For demonstrating conservation of energy and momentum;
4 rubber balls stacked one upon another on a central plastic rod;
the bottommost ball is fixed on the plastic axis,
the next two balls are movable but they can not fall down;
just the uppermost, smallest ball is totally movable on the axis;
diameter of balls: 48/37/24/21 mm
(Delivered with two small spare balls)
If you drop the momentum cannon from a certain height,
the uppermost, smallest ball will fly upwards
with a multiple of the speed of fall v0
and can reach an enormous height.
If you highlight the maximum height on the wall,
you can work out a relation with respect to the different initial heights.