The second moment of area, also known as the moment of inertia of an area, is a geometric property of a shape that reflects its resistance to change. The moment of inertia of an area is expressed in terms of a single number and can be calculated for both two-dimensional and three-dimensional shapes. The moment of inertia of an area is calculated by taking the sum of the product of the area and the square of its distance from a chosen axis of rotation. The moment of inertia is expressed in units of length raised to the fourth power (m⁴, mm⁴, ft⁴, in⁴). For two-dimensional shapes, the moment of inertia can be expressed as a single equation in terms of the x and y axes of the shape's coordinates.

The equation for the moment of inertia of an area with respect to the x-axis is as follows:

where A is the shape's area, and y is the distance from the x-axis. A similar equation can be written for the y-axis:

For three-dimensional shapes, the moment of inertia can be expressed in a polar form, which takes into account the distance from the origin of the shape, as well as the angle of rotation around the origin. The equation for the moment of inertia of an area in polar form is as follows:

where r is the distance from the origin. The moment of inertia of an area has many practical applications in engineering, such as in the design of structures and machines. For example, when designing a bridge, engineers must consider the moment of inertia of the bridge's cross-section to ensure that it is strong enough to withstand the forces of an earthquake. Similarly, when designing a car engine, engineers must consider the moment of inertia of the pistons and other moving parts to ensure that they can handle the forces produced by the engine's combustion. In conclusion, the moment of inertia of an area is an important concept in mechanics and engineering, as it is used to calculate the stresses and strains on an object when the force of an applied torque is known.