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Optional Topic: Newton’s third law and action at a distance
Newton’s third law is completely symmetric in the sense that neither
force constitutes a delayed response to the other. Newton’s third law
does not even mention time, and the forces are supposed to agree
at any given instant. This creates an interesting situation when it
comes to noncontact forces. Suppose two people are holding
magnets, and when one person waves or wiggles her magnet, the
other person feels an effect on his. In this way they can send signals
to each other from opposite sides of a wall, and if Newton’s third law
is correct, it would seem that the signals are transmitted instantly,
with no time lag. The signals are indeed transmitted quite quickly,
but experiments with electronically controlled magnets show that the
signals do not leap the gap instantly: they travel at the same speed
as light, which is an extremely high speed but not an infinite one.
Is this a contradiction to Newton’s third law. Not really. According to
current theories, there are no true noncontact forces. Action at a
distance does not exist. Although it appears that the wiggling of one
magnet affects the other with no need for anything to be in contact
with anything, what really happens is that wiggling a magnet un-
leashes a shower of tiny particles called photons. The magnet
shoves the photons out with a kick, and receives a kick in return, in
strict obedience to Newton’s third law. The photons fly out in all
directions, and the ones that hit the other magnet then interact with
it, again obeying Newton’s third law.
Photons are nothing exotic, really. Light is made of photons, but our
eyes receive such huge numbers of photons that we do not perceive
them individually. The photons you would make by wiggling a
magnet with your hand would be of a “color” that you cannot see, far
off the red end of the rainbow. Book 6 in this series describes the
evidence for the photon model of light.
Discussion questions
A. When you fire a gun, the exploding gases push outward in all directions,
causing the bullet to accelerate down the barrel. What third-law pairs are
involved. [Hint: Remember that the gases themselves are an object.]
B. Tam Anh grabs Sarah by the hand and tries to pull her. She tries to remain
standing without moving. A student analyzes the situation as follows. “If Tam
Anh’s force on Sarah is greater than her force on him, he can get her to move.
Otherwise, she’ll be able to stay where she is.” What’s wrong with this analy-
sis.
C. You hit a tennis ball against a wall. Explain any and all incorrect ideas in the
following description of the physics involved: “According to Newton’s third law,
there has to be a force opposite to your force on the ball. The opposite force is
the ball’s mass, which resists acceleration, and also air resistance.”
Section 5.1Newton’s Third Law
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