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Chapter 4 ENERGY

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What is energy? Energy is the ability to do work (cause change). When a force causes an object to be displaced, work was done upon the object. Examples: brushing your hair, walking down the hall, eating your lunch, yawning.

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What are different forms of energy? http://w ww.eia.go v/kids/en ergy.cfm? page=abo ut_forms _of_ener gy-basics

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Kinetic Energy Energy in the form of motion. Depends on mass and velocity (similar to momentum) Examples: spinning bicycle wheel, a car driving

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Calculating Kinetic Energy Kinetic energy = ½ mass x velocity 2 KE = ½ m x v 2 ↑ mass = ↑ kinetic energy ↑ velocity = ↑ kinetic energy If you double the velocity, the energy quadruples. Joule Mass = kg; velocity = m/s

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Let’s Practice! A 15-kg bicycle carrying a 50-kg boy is traveling at a speed of 5 m/s. What is the kinetic energy of the bicycle (including the boy)? 812.5 Joules (J) The kinetic energy of a boat is calculated at 52,000 J. If the boat has a mass of 39,000 kg, with what velocity is it moving? 1.63 m/s

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Potential Energy Potential energy = stored energy due to position Examples: an apple hanging in a tree; a book on a shelf

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Elastic Potential Energy Energy stored by something that can stretch or compress Examples: a rubberband or a spring

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Chemical Potential Energy Energy stored in chemical bonds between atoms Examples: Food, gasoline

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Gravitational Potential Energy Energy stored in objects above Earth’s surface. GPE = m x 9.8 m/s 2 x h Mass – kg, height - meters ( ↑ h = ↑ GPE) measured in joules

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More Practice! A 0.06-kg tennis ball starts to fall from a height of 2.9 m. How much gravitational potential energy does the ball have at that height? 1.7 J An object of mass 10 kg is raised through a certain height. Its potential energy is increased by 1960 Joules. Find the height of the object initially. 20 m

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Let’s Review KE and PE How are kinetic energy and potential energy different? What are the three types of potential energy? How are elastic potential energy and chemical potential energy different?

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Conservation of Energy Energy is not lost or gained; it changes form. Transforming Energy Often electrical energy is converted to light and thermal energy Examples: light bulb, alarm clock, curling iron, computers Chemical energy is usually converted to kinetic energy or thermal energy. Examples: Your body, food, vehicles

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Conversions PotentialKineticPotential Mechanical energy (ME) = potential energy (PE) + kinetic energy (KE)

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Falling Objects 100% PE 0% KE Falling = change from PE to KE Ground: 0% PE 100% KE

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Swinging Energy

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Conservation of Energy Energy cannot be created or destroyed, ONLY transferred from one to another…..

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Finding Energy Transformations Friction: seems like energy is disappearing, but it is actually converted into different energies, such as thermal energy. Mass into Energy: Nuclear fusion is an example. Remember e=mc 2. A little mass = a lot of energy Mass into Energy Two hydrogen nuclei come together and combine to form one helium Nuclear Fission: Kinetic Energy to thermal energy Nuclear Fission Mass into energy again! Nuclei broken apart… ENORMOUS energy

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Energy Conversions in Your Body Chemical energy to kinetic and thermal energy. Stores energy as fat, example of potential energy Also converts to heat and use this energy to move Maintaining Healthy Weight Must have proper balance between energy contained in food eaten and energy body uses

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