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A force is a push or a pull on an object in a direction. In mechanical systems, forces can cause objects to move or change their motion. If a force is exerted on an object at a distance from its axis of rotation, it creates a torque. Torques can cause objects to rotate or change their rotational motion. In fluids, we think of forces being distributed over areas. Pressure is the force per unit area. Pressure differences in a fluid explain the buoyant force an object feels when it is immersed in the fluid. In fluid systems, a pressure difference can cause fluid to move or change its motion. When electric charges are separated, each charge feels a force of attraction or repulsion because of the presence of the other charges. The space surrounding the electric charges can be thought of as an electric field. There is a potential difference, or voltage, between two points in an electric field. Voltage causes charge to move in electrical systems. The temperature of a body is a measure of the average kinetic energy of the random motion of the particles that make up the body. Thermal energy is the total kinetic energy of all the particles. When two bodies with different temperatures are brought together, thermal energy flows from the hotter body to the colder body. The thermal energy transferred is called heat. In thermal systems, a temperature difference causes heat to flow. The table on the next page summarizes the four energy systems and the prime movers—force, pressure, voltage, and temperature difference. 80 C HAPTER 1 THE PRIME MOVERS Energy System Prime Mover Mechanical Translational Force Units SI English Quantity Moved N lb Torque N•m lb • ft Fluid Pressure N/m lb/in2 Electrical Potential difference V V Charge Thermal Temperature difference °C °F Heat Rotational Mass moved through a distance Mass rotated through an angle Fluid volume or mass C HAPTER 1 SUMMARY 81