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Work and Energy Chapter 12 Section 1 – Work, Power, and Machines Work is done when a force causes an object to move. This meaning is different from the everyday meaning of work. Work is equal to force times distance. W = F x d SI unit is joules. 1 J = 1 Nm = 1 kgm2/s2 Power is the rate at which work is done. SI unit for power is the Watt (W) 𝑃= 𝑊 𝑡 Machines help people by redistributing the work put into them. They change the size or the direction of the input force. Mechanical Advantage – describes how much the machine multiplies force or increases distance. 𝑀𝐴 = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 also 𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 𝑀𝐴 = 𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 If MA >1 then input force is multiplied If MA < 1 the speed or distance is multiplied Section 2 – Simple Machines The most basic machines are called simple machines. Other machines are modifications or combinations of simple machines. There are six types of machines in two families. The Lever Family 1. Levers have a rigid arm and a fulcrum. There are three classes of levers (see Figure 5 pg. 386). Ex. Claw hammer, wheelbarrow, forearm The Lever Family con’t 2. Pulleys increase MA (see Figure 6 pg. 387) The Lever Family con’t 3. Wheel-and-axle is a lever or pulley connected to a shaft – increases force Ex. Steering wheel, screwdriver Inclined Plane Family 4. Inclined Plane – increases force by spreading it over a longer distance. Ex. Ramp Inclined Plane Family con’t 5. Wedge – two inclined planes together. Redirects force. Ex. Axe head, nail Inclined Plane Family cont’d 6. Screw – inclined plane wrapped around cylinder. Ex. Screw, jar lid, spiral staircase Compound machines are made of two or more simple machines. Many are very complex Mechanical Advantage of Simple Machines 𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 or 𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒 𝑀𝐴 = 𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑀𝐴 = Lever: Mechanical Advantage = length of effort arm/ length of resistance arm Pulley: Count the number of rope segments that exert an upward force on the object being moved. Wheel and Axle: Mechanical Advantage = radius of wheel/radius of axle Inclined Plane: Mechanical Advantage = length of slope/ height of slope (Includes wedge height of slope and screw) Section 3 – What is Energy? Energy - the ability to do work. Like work, energy is measured in joules (J). Mechanical Energy – the amount of work an object can do. The sum of the potential and kinetic energy. Most systems also contain nonmechanical energy – energy at the atomic level. Nonmechanical energy does not usually affect systems on a large scale. Potential Energy - energy of position. Elastic potential energy is stored in any stretched or compressed elastic material. The gravitational potential energy of an object is determined by its mass, its height, and g, the free-fall acceleration due to gravity. PE = mgh. Kinetic Energy - energy of motion. Determined by its mass and speed. KE = ½ mv2. Section 4 – Conservation of Energy Energy readily changes from one form to another. In a mechanical system, potential energy can become kinetic energy, and kinetic energy can become potential energy. Mechanical energy can change to nonmechanical energy as a result of friction, air resistance, or other means. Law of Conservation of Energy – energy cannot be created or destroyed, although it may change form. A machine cannot do more work than the work required to operate the machine. Because of friction, the work output of a machine is always less than the work input. Efficiency of a machine is the ratio of the useful work performed by the machine to the work required to operate the machine. No machine can 100% efficiency – friction. ☞All machines require energy input → Perpetual motion machines are impossible in real life due to friction and air resistance.