Simultaneous rotation and
motion through space.
One person might say that the
tipping chair was only rotating
in a circle about its point of
contact with the floor, but
another could describe it as
having both rotation and
motion through space.
2Velocity and Relative
2.1Types of Motion
If you had to think consciously in order to move your body, you would
be severely disabled. Even walking, which we consider to be no great feat,
requires an intricate series of motions that your cerebrum would be utterly
incapable of coordinating. The task of putting one foot in front of the
other is controlled by the more primitive parts of your brain, the ones that
have not changed much since the mammals and reptiles went their separate
evolutionary ways. The thinking part of your brain limits itself to general
directives such as “walk faster,” or “don’t step on her toes,” rather than
micromanaging every contraction and relaxation of the hundred or so
muscles of your hips, legs, and feet.
Physics is all about the conscious understanding of motion, but we’re
obviously not immediately prepared to understand the most complicated
types of motion. Instead, we’ll use the divide-and-conquer technique.
We’ll first classify the various types of motion, and then begin our campaign
with an attack on the simplest cases. To make it clear what we are and are
not ready to consider, we need to examine and define carefully what types
of motion can exist.
Rigid-body motion distinguished from motion that changes
an object’s shape
Nobody, with the possible exception of Fred Astaire, can simply glide
forward without bending their joints. Walking is thus an example in which
there is both a general motion of the whole object and a change in the shape
of the object. Another example is the motion of a jiggling water balloon as
it flies through the air. We are not presently attempting a mathematical
description of the way in which the shape of an object changes. Motion
without a change in shape is called rigid-body motion. (The word “body”
is often used in physics as a synonym for “object.”)
Center-of-mass motion as opposed to rotation
A ballerina leaps into the air and spins around once before landing. We
feel intuitively that her rigid-body motion while her feet are off the ground
consists of two kinds of motion going on simultaneously: a rotation and a
motion of her body as a whole through space, along an arc. It is not
immediately obvious, however, what is the most useful way to define the
distinction between rotation and motion through space. Imagine that you
attempt to balance a chair and it falls over. One person might say that the
only motion was a rotation about the chair’s point of contact with the floor,
but another might say that there was both rotation and motion down and
to the side.
Chapter 2Velocity and Relative Motion
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