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.‘ 54 Chapter4 Force and Motion Elﬁllliﬂl The climbing ropeexerts an
upward tension force l: that balances the
force of gravity. Elﬁlﬂiﬁﬂ Unification of forces is a major
theme in physics. 4.3 Forces When you sit in a chair, . _ , the chair compresses and
The most famihar forces are pushes and pulls you apply With exam an upward force that
your own body, but passive objects can apply forces, too. A car balanws swat?- collides with a parked truck and comes to a stop. Why?
Because the truck exerts a force on it. The Moon circles Earth
rather than moving in a straight line. Why? Because Earth
exerts a gravitational force on it. You sit in a chair and don‘t
fall to the floor. Why not? Because the chair exerts an
upward force on you, countering gravity. Some forces, like those you apply with your muscles, can
have values that you choose, Other forces take on values de—
termined by the situation. When you sit in the chair
shown in Fig. 4.6, the downward force of gravity on you
causes the chair to compress slightly. The chair acts like . a spring and exerts an upward force. When the chair W Acompresston force.
compresses enough that the upward force is equal in magnitude to the downward force of gravity, there’s no net force and you sit without accel~
crating. The same thing happens with tension forces when objects are suspended from
ropes or cables4the ropes stretch until the force they exert balances the force of gravity
(Fig. 4.7). Forces like the pull you exert on your rolling luggage, the force of a chair on your
body, and the force a baseball exerts on a bat are contact forces because the force is ex—
erted through direct contact. Other forces, like gravity and electric and magnetic forces,
are action-at—a—distance forces because they seemingly act between distant objects,
like Earth and the Moon. Actually, the distinction isn’t clear-cut; at the microscopic
level, contact forces involve action—at—a-distance electric forces between molecules.
And the action—at—a-distance concept itself is troubling. How can Earth “reach out”
across empty space and pull on the Moon? Later we’ll look at an approach to forces that
avoids this quandary. The Fundamental Forces Gravity, tension forces, compression forces, contact forces, electric forces, friction
forces—how many kinds of forces are there? At present, physicists identify three basic
forces: the gravitational force, the electroweak force, and the strong force. Gravity is the weakest of the fundamental forces, but because it acts attractively be
tween all matter, gravity’s effect is cumulative. That makes gravity the dominant force in
the large—scale universe, determining the structure of planets, stars, galaxies, and the
universe itself. The electroweak force subsumes electromagnetism and the weak nuclear force.
Virtually all the nongravitational forces we encounter in everyday life are electromag~
netic, including contact forces, friction, tension and compression forces, and the forces
that bind atoms into chemical compounds. The weak nuclear force is less obvious,
but it’s crucial in the Sun’s energy production—#providing the energy that powers life
on Earth. The strong force describes how particles called quarks bind together to form protons,
neutrons, and a host of less-familiar particles. The force that joins protons and neutrons to
make atomic nuclei is a residue of the strong force between their constituent quarks. Al-
though the strong force is not obvious in everyday life, it is ultimately responsible for the
structure of matter. If its strength were slightly different, atoms more complex than helium
would not be possible, and the universe would be devoid of life! Unifying the fundamental forces is a major goal of physics. Over the centuries we’ve
come to understand seemingly disparate forces as manifestations of a more fundamental
underlying force. Figure 4.8 suggests that the process continues, as physicists attempt ﬁrst
to unify the strong and electroweak forces, and then ultimately to add gravity to give a
“Theory of Everything.” ...
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- Spring '07