Motion
In physics,
motion is a change in position of an object with respect to time and its reference point. Motion is typically described in terms of displacement, direction, velocity, acceleration, and time.
Motion is observed by attaching a frame of reference to a body and measuring its change in position relative to that frame.
If the position of a body is not changing with the time with respect to a given frame of reference the body is said to be
at rest,
motionless,
immobile,
stationary, or to have constant (time-invariant) position. An object's motion cannot change unless it is acted upon by a force, as described by Newton's first law. Momentum is a quantity which is used for measuring motion of an object. An object's momentum is directly related to the object's mass
and velocity, and the total momentum of all objects in an isolated
system (one not affected by external forces) does not change with time,
as described by the law of conservation of momentum. The study of motion deals with (1) The study of motion of solids (mechanics). (2) study of motion of fluids (fluid mechanics)
Laws of motion
Main article: Mechanics
In physics, motion in the universe is described through two sets of apparently contradictory laws of mechanics. Motions of all large scale and familiar objects in the universe (such as projectiles, planets, cells, and humans) are described by classical mechanics. Whereas the motion of very small atomic and sub-atomic objects is described by quantum mechanics.
Types of Motion
 |
| a / Rotation. |
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.
 |
| b / Simultaneous rotation and
motion through space. |
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-andconquer
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.
 |
| c / 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. |
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.
 |
| d / The leaping dancer's motion is
complicated, but the motion of her
center of mass is simple. |
It turns out that there is one particularly natural and useful way
to make a clear definition, but it requires a brief digression. Every
object has a balance point, referred to in physics as
the center of
mass. For a two-dimensional object such as a cardboard cutout, the
center of mass is the point at which you could hang the object from
a string and make it balance. In the case of the ballerina (who is
likely to be three-dimensional unless her diet is particularly severe),
it might be a point either inside or outside her body, depending
on how she holds her arms. Even if it is not practical to attach a
string to the balance point itself, the center of mass can be defined
as shown in figure e.
 |
| e / No matter what point you
hang the pear from, the string
lines up with the pear's center
of mass. The center of mass
can therefore be defined as the
intersection of all the lines made
by hanging the pear in this way.
Note that the X in the figure
should not be interpreted as
implying that the center of mass
is on the surface - it is actually
inside the pear. |
Why is the center of mass concept relevant to the question of
classifying rotational motion as opposed to motion through space?
As illustrated in figures d and f, it turns out that the motion of an
object's center of mass is nearly always far simpler than the motion
of any other part of the object. The ballerina's body is a large object
with a complex shape. We might expect that her motion would be
much more complicated than the motion of a small, simply-shaped
object, say a marble, thrown up at the same angle as the angle at
which she leapt. But it turns out that the motion of the ballerina's
center of mass is exactly the same as the motion of the marble. That
is, the motion of the center of mass is the same as the motion the
ballerina would have if all her mass was concentrated at a point. By
restricting our attention to the motion of the center of mass, we can
therefore simplify things greatly.
 |
| f / The same leaping dancer, viewed from above. Her center of
mass traces a straight line, but a point away from her center of mass,
such as her elbow, traces the much more complicated path shown by the
dots. |
We can now replace the ambiguous idea of "motion as a whole
through space" with the more useful and better defined concept
of "center-of-mass motion." The motion of any rigid body can be
cleanly split into rotation and center-of-mass motion. By this definition,
the tipping chair does have both rotational and center-of-mass
motion. Concentrating on the center of mass motion allows us to
make a simplified model of the motion, as if a complicated object
like a human body was just a marble or a point-like particle. Science
really never deals with reality; it deals with models of reality.
Note that the word "center" in "center of mass" is not meant
to imply that the center of mass must lie at the geometrical center
of an object. A car wheel that has not been balanced properly has
a center of mass that does not coincide with its geometrical center.
An object such as the human body does not even have an obvious
geometrical center.
 |
| g / An improperly balanced
wheel has a center of mass that
is not at its geometric center.
When you get a new tire, the
mechanic clamps little weights to
the rim to balance the wheel. |
It can be helpful to think of the center of mass as the average
location of all the mass in the object. With this interpretation, we
can see for example that raising your arms above your head raises
your center of mass, since the higher position of the arms' mass
raises the average.
 |
| i / A fixed point on the dancer's body follows a trajectory that is
flatter than what we expect, creating an illusion of flight. |
 |
| h / This toy was intentionally
designed so that the mushroomshaped
piece of metal on top
would throw off the center of
mass. When you wind it up, the
mushroom spins, but the center
of mass doesn't want to move,
so the rest of the toy tends to
counter the mushroom's motion,
causing the whole thing to jump
around. |
Ballerinas and professional basketball players can create an illusion of
flying horizontally through the air because our brains intuitively
expect them to have rigid-body motion, but the body does
not stay rigid while executing a grand jete or a slam dunk. The legs
are low at the beginning and end of the jump, but come up higher at
the middle. Regardless of what the limbs do, the center of mass will
follow the same arc, but the low position of the legs at the beginning
and end means that the torso is higher compared to the center of
mass, while in the middle of the jump it is lower compared to the
center of mass. Our eye follows the motion of the torso and tries
to interpret it as the center-of-mass motion of a rigid body. But
since the torso follows a path that is flatter than we expect, this
attempted interpretation fails, and we experience an illusion that
the person is flying horizontally.