Inertia

Inertia, the property of matter that causes it to resist any change of its motion in either direction or speed. This property is accurately described by the first law of motion of the English scientist Sir Isaac Newton: An object at rest tends to remain at rest, and an object in motion tends to continue in motion in a straight line unless acted upon by an outside force. For example, passengers in an accelerating automobile feel the force of the seat against their backs overcoming their inertia so as to increase their velocity. As the car decelerates, the passengers tend to continue in motion and lurch forward. If the car turns a corner, then a package on the car seat will slide across the seat as the inertia of the package causes it to continue moving in a straight line.

Any body spinning on its axis, such as a flywheel, exhibits rotational inertia, a resistance to change of its rotational speed. To change the rate of rotation of an object by a certain amount, a relatively large force is required for an object with a large rotational inertia, and a relatively small force is required for an object with a small rotational inertia. Flywheels, which are attached to the crankshaft in automobile engines, have a large rotational inertia. The engine delivers power in surges; the large rotational inertia of the flywheel absorbs these surges and keeps the engine delivering power smoothly.

An object's inertia is determined by its mass. Newton's second law states that a force acting on an object is equal to the mass of the object multiplied by the acceleration the object undergoes. Thus, if a force causes an object to accelerate at a certain rate, then a stronger force must be applied to make a more massive object accelerate at the same rate; the more massive object has a larger amount of inertia that must be overcome. For example, if a bowling ball and a baseball are accelerated so that they end up rolling at the same speed, then a larger force must have been applied to the bowling ball, since it has more inertia.

Matter

Matter, in science, general term applied to anything that has the property of occupying space and the attributes of gravity and inertia. In classical physics, matter and energy were considered two separate concepts that lay at the root of all physical phenomena. Modern physicists, however, have shown that it is possible to transform matter into energy and energy into matter and have thus broken down the classical distinction between the two concepts (see Mass; Relativity). When dealing with a large number of phenomena, however, such as motion, the behavior of liquids and gases, and heat, scientists find it simpler and more convenient to continue treating matter and energy as separate entities.

Certain elementary particles of matter combine to form atoms; in turn, atoms combine to form molecules. The properties of individual molecules and their distribution and arrangement give to matter in all its forms various qualities such as mass, hardness, viscosity, fluidity, color, taste, electrical resistivity, and heat conductivity, among others. See Antimatter; Chemistry; Electricity; Heat; States of Matter.

In philosophy, matter has been generally regarded as the raw material of the physical world, although certain philosophers of the school of idealism, such as the Irish philosopher George Berkeley, denied that matter exists independent of the mind. See Greek Philosophy; Kant, Immanuel. Most modern philosophers accept the scientific definition of matter.

Energy

Energy, capacity of matter to perform work as the result of its motion or its position in relation to forces acting on it. Energy associated with motion is known as kinetic energy, and energy related to position is called potential energy. Thus, a swinging pendulum has maximum potential energy at the terminal points; at all intermediate positions it has both kinetic and potential energy in varying proportions. Energy exists in various forms, including mechanical (see Mechanics), thermal (see Thermodynamics), chemical (see Chemical Reaction), electrical (see Electricity), radiant (see Radiation), and atomic (see Nuclear Energy). All forms of energy are interconvertible by appropriate processes. In the process of transformation either kinetic or potential energy may be lost or gained, but the sum total of the two remains always the same.

A weight suspended from a cord has potential energy due to its position, inasmuch as it can perform work in the process of falling. An electric battery has potential energy in chemical form. A piece of magnesium has potential energy stored in chemical form that is expended in the form of heat and light if the magnesium is ignited. If a gun is fired, the potential energy of the gunpowder is transformed into the kinetic energy of the moving projectile. The kinetic mechanical energy of the moving rotor of a dynamo is changed into kinetic electrical energy by electromagnetic induction. All forms of energy tend to be transformed into heat, which is the most transient form of energy. In mechanical devices energy not expended in useful work is dissipated in frictional heat, and losses in electrical circuits are largely heat losses.

Empirical observation in the 19th century led to the conclusion that although energy can be transformed, it cannot be created or destroyed. This concept, known as the conservation of energy, constitutes one of the basic principles of classical mechanics. The principle, along with the parallel principle of conservation of matter, holds true only for phenomena involving velocities that are small compared with the velocity of light. At higher velocities close to that of light, as in nuclear reactions, energy and matter are interconvertible (see Relativity). In modern physics the two concepts, the conservation of energy and of mass, are thus unified.

See also Bioenergetics.