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Energy Objectives 1. Distinguish between kinetic and potential energy. 2. Calculate kinetic energy. 3. Describe different forms of potential energy. 4. Calculate gravitational potential energy. 2 5.Describe how energy can be transferred from one form to another. 6.Explain how the mechanical energy of a system is the sum of the kinetic and potential energy. 7. Discuss the law of conservation of energy.

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Nature of Energy Energy is the ability to cause change 1. kinetic energy (E k ) - energy in the form of motion -depends on an objects mass and speed ~increase mass, increase kinetic energy ~increase speed, increase kinetic energy 3

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Nature of Energy 1. Kinetic energy (E k ) -shown by the relationship kinetic energy = ½ mass x speed 2 E k = ½ mv 2 -SI unit is the Joule (J), or kgm 2 /s 2, or Nm -the faster you are moving, the more kinetic energy you have, helping you overcome gravity 4

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Which is more important to E k, speed or mass? -if you double the mass, you only double that component E k = ½ mv 2 E k = ½ mv 2 = ½ (2kg)(1m/s) 2 = ½ (4kg)(1m/s) 2 = (1kg)(1m 2 /s 2 ) = (2kg)(1m 2 /s 2 ) -if you double the velocity, you quadruple that component E k = ½ mv 2 E k = ½ mv 2 = ½ (1kg)(1m/s) 2 = ½ (1kg)(2m/s) 2 = (0.5kg)(1m 2 /s 2 ) = (0.5kg)(4m 2 /s 2 ) 5

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Kinetic Energy: Example 1 A jogger whose mass is 60 kg is moving at a speed of 3 m/s. What is the joggers E k ? Known variables: m = 60 kg v = 3 m/s Unknown variable:E k = ? Formula: E k = ½ mv 2 =1/2(60kg)(3 m/s) 2 =30 kg(3 2 m 2 /s 2 ) =30 kg(9 m 2 /s 2 ) =270 kg m 2 /s 2 6

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Kinetic Energy Practice Remember: J = kg·m 2 /s 2 1.What is the E K of a 1500 kg car moving at 18 m/s? 2.A 4 kg rock is rolling 10 m/s. Find the E K. 3.A rolling ball has 18 J of energy and is rolling 3 m/s. What is its mass? 4.A 4 kg bird has 8 J of kinetic energy. How fast is it flying? 7

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Energy Homework 1 1. Two trucks have the same velocity but different masses. The truck with the greater mass has ___________ E K than the other truck. 2. __________ is the ability to cause change. 3. A __________ is a unit of measure of all forms of energy. 4. Which has more kinetic energy: A 25 kg mass or a 30 kg mass? 5. Which has more kinetic energy: Two 10 kg masses, one going 75 m/s and one going 45 m/s? 6. Which has more kinetic energy: A car at rest or a car rolling down a hill? 8

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2. potential energy - energy in a motionless object, giving it the potential to cause change. a. elastic potential energy - energy stored by things that compress or stretch b. chemical potential energy - energy stored in chemical bonds between atoms 9

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2. potential energy (E P ) - energy in a motionless object, giving it the potential to cause change. 10 c. gravitational potential energy (E GP )- energy stored by things that are above Earth. -amount depends on its mass, the acceleration due to gravity and its height above ground - E GP = mass x 9.8 m/s 2 x height E GP = mgh

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Potential Energy: Example 1 Suppose a ceiling fan has a mass of 7 kg and is 4 m above the ground. What is its gravitational potential energy? Known variables:m = 7 kg g = 9.8 m/s 2 h = 4 m Unknown variable: E GP = ? J Formula:E GP = mgh =(7 kg)(9.8 m/s 2 )(4 m) =274 kg·m 2 /s 2 =274 J 11

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Potential Energy Practice Remember: J = kg·m 2 /s 2 1. Calculate the E GP of a 0.52 kg bird flying at an altitude of 550 m. 2. A diver has 3400 J of E GP after stepping up onto a platform that is 6.0 m above the water. What is the divers mass? 3. Before ascending the cliff, a 65 kg rock climber has a E GP of J. How high was he from the base of the cliff? 12

