As seen on Youtube: https://www.youtube.com/watch?v=fDlABg2MQqI
It's a Gauss Gun, (10 mm cube Neodymium magnet), with the Magnet below, off the way of the...Show more spheres, a Newton Cradle inside, and a Ratchet!
Simple. But SOMEONE had to think of it! ;-)
And as the Ratchet keeps the roller balls, (9.5 mm diameter, for ball bearings), in place, you can re-load it for ever. The wheels are kept in place by slightly heating the axis, then, pressing.