Second Law of Motion Device
To demonstrate that regardless of horizontal forces two objects will still be affected in the same way by gravity.
Two steel balls
Some kind of clamping device to hold apparatus level
Place the apparatus onto a clamp and make sure that it is level with the ground. Pull the trigger back and lock it in place, Place the one ball onto the metal piece by threading the metal piece through the hole in the ball and place the other ball onto the apparatus in the small holders to keep it from rolling off. Release the trigger and both should land on the surface at the same time.
If you are using a table instead of the floor make sure that the table is long enough so that both balls will land on the table and that one won't fly off to the ground, Also make sure that there is no one in the way who may be hit by the ball that is projected outwards.