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.


Velocity, in physics, is a rate of linear motion of a body in a particular direction. Velocity is a vector—that is, it has both magnitude and direction. The magnitude of velocity, known as speed, is usually expressed in terms of distance covered per unit of time (usually the hour or second), such as feet per second or kilometers per hour. Velocity can be constant, or it can be accelerated. Constant, or uniform, velocity can be determined simply by dividing the distance covered by the time taken to travel that distance. When an object is accelerated, the object's velocity vector changes per unit of time. Acceleration can change the direction of the velocity vector, or change the magnitude of the velocity vector, or change both the direction and magnitude of the velocity vector. See Acceleration; Mechanics.