Newton's 3rd Law Feature Summary

Type:
From Physics Research
Title:
Newton's 3rd Law
Description:
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.)
Image:
image credit: <a href="http://www.af.mil/photos/mediagallery.asp?galleryID=2" target="_blank">U.S. Air Force</a>; <a href="http://www.af.mil/photos/mediagallery.asp?galleryID=41&page=7" target="_blank">image source</a>; <a href="http://www.compadre.org/informal/images/features/AF-helicopter-large.jpg" target="_blank">larger image</a>
image credit: U.S. Air Force; image source; larger image
Image URL:
http://www.compadre.org/Informal/images/features/AF-helicopter-large.jpg
Featured:
May 1, 2012 - May 16, 2012