Klinger Scientific 2nd Law of Motion Apparatus 

To demonstrate the Second Law of Motion simply and accurately, two identical balls-one drilled, and the other solid is used. When the release lever is triggered, the internal spring accelerates one ball and releases the other, showing that two balls dropped from the same height hit the ground at the same time even though one is traveling forward when the balls are released. Support stand and clamp required, but not included. With instructions.