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Ch. 13 Work and Energy Work, Power and Machines • Work - Amount of energy transferred when a force causes an object to move in the direction of the applied force. – Work = Force x distance –W= Fxd -work is measured in Joules (J) * 1J = 1N x 1m or 1J = 1 Nm • Ex. A student lifts a 12N book 1.5m. How much work did he do on the book? • Ex. A student pushes against a desk with 50N of force. The desk doesn’t move. How much work did he do on the desk? • Ex. A mechanic uses a lift to raise a 1,200 kg car .50 m off the ground. How much work does the lift do on car? • Power - The rate at which work is done – Power = Work / time – P= W / t Power is measured in watts (W) Kilowatts(kW)=1000W (what electric companies use) Can also measure in horsepower * 1hp = 750W Power Example • 1. While rowing across the lake during a race, John does 3,960 J of work on the oars in 60.0 s. What is his power output in watts? P = W/t P = 3,960J/ 60.0s P = 66.0 W Machines • Machine - Any device that helps make work easier -Makes work easier by changing the size or direction of the applied force. • 2 forces involved in using a machine 1. effort force (Fe) ( also called input force) - the force applied to a machine (your force) 2. resistance force (Fr) (also called output force) - the force applied by the machine -usually equal to the weight of the object being moved We use machines to overcome gravity and friction Mechanical Advantage • Mechanical Advantage (MA) The number of times a machine multiplies the effort force. -Input force (effort force) -Force you put in -Output force (resistance force) -Force machine does -Input distance -Distance you do -Output distance -Distance machine does MA equation • Ratio that measures how much a machine multiplies force or distance. MA = output force or input distance input force output distance MA that is greater than 1 means the machine multiplies the input force MA that is less than 1 means the machine increases the distance or speed • Ex. A student lifts a 12 N book 1.5 m in 1.5 s and carries the book 5 m across the room in 7 s. • How much work does the student do on the book? • What is the power output of the student? Simple Machines • 6 simple machines 2 Families of simple machines Lever family and Inclined plane family Lever Family 1. Lever Made up of a rigid beam (arm) and a fulcrum(fixed point). 3 classes: st 1 class lever Fulcrum is in the middle. Input on one end, and the Output on the other end. Ex. 1st class lever 2nd class lever Output is in the middle. Input on one end, and the Fulcrum on the other end. Ex. 2nd class lever 3rd class lever Input is in the middle. Output on one end, and the Fulcrum on the other end. Ex. 3rd class lever 2. Pulley - Changes the direction of the force. The middle is like the fulcrum The wheel is like the short end of the lever arm Pulley cont. Single pulley Doesn’t magnify force, it just changes direction. MA = 1 Moving pulley Magnify force, but doesn’t change direction. -can increase MA 3. Wheel and Axel -a lever/pulley (wheel) connected to a rigid arm (axel) -Changes a small input force into a large output force. -Ex. Wheel and axel examples Inclined Plane Family 4. Inclined plane -A slanted board used to lift an object -It increases the distance but decreases the input force. -MA is equal to the length of the plane divided by the height -Ex. Inclined plane 5. Wedge- modified inclined plane -Two inclined planes, back to back -changes a downward force into two forces directed out to the sides Ex. Ex. wedge 6. Screw - Inclined plane wrapped around a cylinder -increases distance so you have a small input force The closeness of the ridges determine The MA - far apart = small force, large distance - close together = large force, short distance Ex. Examples of screws Compound Machine • Combination of 2 or more simple machines Energy Energy- the ability to do work. Measured in Joules (J) Potential Energy • Potential Energy (PE) Stored energy Potential Energy • 2 types of Potential Energy (PE) 1. Elastic PE - energy stored in compressed or stressed material 2. Gravitational PE - energy stored because of the location of the object - amount of gravitational PE (GPE) depends on mass and distance of the objects - Gravitational PE = mass x gravity x height Kinetic Energy • Kinetic Energy (KE) - Energy of motion - Kinetic Energy = 1/2 mv2 Potential and Kinetic Energy Mechanical Energy • Mechanical Energy -when you can see the change from PE into KE. - large scale Non-mechanical Energy • Non-mechanical Energy -when you can not see the change from PE into KE. - small scale (dealing with atoms) Conservation of Energy • Conservation of Energy Energy of transformation - PE KE PE KE -When energy is transformed some of the energy is changed into other forms of energy. Conservation of Energy Law of Conservation of Energy - Energy can not be created or destroyed it just changes form Forms of Energy 1. Chemical Energy - Depends on the position of atoms in the substance -Non mechanical energy - PE when it is the substance - KE when bonds break Ex. 2. Electric Energy - Flow of electron particles -Non mechanical energy -KE because the electrons are moving Ex. 3. Nuclear Energy -Energy stored by forces holding nucleus together - fusion- combining two nuclei - fission - splitting a nucleus - PE 4. Solar Energy -Energy that plants use to create food (chemical energy) -Non mechanical energy -PE 5. Light Energy -Electromagnetic waves that travel over great distances - mechanical energy -KE