Level: Elementary to H.S.
Time involvement: 10 to 20 minutes.
The “Third Law” of motion proclaimed by Sir Isaac Newton in 1686 stated that “For every action there is an equal and opposite reaction”. Swinging a baseball bat at a baseball is proof positive that the law works. This demonstration uses two small wooden carts held together by a releasable spring. Several metal weights may be placed equally on the pair of carts or loaded on just one cart. This gives the flexibility of showing how the force which pushes equally on each cart will have a dramatically different effect on the speed of each cart.
Discussion: A rocket ship “parked” in outer space cannot move itself, but by firing a rocket engine and forcing the gasses to eject at a high velocity it can move itself in the opposite direction. Interestingly, the Newton principle also explains how a helicopter can fly by “grabbing” air molecules above its’ rotor blades and by “throwing” them downward can create an upward lift. The same principle applies to an airplane wing.
Notes: Kids love this demonstration and have fun releasing the carts. A smooth floor is required. Twelve inch floor tiles can provide a handy ruler to measure how far each cart travelled. A person with a few shop tools and some patience can easily duplicate the design.