On a January morning in 1993, the Space Shuttle Endeavour blasted off the coast of Florida carrying a high-tech communication satellite, and X-ray spectrometer, and a locker full of toys.
Toys! The shuttle was full of cool toys for the purpose of teaching the concepts of force and motion.
How did the experiments turn out?
Toy: This experiment uses one of those toy cars with a spring inside that you can wind up by pulling the car backward on a surface. When you let go of the car, the tension in the spring is released and propels the car forward. The experiment also uses a vertical, circular track.
How does it work on Earth?
Wind the car up, place it inside the vertical, circular track, and let go. The car speeds forward, up the track and around. It keeps looping around the track until the spring is unwound and friction slows it. Eventually, the car will no longer be able to loop to the top of the track and around, and gravity will make it fall down.
Why was the car at first, when it went fast, able to loop around the track, but not later after it had slowed?
Answer: At first, the car's great speed around the circular track created a lot of centrifugal force. That force pushed it hard against the track, and the downward force of gravity could not pull it off. Only after the car had slowed and the centrifugal force became reduced was gravity able to make it fall.
A general note: Centrifugal force is felt whenever you go quickly around a curve -- on a merry-go-round, in a car speeding around a tight curve, or when you swing an object tied to a string in circles overhead.
Toy: A propeller-driven toy submarine.
How does it work on Earth?
The toy submarine, like most ships, is moved through water by a propeller.
As the submarine moves through water it remains in its usual, upright position. The reason is that the submarine is bottom heavy. Gravity pulls harder on its heavier bottom section than on its lighter top section. There is no danger of capsizing.
The astronauts turned on the propeller of the toy submarine and let it float freely in the air of the space shuttle cabin. The submarine began to spin -- in the direction opposite that of the turning propeller.
Why? According to Newton, to every action there is an equal and opposite reaction. In the space shuttle, one part of the toy submarine (the propeller) turned one way; the rest of the submarine turned in the opposite direction.
Lesson learned: If ships on the Earth's oceans, lakes, and rivers were not bottom-heavy, the turning of their propellers would make them spin, as the astronauts found out, and they'd capsize!
In Earth orbit, where gravity cannot stabilize the submarine, the body spins in the direction opposite that of the propeller.
An additional note: There are many other familiar examples of Newton's law of action-reaction.
For instance, when you shoot a rifle, the bullet comes out in one direction while the rifle recoils in the opposite direction.