March 1, 2009 Issue

Physics To Go 68 - Martial arts/action-reaction

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Physics in Your World

Newton's Third Law of Motion image
image © Institute of Physics; image source; larger image

Newton's Third Law of Motion

The woman's hand exerts a force that can break a board. But what about the force that hurts her hand?
-- That's the reaction force, equal and opposite to the force she exerts. The faster she moves her hand, the more boards she can break, but also the more damage she can do to herself.
-- To find out how she learned to break boards, visit Kung Fu Science.
-- For examples of reaction forces, see Newton's Third Law of Motion from The Physics Classroom.

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Physics at Home

How To Demonstrate Newton's Third Law of Motion

For an example of action reaction forces, see the video Newton's 3rd Law.


From Physics Research

Light Bends Glass image
image credit: Weilong She; image source; larger image

Light Bends Glass

A long-standing issue in physics has been what happens to the momentum of a photon as it passes from a transparent material into the air.
-- Weilong She investigated this question by firing laser light through fine glass fibers, including the ones shown in the photo, which are only 0.5 µm wide and 1.5 mm long (a human hair is about 100 µm wide).
-- The change in the photon's momentum determines, according to Newton's third law, whether the fiber recoils backwards or stretches forwards.
-- As the photo shows, this experiment indicates the fiber recoils backwards, but the issue persists; to learn about it, see the American Physical Society's Physical Review Focus article Light Bends Glass.

Worth a Look

Momentum Conservation Principle

Visit Momentum Conservation Principle to find out about the connection between action reaction forces and the conservation of linear momentum.

(This feature was updated on August 14, 2013.)

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