Everything about Rotate totally explained
A
rotation is a movement of an object in a circular motion. A two-
dimensional object rotates around a
center (or
point)
of rotation. A three-dimensional object rotates around a line called an
axis. If the axis of rotation is within the body, the body is said to rotate upon itself, or
spin—which implies relative
speed and perhaps free-movement with
angular momentum. A circular motion about an external point, for example the
Earth about the
Sun, is called an
orbit or more properly an
orbital revolution.
Mathematics
Mathematically, a rotation is, unlike a
translation, a
rigid body movement which keeps a point fixed. This definition applies to rotations within both two and three dimensions (in a plane and in space, respectively.) A rotation in three-dimensional space keeps an entire line fixed, for example a rotation in three-dimensional space is a rotation around an axis. This follows from
Euler's rotation theorem.
All rigid body movements are rotations, translations, or combinations of the two.
If a rotation around a point or axis is followed by a second rotation around the same point/axis, a third rotation results. The reverse (
inverse) of a rotation is also a rotation. Thus, the rotations around a point/axis form a
group. However, a rotation around a point or axis and a rotation around a different point/axis may result in something other than a rotation, for example a translation.
Rotations around the
x,
y and
z axes are called
principal rotations. Rotation around any axis can be performed by taking a rotation around the
x axis, followed by a rotation around the
y axis, and followed by a rotation around the
z axis. That is to say, any spatial rotation can be decomposed into a combination of principal rotations.
In
flight dynamics, the principal rotations are known as
pitch,
roll and
yaw (known as
Tait-Bryan angles). This terminology is also used in
computer graphics.
Astronomy
In
astronomy, rotation is a commonly observed phenomenon.
Stars,
planets and similar bodies all spin around on their axes (the plural of
axis). The rotation rate of planets in the solar system was first measured by tracking visual features.
Stellar rotation is measured through
Doppler shift or by tracking active surface features.
This rotation induces a
centrifugal acceleration in the reference frame of the Earth which slightly counteracts the effect of gravity the closer one is to the
equator. One effect is that an object weighs slightly less at the equator. Another is that the Earth is slightly deformed into an
oblate spheroid.
Another consequence of the rotation of a planet is the phenomenon of
precession. Like a
gyroscope, the overall effect is a slight "wobble" in the movement of the axis of a planet. Currently the tilt of the
Earth's axis to its orbital plane (
obliquity of the ecliptic) is 23.45 degrees, but this angle changes slowly (over thousands of years). (See also
Precession of the equinoxes and
Pole star.)
Rotation and revolution
While revolution is often used as a synonym for rotation, in many fields, particularly astronomy and related fields, revolution, often referred to as orbital revolution for clarity, is used when one body moves around another while rotation is used to mean the movement around an axis. Moons revolve about their planet, planets revolve about their star (such as the Earth around the Sun); and stars slowly revolve about their
galaxial center. The motion of the components of
galaxies is complex, but it usually includes a rotation component.
The Moon makes one complete rotation during one complete orbital revolution around the Earth (an effect called
tidal locking) so that the same side of the Moon always faces the Earth (the other side is called the
far side of the Moon).
Retrograde rotation
Most
planets in our
solar system, including
Earth, spin in the same direction as they orbit the
Sun. The exceptions are
Venus and
Uranus. Uranus rotates nearly on its side relative to its orbit. Current speculation is that Uranus started off with a typical prograde orientation and was knocked on its side by a large impact early in its history. Venus may be thought of as rotating slowly backwards (or being "upside down"). The
dwarf planet Pluto (formerly considered a planet) is anomalous in this and other ways.
Physics
The speed of rotation is given by the
angular frequency (rad/s) or
frequency (
turns/s, turns/min), or
period (seconds, days, etc.). The time-rate of change of angular frequency is angular acceleration (rad/s²), This change is caused by
torque. The ratio of the two (how heavy is it to start, stop, or otherwise change rotation) is given by the
moment of inertia.
The
angular velocity vector also describes the direction of the axis of rotation. Similarly the torque is a vector.
According to the
right-hand rule, the direction away from the observer is associated with clockwise rotation and the direction towards the observer with counterclockwise rotation, like a
screw.
Aviation
In
flight dynamics, the principal rotations are known as
pitch,
roll and
yaw. The term rotation is also used in aviation to refer to the upward pitch (nose moves up) of an aircraft, particularly when starting the climb after takeoff.
Amusement rides
Many
amusement rides provide rotation. A
Ferris wheel and
observation wheel have a horizontal central axis, and parallel axes for each gondola, where the rotation is opposite, by gravity or mechanically. As a result at any time the orientation of the gondola is upright (not rotated), just translated. The tip of the translation vector describes a circle. A
carousel provides rotation about a vertical axis. Many rides provide a combination of rotations about several axes. In
Chair-O-Planes the rotation about the vertical axis is provided mechanically, while the rotation about the horizontal axis is due to the
centripetal force. In
roller coaster inversions the rotation about the horizontal axis is one or more full cycles, where inertia keeps people in their seats.
Sports
Rotation, usually called
spin, plays a role in many sports.
Topspin and
backspin in
tennis.
English,
follow and
draw in
billiards and pool.
Curve balls in baseball and
spin bowling in cricket.
Table tennis paddles are specialized to allow players to spin the ball as they hit it.
Further Information
Get more info on 'Rotate'.
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