Just look up "Gauss Gun." Or I can give you a reaaaaaally simple one right here.
Step 1) Get a ruler, four magnets (not quite as big as the width of the ruler), ten metal ball bearings (about as big as the magnets), and some tape.
Step 2) configure the magnets and ball bearings on the ruler like this ([]=magnet, O=ball bearing)
O []OO []OO []OO []OOO
They should be spaced so that the last ball bearing in each chain is being pulled by the other magnet, but not quite strong enough to rip it from the ball bearing it's currently magnetized to, and the last ball bearing on the end is near the edge of the ruler
Step 3) tape the magnets down
Step 4) roll the free ball bearing at the first magnet. When it hits, momentum is conserved, and the ball bearing on the end of the chain is knocked free and instantly sucked to the other magnet. When it collieds, the ball bearing on the end of that chain will be knocked free, and the physics continues from there until the final ball bearing is launched off the ruler.
Step 5) Don't shoot your eye out.
To a point. It basically operates on the principle of conservation of momentum. That is, the speed of the ball bearing hitting the magnet will be the speed of the ball bearing that leaves the chain. The magnets increase the speed by pulling on the ball bearings, and that's how you get up to speeds faster than you would by just flicking the ball bearing with your finger. I don't know at what point, if at all, the ball bearing would be going too fast for the magnet to really add any bonus, but adding more at least shouldn't make it slower. You'd need to upgrade to a yardstick, however.
Step 1) Get a ruler, four magnets (not quite as big as the width of the ruler), ten metal ball bearings (about as big as the magnets), and some tape.
Step 2) configure the magnets and ball bearings on the ruler like this ([]=magnet, O=ball bearing)
O []OO []OO []OO []OOO
They should be spaced so that the last ball bearing in each chain is being pulled by the other magnet, but not quite strong enough to rip it from the ball bearing it's currently magnetized to, and the last ball bearing on the end is near the edge of the ruler
Step 4) roll the free ball bearing at the first magnet. When it hits, momentum is conserved, and the ball bearing on the end of the chain is knocked free and instantly sucked to the other magnet. When it collieds, the ball bearing on the end of that chain will be knocked free, and the physics continues from there until the final ball bearing is launched off the ruler.
Step 5) Don't shoot your eye out.