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We say that force is the prime mover in a mechanical system because a net force acting on an object changes the object’s motion. If the force acts in the direction of the object’s velocity or in the opposite direction, the force does work on the object. For example, when a tennis racket strikes a ball, it exerts a force in the direction opposite the ball’s initial velocity. The racket does work on the ball and changes its direction and speed. You can calculate the work done on the tennis ball by using the force applied by the racket and the distance the ball moves while being pushed by the racket. A tennis racket exerts a force on a ball and does work on the ball. Forces are distributed over areas in fluid systems. The force per unit area is pressure. Pressure (or pressure difference) is the prime mover in fluid systems. If the pressure in a fluid is high enough, it can cause a boundary of the fluid to move, doing work on the boundary. For example, in the power stroke of an internal-combustion engine, an electric spark ignites the fuel-air mixture in a cylinder. The mixture burns rapidly, and the gas pressure inside the cylinder increases by as much as 600 psi. This pressure acting over the surface of the piston forces the piston to move down the cylinder. During the power stroke, the expanding gas does work on the piston. You can use the pressure and the volume to calculate the work done by the gas. 82 C HAPTER 2 WORK In the power stroke of an internal-combustion engine, a fuel-air mixture burns rapidly in a cylinder. The hot, high-pressure gas expands and does work on a piston. In an electrical system, charges (electrons) move through a circuit because coulomb forces are exerted on the charges. Coulomb forces are caused by the presence of other charges. These other charges create an electric field and a potential difference. Potential difference is the prime mover in electrical systems. In the circuit below, a current (or flow of charge) drives a DC motor for a model boat. The source of the potential difference that moves the charge is a 6-V battery. The battery does work in moving the charge. You can use the potential difference and the amount of charge moved to calculate the work done by the battery. In a circuit for a model boat motor, a battery does work in moving charge through a potential difference. C HAPTER 2 WORK 83