Equilibrium of physics

 Statics is concerned with bodies that are in equilibrium, which is the state of an object when it is not accelerated; a body that is at rest or moving at constant velocity is therefore in equilibrium.

For an object to be in equilibrium it is necessary for all the forces acting on it to cancel each other out exactly.

Center of gravity

The concept of center of gravity is used in determining the stability of an object’s equilibrium: The center of gravity is the point where the entire weight of an object can be considered to be concentrated. A disk, for example, has its center of gravity at the center, whereas that of a rigid triangular sheet lies at the point of intersection of the lines that join the vertices of the triangle to the midpoints of the opposite sides. The center of gravity of an irregularly-shaped flat object can be found by suspending it from any two points on it and marking the point of intersection of a plumb line suspended from each point in turn.

The principle of moments

The reason an object falls over when it is disturbed from its unstable equilibrium is that the force of gravity acts as a torque, turning the object on an axis. The strength of this turning effect, the moment of the force, can be measured at any point on the object and varies according to the distance of the point from the object’s axis. For any given point, the moment of the force equals the magnitude of the force multiplied by the perpendicular distance from the point to the line along which the force acts. The principle of moments states that when a body is in rotational equilibrium, the algebraic sum of all the torques acting on the body about all its axes is zero.

Practical applications

The principle of moments is used for working out the strength of materials needed to construct bridges and buildings.

For example, frameworks consisting of structural members are often found in the roofs of houses and in some forms of bridges; using the principle of moments and knowing the forces at work on the framework or truss, it is possible to discover which members are under compression (called struts) and which are in tension (called ties). Materials of suitable strength and thickness can then be chosen for the construction.

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