This Newton's Second Law of Motion Apparatus is made of solid metal and designed to last many years in the class room. This is spring driven heavy to demonstrates Newton's Second law of Motion. The unit is based on a substantial metal casting and the entire mechanism is visible. The spring gun offers two tensions. The 7.5" long unit has a 0.5" dia rod so it may be horizontally clamped to a support stand or table clamp. Complete with two steel balls ( One drilled ) This shows that both balls will hit the floor at the same time.