| WELCOME
TO FORCE AND MOTION
The world is filled with motion. Some motion happens without
human intervention: Earth revolves around the Sun, snowakes
fall to the ground, waves surge across the sea, salmon swim
up rivers to fulll their destinies. Other motions are under
our control: clock hands faithfully monitor time, jet planes
streak across the sky, baseballs fly over center eld fences,
bicycles race in the Tour de France. Both natural and designed
motions are part of our perception of the world—there
is nothing special about them.
What we take for granted is often worthy of contemplation,
in part because it is so commonplace. Familiarity can breed
a sense of innate understanding where none really exists.
We rarely question what makes things move, often resorting
to the popular nonexplanation, “That’s the way
the world works.”
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Forces make things move or, more accurately, make things
change their motion. The two natural forces that affect most
of the motion we are aware of are the force of gravity and
the electromagnetic force. If a peanut slips between your
fingers in the backyard, the force of gravity will pull it
to the ground. If the same peanut happens to fall onto the
picnic table, the force of gravity will still pull the peanut,
but it will not fall to the ground. Why? Because the table
is pushing up against the peanut with a force exactly equal
to the force exerted by gravity pulling the peanut down. The
force opposing the force of gravity is the electromagnetic
force, expressed in countless molecular interactions in the
matter in Earth and the table.
These ideas seem to conflict. Over here force results in change
of motion. But over there force produces no change of motion.
Welcome to force and motion. This is where you and your students
will start to uncover explanations for why things work in
the ways we expect them to work.
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