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readme.doc
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1987-04-21
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Instructions for GravCalc v1.1
by S. Osborn 1988
Written using Fast Basic by Computer Concepts.
GravCalc, (as the name suggests) determines the gravitational
force between two bodies and then calculates their subsequent
motion through 2-dimensional space (3D space, apart from being
pointless for two bodies, is a little complicated to represent
on a 2D screen).
It uses the basic 'A' level physics formula:
F = -GMm/r^2 (G, grav. constant = 6.67E-11)
and Newton's laws of motion.
GravCalc is intended for use by those who wish to experiment
with and observe the laws of gravity. It is simple to use, but
a short explanation of the input parameters follow. Engineering
format (eg 1.234*10^6 or 1.234 E6) and SI units are used
throughout.
1. Scales
Distance across screen in metres.
Time step; this sets the intervals at which the computer
recalculates the velocities of the bodies. A large number gives
innacurate results but takes less time to show the path of the
bodies. It is important to set a reasonable scale compared to the
distances the bodies move (don't set a 1m scale when plotting the
orbit of the moon).
2. Body Mass and Radius
Mass of the body in kg.
Radius in m. Note that the raduis serves no real purpose except
to conform to the scales.
3. Coordinates
x and y coordinates are relative to the centre of the screen and
are in metres (not pixels!), ie x=-1 y=+1 would mean left and up
a bit from the centre.
4. Velocity components
The 'x' velocity component is the speed with which the body
moves West to East (a negative number means East to West).
The 'y' velocity component is the speed with which the body
moves South to North (a negative number means North to South).
5. Options
Execute; this starts the program. Press any key to stop.
With Reset Data to Input off the program continues from where
it left off, regardless of alterations to the input.
With Display Data on information concerning the bodies is
displayed during execution. This slows down the program.
With Trace Path on the bodies leave a trail no the screen
showing their path. With this OFF the program slows down.
Load and Save will store your data on disk. There are also
some examples to look at already on the disk.
Important Notes:
It is no good just giving any old number to the computer since
you are unlikely to see anything and may even cause an error
after which the program will be unable to continue. For instance
never give more than a sign and two digits in an "E___".
(9 E+99 is quite big enough!)
Look at the examples, they will make using GravCalc clearer.
An interesting point to note is that GravCalc does not detect
collisions. You can tell when one has occured as the two bodies
will fly apart. This is because of the way GravCalc calculates
in small steps and assumes that all mass is concentrated at a
point (regardless of radius).
Finally: remember the context in which Gravcalc works. We are
assuming a perfect world (no external forces such as friction or
other grav. forces). Also rembemer EVERYTHING is to a scale:
GravCalc is not working in REAL time but it is, in scaled time.
For instance in ATOMIC.DAT it takes the two tiny bodies 18 days
to coincide, although everything happens on the screen in a few
seconds.
The example files:
MOONORBI.DAT (Default on loading) Represents the approx. orbit
of the Moon. However one orbit seems to take too
long! If you have any suggestions, please let me
know. (-the Sun & other planets perhaps?)
ATOMIC.DAT Very small scale (eg two H atoms)
SMALL.DAT Attraction of man sized objects
COMET.DAT A very elliptical and eccentric orbit.
BISYSTEM.DAT Two large (Earth sized) bodies spin about each other.
SATELITE.DAT A satelite orbits the Earth at 100 km.
LOOPER.DAT An interesing pattern repeated.
BALEARTH.DAT The path of a ball thrown on the surface of the Earth
BALLMOON.DAT The path of a ball thrown on the surface of the Moon.
These last two are intended for comparison. All
factors are the same (except of course mass of Moon/Earth) but scales
are different.
PLEASE NOTE:
If you are satisfied with the program please pass it on to
friends or colleagues. If you have any questions or would like a
copy of the source program (Fast Basic) please send a disk (if
you want source program) and 1 pound (for my time and postage -
cheques etc made out to Steven Osborn) to:
Gravcalc info,
4 Arley Hill,
Cotham,
Bristol BS6 5PS,
England.