The axis of rotation is an imaginary line through a 3D object that it can rotate around. 2D objects can only rotate around a point, not an axis. A 3D object cannot rotate around an interior point and can have multiple internal rotation axes.
Axis of rotation is a term used in many branches of engineering, mathematics and science. It is used to describe a type of movement for objects existing in three-dimensional (3D) space. It is an imaginary straight line drawn through a 3D object about which the object can rotate or rotate. Rotation models are used for both theoretical and practical applications.
Rotation around an axis can only occur in three-dimensional space, which means that an object has length, width, and depth. Objects in two-dimensional (2D) space can rotate, but rotation occurs about a point, inside or outside, on a flat plane, not an axis. The image can’t truly rotate around an axis since it doesn’t exist in all three dimensions: it has length and width, but no depth. If a 2D object is plotted on a graph, it can be “flipped” around the X or Y axis, but this simply flips or inverts the image. The image is flat and would disappear at some point if rotated through a full rotation around one of the axes.
Conversely, a 3D object can rotate around an axis but cannot rotate around an interior point. This would cause the object to rotate in many directions at once, along every line that intersects that point, and tear. Therefore, a 3D object can only rotate around a line or axis. If a 3D object moves around an external point, this is called a revolution, not a rotation. For example, the Earth rotates on an internal axis and revolves around the sun.
Existing objects in 3D space can have more than one internal rotation axis, but cannot rotate around two of these axes at the same time. For example, if the Earth has a new axis of rotation perpendicular to its original axis, it cannot rotate in opposite directions on both axes at the same time. As it attempts to rotate in both directions, it will find a balance point between the two and create a third axis of rotation along that line.
Objects that cannot perform a full rotation or rotation can still be considered as rotating about an axis. Ankles and knees, as well as mechanical parts and many other objects, can perform a limited rotation around a given inside line. Because they are 3D objects, they are referred to as having an axis of rotation, even though the rotation itself is limited.
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