Book: #1
Book: #2

THE PROCEDURE: 

A spring gun is aimed at a wooden ball or the stuffed monkey at a distance L away from it. (The angle to the horizontal can be varied.) The apparatus is then connected to a DC power supply. The spring gun is cocked with the brass ball and a trip wire is set in front of the gun. The DC power is turned on to 20 volts. The wooden ball is attached to the electromagnet. When the gun is fired the electromagnet releases the wooden ball at the moment the brass ball leaves the spring gun. Regardless of the angle to the horizontal at which the gun is positioned, the wooden ball and the brass ball collide in midair.