A helix (pl: helices), from the Greek word έλιξ, is a three-dimensional, twisted shape. Common objects formed like a helix are a spring, a screw, and a spiral staircase (though the last would be more correctly called helical). Helices are important in biology, as the DNA molecule is formed as two intertwined helices, and many proteins have helical substructures, known as alpha helices.Helices can be either right-handed or left-handed. With the line of sight being the helical axis, if clockwise movement of the helix corresponds to axial movement away from the observer, then it is a right-handed helix. If counter-clockwise movement corresponds to axial movement away from the observer, it is a left-handed helix. Handedness (or chirality) is a property of the helix, not of the perspective: a right-handed helix cannot be turned or flipped to look like a left-handed one unless it is viewed through a mirror, and vice versa.
Here is another test for handedness: first grip the helix with your right hand and direct your thumb parallel to the axis of the helix. Then curl your fingers toward your palm, following the path of the spiral as if the helix were a set of rails that your fingers must slide along. If this causes your entire hand to move in the same direction as your thumb is pointing, then the helix is right-handed. If not, it is left-handed. Try this test on the left-handed helix in the picture below; in this case, your hand should move in the direction opposite to the way your thumb points.
Here is another test for handedness: first grip the helix with your right hand and direct your thumb parallel to the axis of the helix. Then curl your fingers toward your palm, following the path of the spiral as if the helix were a set of rails that your fingers must slide along. If this causes your entire hand to move in the same direction as your thumb is pointing, then the helix is right-handed. If not, it is left-handed. Try this test on the left-handed helix in the picture below; in this case, your hand should move in the direction opposite to the way your thumb points.
Most hardware screws are right-handed helices. The alpha helix in biology as well as the A and B forms of DNA are also right-handed helices. The Z form of DNA is left-handed.
A double helix typically consists geometrically of two congruent helices with the same axis, differing by a translation along the axis, which may or may not be half-way.
A conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of the angle indicating direction from the axis. An example of a helix would be the Corkscrew roller coaster at Cedar Point amusement park.
A circular helix has constant band curvature and constant torsion. The pitch of a helix is the width of one complete helix turn, measured along the helix axis. A curve is called a general helix if its tangent makes a constant angle with a fixed line in space.
A double helix typically consists geometrically of two congruent helices with the same axis, differing by a translation along the axis, which may or may not be half-way.
A conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of the angle indicating direction from the axis. An example of a helix would be the Corkscrew roller coaster at Cedar Point amusement park.
A circular helix has constant band curvature and constant torsion. The pitch of a helix is the width of one complete helix turn, measured along the helix axis. A curve is called a general helix if its tangent makes a constant angle with a fixed line in space.
ไม่มีความคิดเห็น:
แสดงความคิดเห็น