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.


Weight, measure of the gravitational force exerted on an object (see Gravitation). The weight of an object may be determined by using either a comparative method, as with a chemical-laboratory balance, or by measuring the gravitational force directly by means of a spring scale, such as the familiar bathroom scale (see Mass). The deflection of a spring scale depends on the local gravitational attraction; therefore, a spring scale would record different weights for the same mass (amount of matter) at locations with a different gravitational attraction. One kilogram of mass, for example, might weigh 2.2 lb at sea level at latitude 45° North, but slightly less on the top of a high mountain or at the equator, although the mass remains unchanged. More dramatically, an object with 1 kg of mass might weigh 2.2 lb on earth but only 0.35 lb on the moon. This problem is avoided by using the so-called beam scale such as that employed in the chemical-laboratory balance. In this, two pans are suspended from a cross beam that rests on a fulcrum, and a pointer is used to indicate balance when equal masses are in both pans. Because both masses are subject to the same gravitational attraction, weighing is performed by comparison and is independent of the specific magnitude of the local gravitational attraction.