction between any two objects. The strength of that force depends on the distance between the two objects, the mass of each object, and a gravity constant. This force can be represented by the formula Force = (g * M1 * M2)/D2
where g= gravity constant, M1 is the mass of the first object, M2 is the mass of the second object, and D is the distance between them.
Accordingly, as the two objects come closer the gravitational pull increases.
Also, as the mass of either or both objects increases, the gravitational pull will increase.
How much a given gravitational pull affects an object depends on that object's mass. A small pull on a heavy object will cause very little change in the object's speed and position (a small acceleration). A large pull on a light object will cause a strong acceleration.
#Using Gravitation Ltd.
*File Operations*
Solar systems can be opened or saved via the OPEN and SAVE commands in the File menu.
*About the Editor*
The Graphic Editor, located under the Edit menu, is used to design or modify model solar systems. It allows you to do the following:
Create and delete planets (Max. 10 Planets).
Specify Mass for each planet. Mass must be in the range of 1..900.
Specify X and Y location for each planet. Units are pixels, with 0,0 being center screen.
Specify X Velocity and Y Velocity for each planet (-10..10). Units are pixels per frame.
Specify Trace Enable for each planet. This selection will leave a trail behind the planet when applicable.
Specify Stationary for each planet. Stationary will lock that object in place so that it is unaffected by the gravitational pull of other objects.
Two displays in the editor allow you to see what effects your changes will have. The first and larger one is the Planet Display, which is a reduced scale view of the screen as it will look when you leave the editor. The second is the Velocity Vector Display. This display lets you see the starting direction and speed of the currently selected planet.
*Using the Editor*
Buttons
Create: Creates a new planet and selects it.
Delete: Erases currently selected planet and selects next planet.
Next: Select next planet.
Prev. : Select previous planet.
O.K. : Leave the editor and return to main program.
Trace: Will leave a trail behind the currently selected planet.
Stationary: Locks the current planet in position.
Edit boxes
Used to enter Mass, X and Y location, and X and Y velocity for each planet.
Planet Display
The planet display, the large grid in the upper right of the editor, gives a graphical representation of the current solar system as it will appear on your screen. This display is also a control in the sense that planets can be both clicked on to select them and view their data, and also dragged around the screen to move them. The selected planet is shown with a rectangle around it. The three round buttons located below the planet display, 1x, 2x and 4x, control the viewing distance for the planet display (1x = close up view, 4x = viewing from far away. - Try it, you'll see).
Velocity Vector Display
The velocity vector display complements the planet display by providing a graphical representation of the velocity(Speed) and direction of the selected planet. This display can also be changed by clicking in it. The VVD will reflect the settings of the X velocity and Y velocity text boxes and vice versa.
Although all of this may seem complicated, I think it you will find it all very clear when you see it and work(Play?) with it. Just click around and try things.
*Go Menu Commands*
Run
Starts the current simulation.
Set Pause At...
Set pause at brings up a dialog box that allows the simulation to automatically stop,stop and beep, or repeat at a specified frame. The preset frame number is the maximum number of frames that can be recorded by the Instant Replay device (see below). Using a larger number will mean that early replay tape will be overwritten.
Limit Speed
Limit Speed will limit the simulation to a maximum of 30 frames per second. This will keep simulations from going too fast, especially when run on powerful Macintoshes. Users of standard 68000 based Macs will rarely need to use this feature. Limit speed applies to normal Run mode and also to Instant Replay Play mode (see instant replay below).
Anti Drift
Anti drift calculates a Zero Momentum Frame. That is, anti drift will zero the total velocity of the model solar system. This will help keep the system from drifting off of the screen. Anti Drift is a one time calculation done only at frame zero. Therefore, for it to take effect, you must be at frame zero or use the Reset command or enter/exit the editor to get to frame zero. Anti drift will have no effect if any planets are locked in place with stationary. Indeed, the whole purpose of the anti drift feature is to avoid having to lock any planets in place, yet still have the simulation stay centered on the screen.
Reset
Reset clears the screen and returns the simulation to its starting point (frame zero). All replay tape is erased.
*Instant Replay Menu Commands*
Instant Replay
Selecting instant replay will bring up an instant replay palette in the upper right corner of the screen. This device will let you review the previous 2400 frames. The five buttons in the palette from left to right are...
Rewind: Rewinds the "tape" back to the beginning at high speed.
Stop: Stops the tape.
Play: Runs the tape forward at normal speed(limited by the limit speed command).
Fast Forward: Moves the tape forward at high speed.
Exit Playback: Returns to the main simulation.
The primary purpose of the replay device is to allow you to review simulations at speeds greater than are possible when first calculating them. The replay device does not have to calculate as it displays frames and is therefore much faster. Remember, only 2400 frames can be stored. Additional frames will take the place of the oldest frames. The scroll bar on the bottom of the replay box can be clicked on to quickly move through the replay tape.
Normal Play
Returns the program to the main simulation at the frame it was at when instant replay was entered. This is the same as clicking on the Exit Playback button.
*Display Menu Commands*
Trace On
Trace On activates the tracing of all planets that have been selected as Trace Enable in the editor. This will leave a trail of dots behind those planets.
Clear Trace
Clear trace will erase all previous trace dots.
Normal Zoom
Zoom Out 2x
Zoom Out 4x
These three items control the scale at which the solar system is displayed. Normal Zoom is a close up view, and Zoom Out 4x is a distant view which will allow you to see a larger area.
Frame Counter
This command brings up a counter in the upper left of the screen which shows how many frames have passed since the simulation began.
*Collisions etc.*
When two or more planets collide their mass and momentum will be added to form a single new planet. Stationary or trace attributes will be combined into the Or of the original planets. Because the number of calculations required increases geometrically as the number of planets is increased, large simulations will run slowly. Use instant replay to review them at high speed after they have been calculated.
*Gravitation Documents.*
Numerous Gravitation Ltd. solar systems are available on bulletin boards and online services. Please take the time to upload solar systems that you design to any available services. This gives others a chance to share in your creativity. If your service has a keyword search capability, please include the keyword " Gravitation " with your file so that users can quickly find all available files. If your online service does not have a copy of Gravitation Ltd. 4.0, please upload it - this would be especially appreciated. Thank you.
*Technical Stuff*
Compatibility
Gravitation Ltd. 4.0 should run on 512K, 512KE, Plus, SE, Mac II.
Memory usage and Multifinder.
Gravitation Ltd. 4.0 requires about 400 K to run. Although large screen Macs may require more memory (Up to about 650 K.). If insufficient memory is available the program should alert you when you start it up. Within these memory guidelines, it should be fine with Multifinder. Future compatibility, both with software and hardware, has been strived for but cannot be guaranteed (Naturally).
*The Bottom Line*
This program is Shareware. I have put many hours into creating a simulation that I hope you will find interesting. In return for this, please write me and if possible, enclose a small sum of money for my efforts. If you can't afford to send even a small amount, then just send me your Mac. (Just kidding). Seriously though, even a simple letter or post card would be greatly appreciated. This lets me know where my program has traveled and what people think of it. Thank you for your honesty and support.
Jeff Rommereide
343 Elma Ave.
Laurel Springs, N.J. 08021
Genie: J.Rommereide
Compuserve: 73710,467
Version Information.
1.0 Original Program. Dec. 1987
2.0 Upgraded to take advantage of Mac II and other large screens, minor feature additions, minor bug fixes. April 1988.
2.5 Enhanced speed (up to 40%), ability to open and save model solal systems, cleaner editor, minor bug fixes. May 1988.
