(physics) A vector measure of the rotation of an object about an axis; the vector points along the axis according to the right-hand rule; the length of the vector is the rotation angle, in degrees or radians.
| Sci-Tech Dictionary: angular displacement |
(physics) A vector measure of the rotation of an object about an axis; the vector points along the axis according to the right-hand rule; the length of the vector is the rotation angle, in degrees or radians.
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| Sports Science and Medicine: angular displacement |
The difference between the initial and final angular position of a moving body. It is measured in radians. Angular displacement has both magnitude and direction. Conventionally, clockwise movements are described as positive (+) and anticlockwise movements as negative (−). Also, angular displacements of human body segments usually indicate the type of joint movement (e.g. flexion or extension).

| Wikipedia: Angular displacement |
Angular displacement of a body is the angle in radians (degrees, revolutions) through which a point or line has been rotated in a specified sense about a specified axis.
When an object rotates about its axis, the motion cannot simply be analyzed as a particle, since in circular motion it undergoes a changing velocity and acceleration at any time (t). When dealing with the rotation of an object, it becomes simpler to consider the body itself rigid. A body is generally considered rigid when the separations between all the particles remains constant throughout the objects motion, so for example parts of its mass are not flying off. In a realistic sense, all things can be deformable, however this impact is minimal and negligible. Thus the rotation of a rigid body over a fixed axis is referred to as rotational motion.
In the example illustrated to the right, a particle on object P at a fixed distance r from the origin, O, rotating counterclockwise. It becomes important to then represent the position of particle P in terms of its polar coordinates (r, θ). In this particular example, the value of θ is changing, while the value of the radius remains the same. (In rectangular coordinates (x, y) both x and y vary with time). As the particle moves along the circle, it travels an arc length s, which becomes related to the angular position through the relationship:

Angular displacement is measured in radians rather than degrees. This is because it provides a very simple relationship between distance traveled around the circle and the distance r from the centre.

For example if an object rotates 360 degrees around a circle radius r the angular displacement is given by the distance traveled the circumference which is 2πr Divided by the radius in:
which easily simplifies to θ = 2π. Therefore 1 revolution is 2π radians.
When object travels from point P to point Q, as it does in the illustration to the left, over δt the radius of the circle goes around a change in angle. Δθ = Δθ2 − Δθ1 which equals the Angular Displacement.
In three dimensions, angular displacement has a direction and a magnitude. The direction specifies the axis of rotation; the magnitude specifies the rotation in radians about that axis (using the right-hand rule to determine direction). Despite having direction and magnitude, angular displacement is not a vector because it does not obey the commutative law.[1]
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