Gravity, in the most fundamental sense, is not
really a force between objects with mass.
Einstein discovered that gravity is an effect on
space itself! The mass of the sun, for example,
bends the space around it, so that the Earth
travels in an ellipse rather than a straight line
(as it would in a "flat" space). You can make a
model of this with a large rubber sheet (or a
springy fabric). Pull the sheet taught, and put a
heavy weight in the center. This will bend the
rubber, so that when you roll a marble on the
sheet, it will "orbit" around the "sun" in the
Now, to make the model more like a
black hole: cut a circular hole in the center of
the sheet, and pull the edges of the hole down.
You can still get things to orbit the hole (as
things can orbit real black holes), and there are
also trajectories that will spiral in to the black
hole, never to be seen again.
If you want
to go one step further, instead of just doing a
Swartzchild black hole (the non-rotating kind) you
could do a rotating one (a Kerrblack hole). This
kind of black hole actually drags the space with
it as it rotates. (Imagine the hole in the rubber
sheet rotating with the edges of the sheet fixed -
this would also alter the path of the marbles on
the sheet - and if you drew a grid on the sheet
you would also have a picture of how the space was
I imagine you're expected to do
more than just a demonstration for your science
project. You'll want to have some sort of
testable hypothesis as well. Perhaps with the
model, you could test which types of marble
trajectories escape the black hole and which types
will spiral in. Does the angle you throw the
marble at matter? What about the speed?
hope this is helpful. Don't limit yourself to
these ideas. Have fun &good luck on your project!
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