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The perpendicular axis theorem (or plane figure theorem) states that for a planar lamina with a uniform mass distribution, the moment of inertia about an axis perpendicular to the plane of the lamina is equal to the sum of the moments of inertia about two mutually perpendicular axes in the plane of the lamina, which intersect at the point where the perpendicular axis passes through.
The parallel axis theorem, also known as Huygens–Steiner theorem, or just as Steiner's theorem, [1] named after Christiaan Huygens and Jakob Steiner, can be used to determine the moment of inertia or the second moment of area of a rigid body about any axis, given the body's moment of inertia about a parallel axis through the object's center of gravity and the perpendicular distance between ...
Diagram for geometric proof. This proof is valid only if the line is not horizontal or vertical. [5] Drop a perpendicular from the point P with coordinates (x 0, y 0) to the line with equation Ax + By + C = 0. Label the foot of the perpendicular R. Draw the vertical line through P and label its intersection with the given line S.
However, it is often easier to derive the second moment of area with respect to its centroidal axis, , and use the parallel axis theorem to derive the second moment of area with respect to the ′ axis. The parallel axis theorem states ′ = + where is the area of the shape, and; is the perpendicular distance between the and ′ axes. [4] [5]
Secant-, chord-theorem. For the intersecting secants theorem and chord theorem the power of a point plays the role of an invariant: . Intersecting secants theorem: For a point outside a circle and the intersection points , of a secant line with the following statement is true: | | | | = (), hence the product is independent of line .
The line segments OT 1 and OT 2 are radii of the circle C; since both are inscribed in a semicircle, they are perpendicular to the line segments PT 1 and PT 2, respectively. But only a tangent line is perpendicular to the radial line. Hence, the two lines from P and passing through T 1 and T 2 are tangent to the circle C.
Let PQ be a line perpendicular to line OQ defined by angle , drawn from point Q on this line to point P. OQP is a right angle. Let QA be a perpendicular from point A on the x -axis to Q and PB be a perpendicular from point B on the x -axis to P. ∴ {\displaystyle \therefore } OAQ and OBP are right angles.
The principal axis theorem states that the principal axes are perpendicular, and gives a constructive procedure for finding them. Mathematically, the principal axis theorem is a generalization of the method of completing the square from elementary algebra .