Physics Chapter 4 Vocabulary 4.1 Work: the action that results when a force causes an object to move in the direction of the force; W = F x D Joule: the unit used to express work and energy; equivalent to the newton-meter (N*m) Power: the rate at which work is done; P = W/t Watt: the unit used to express power; equivalent to joules per second (J/s) 4.2 machine: a device that helps make work easier by changing the size or direction (or both) of a force. Work input: the work done on a machine; the product of the input force and the distance through which it is exerted. Work output: the work done by machine; the product of the output force and the distance through which it is exerted. Mechanical advantage: a number that tells how many times a machine multiplies force; can be calculated by dividing the output force by the input force. Mechanical efficiency: a comparison expressed as a percentage of a machine’s work out put with the work input; can be calculated by dividing work output by work input and then multiplying by 100. 4.3 Lever: simple machine consisting of a bar that pivots at a fixed point, called a fulcrum; there are three classes of levers based on where the input force, output force, and fulcrum are placed in relation to the load. Inclined plane: a simple machine that is a straight, slanted surface; a ramp Wedge: a simple machine that is a double incliend plane that moves; a wedge is often used for cutting. Screw: a simple machine that is an inclined plane wrapped in a spiral. Wheel and axle: a simple machine consisting of two circular ojects of different sizes; the wheel is the larger of the two circular objects. Pulley: a simple machine consisting of a grooved wheel that holds a rope or a cable; there are two knds of pulleys—fixed and movable. Compound machine: machine that is made of two ro more simple machines.