Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Work (physics), product of a force applied to a body and the displacement of the body in the direction of the applied force. While work is done on a body, there is a transfer of energy to the body, and so work can be said to be energy in transit. The units of work are identical to those of energy. If, for example, an object is lifted from the floor to the top of a table, work is done in overcoming the downward force of gravity, and the energy imparted to the body as work will increase its potential energy. Work is also expended when a force accelerates a body, such as the acceleration of an airplane because of the thrust forces developed by its jet engines. The force need not be simply a mechanical force (see Mechanics) as in the case of lifting a body or accelerating a plane by jet reaction; it can also be an electrostatic, electrodynamic, or surface-tension force (see Electricity; Surface Tension). On the other hand, if a constantly acting force does not produce motion, no work is performed. Holding a book steadily at arm's length, for example, does not involve any work, irrespective of the apparent effort required.

In the commonly used English system of units, the unit of work is the foot-pound (ft-lb), which is equal to the amount of work required to raise a mass of 1 lb through an elevation of 1 ft at sea level and 45° latitude. In the CGS system, a force of 1 dyne moving through 1 cm corresponds to 1 erg of work. The more commonly used practical unit of work or energy in the CGS system is the joule, which equals 107 ergs. The rate of doing work is known as the power. The application of 550 ft-lb/sec, for example, corresponds to 1 horsepower, abbreviated hp, in the English system.