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Work & Power Notes Ch 8 Work • Work is done when a force is applied over a distance. • W = F•d in N (Newtons) in m (meters) Work is measured in Joules (J) 1 J = 1N•1m Discuss with your table: • Is holding a heavy book considered work? • Not in physics! But why? Lifting a book is work: • Work is done on the book when: 1. A force is applied 2. The book moves due to the force • If you just hold the book, you are NOT doing work on it, even though you may become tired. • The work that is done is on your muscles as they flex or stretch. Think about it! Work is done by lifting a book… Q: How much more work is done when lifting two books the same distance? A: Twice as much: 2F•d Q: How much more work is done when lifting two books twice as far? A: 4 times as much as the original book: 2F•2d Discuss with your table group… • When carrying a box up stairs, you do the same work (W = F d ) whether you run or walk. – Force (weight of the box) is the same. – Distance up the stairs is the same. • Why are you more tired after running up the stairs than when you walk? • Hint: What’s different between the two situations? Power! • Power is the rate at which work is done. • P=W t • Power is measured in Watts (W) • 1 watt = 1 joule 1 second Types of Energy Notes Chapter 8 When work is done to lift a hammer, the hammer acquires the ability to do work on the object it hits when it falls. When an archer pulls back on a bowstring, the bent bow acquires the ability to do work on the arrow. • In both cases, “something” has been acquired that enables the object to do work. – Compression of atoms – Physical separation of attracting bodies – Rearrangement of electrical charges Mechanical Energy (ME) • Energy that enables an object to do work • ME is most often – Potential Energy (PE) -- The energy due to an object’s position – Kinetic Energy (KE) -- The energy due to an object’s motion • Energy is measured in Joules (J) Potential Energy (PE) • Gravitational Potential Energy (GPE) • Is the energy of position above the earth’s surface • GPE = mg • h • where: m = mass, g = acceleration due to gravity, and h = height above the Earth’s surface Potential Energy • Potential energy is energy that has the potential to do work. • The energy is considered in a “stored” state. • Chemical energy in fuels, electric batteries, and the food we eat is potential energy due to the positions of the electrical charges at a submicroscopic level! Potential Energy (PE) • PE is stored energy • To elevate an object, work must be done to lift the object against the pull of gravity • How much work must be done to elevate the object is also the amount of PE it has! • Remember that weight = mass•gravity, or • w = mg … and this is the force (F) of an object due to its mass! Gravitational Potential Energy (GPE) • Is the energy of position above the earth’s surface • GPE = Work • GPE = F • d • GPE = mg • h • where: m = mass, g = acceleration due to gravity, and h = height above the Earth’s surface Kinetic Energy (KE) • If an object is moving, it can do work! • Kinetic energy is the energy of motion, and depends on the mass of an object and its speed. • KE = ½ mv2 • KE = Work required to start or stop an object from moving • KE = W • ½ mv2 = F•d Conservation of Energy • Whenever work is done, energy changes. • Law of Conservation of Energy: Energy is not created or destroyed; energy is only transformed. • ME = KE + PE • The total ME is conserved, even if PE and KE change. ME is conserved • Consider a cat on a ledge. • The cat has PE due to its position • If the cat jumps off the PE changes and is transferred to KE as the cat gains speed. • Just before the cat hits the ground its energy is all in the form of KE