image credit: U.S. Air Force; image source; larger image
Newton's 3rd Law
What is the purpose of the small rotor on the back of this helicopter? In fact, it's needed because of Newton's third law--for every action force there is an equal and opposite reaction. The helicopter puts a force on the main rotor shaft to make it turn, and the rotor shaft puts an equal and opposite force back on the helicopter. For an example of reaction forces, see this video.
This reaction force creates a torque that, by itself, would make the helicopter spin in the opposite direction as the rotor. To counter this torque, the small rotor on the tail creates an opposing torque, so the two are balanced.
(This feature was updated on September 22, 2013.)
Check out this PBS video to use the concept of torque to explain how one dancer can balance on top of the head of another--and on point!
(This feature was updated on August 18, 2016.)