Matter Anything that has mass, takes up space, and is made of particles. Solid The state of matter in which the substance has both a definite volume and a definite shape. Particles can not move around one another. Liquid The state of matter in which the substance has a definite volume, but no definite shape. Particles move freely. Gas The state of matter in which the substance has neither a definite volume nor a definite shape. Particles move rapidly and freely. Fluid Any substance that flows. Can be a liquid or a gas. Viscosity The measure of how fast a fluid will flow; the "thickness" or "thinness" of a fluid. Flow Rate The volume of fluid that passes a point in a pipe or tube in a certain amount of time. Mass The amount of matter in a substance. Weight The force of gravity exerted on a mass. Density The amount of mass in a certain unit volume of a substance (mass divided by volume). Average Density The total mass of an object divided by the total volume. Buoyancy The tendency to rise or float in a fluid. Force A push or pull, or anything, that can cause a change in motion. Gravity The attractive force between masses. Capacity The largest amount that can be held by a container. Pressure The force acting perpendicular to a surface area. Compressibility The ability to be squeezed into a smaller volume. Compressible Capable of being squeezed into a smaller volume. Incompressible Not capable of being squeezed into a smaller volume. Hydraulics The study of pressure in liquids. Pneumatics The study of pressure in gases. Pump A device that moves a fluid through a hose, pipe or into a container. Compressor A device that compresses air. Balanced Forces When all the forces on an object are equal and opposite, such that the movement will not change.