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Energy Homework 2 1. Two baseballs have the same mass. The ball closer to the ground has __________ GPE than the other ball. 2. The _________ energy of an object depends on its mass and height. 3. The energy of food and other fuels is __________ potential energy. 4. What is the difference between kinetic and potential energy? 5. Which has more potential energy: a 25 kg mass or a 30 kg mass at the top of a hill? 6. Which has more potential energy: a car at the top of the hill or the bottom of a hill? 13

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Kinetic and Potential Energy Practice Remember: J = kg·m 2 /s 2 1.Calculate the potential energy of a 5 kg object sitting on a 3 m ledge. 2.A 10 kg rock is at the top of a 20 m tall hill. How much potential energy does it have? 3.How high up is a 3 kg object that has 300 J of energy? 4.An object with 20 J of energy sits on a cliff that is 15 kg high. What is the mass of the object? 5.A skater travels 150 m with 38 J of energy. How fast is he going? 14

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Energy Quiz 1 1.Label each of the following as either kinetic or potential energy: a. Car traveling 45 mi/hr b. Rock on a 5m ledge 2.Which has more potential energy: plane on the ground or a plane in the air? 3.What are the units for energy? 4.What kind of energy is gained from a. A to C? b. D to F? 5. An 8 kg object has 64 J of energy. How fast is it going? 15

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Conservation of Energy 16 top = all potential (no kinetic) middle = ½ potential + ½ kinetic bottom = all kinetic (no potential) Law of Conservation of Energy: energy may change from one form to another, but the total amount of energy never changes

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Since energy can only be transformed, then for an object thrown into the air or falling, E K (at the bottom)= E GP (at the top) and Total Energy (E T ) = E K + E GP What happens to the kinetic energy as the potential energy increases? -as potential energy increases, kinetic energy decreases and vice versa 17

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Energy Transformations energy conversions-energy changing from one form to another 1. chemical energy (as seen earlier) -fuels store energy -chemical PE thermal energy and/or kinetic energy 2. electrical energy (aka electricity) -lightbulbs light + thermal energy -alarm clocks sound + light + thermal energy -video games sound + light + thermal energy 19

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Energy Transformations 3. thermal energy (heat energy) -heat burns -end product of many transformations 4. nuclear energy (from radiation, fission, fusion) -produces huge amounts of energy -fusion in the sun heat + light + radiation -fission in power plants heat (steam) electricity 20

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Energy Transformations 5. radiant energy (light energy) -aka electromagnetic radiation -sun chemical potential (photosynthesis) 6. mechanical energy (can become work) -energy of an objects motion (E K ) or position (E GP ) -swinging, falling, tossing a ball 21

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23 In energy conversions, friction converts some of the mechanical energy into thermal energy (heat) and sometimes sound.

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Energy Homework 3 Match the following: Energy increase or decrease? 7.A force pushes something into the air. 8.When your car brakes. What kind of energy transfer? 9.A nuclear power plant. 10.A battery powered flashlight. 24

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Conservation of energy in your body. 1. chemical potential energy from food stored in your body is used to fuel the processes that keep you alive; a few include -making your heart beat -digesting food -regulate body temperature -muscle movement -breathing 25

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2. The food Calorie is used to measure how much energy you get from various foods. -One Calorie is equivalent to about 4180 J of energy -every gram of fat consumed can supply 9 C of energy -carbs and proteins can supply 4 C of energy Discussion Is energy lost when a car burns gasoline? Explain. 26

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Energy Review 2 1. The type of energy that is greatest at the bottom of a swings path is ____________. 2. The type of energy due to the position and motion of an object is _____________. 3. What is the unit for energy? 4. Describe the law of conservation of energy. 5. Identify the energy transformations for the following: a. A jet plane rapidly accelerates on the runway. b. A swallow sitting on the fence sings a song. c. An electric blanket warms a bed on a cold night. 6. A hammer falls off the roof and strikes the ground. If it fell from twice the height, how would the kinetic energy compare? 27

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More Practice Problems – GPE and KE Remember: J = kg·m 2 /s 2 1. Calculate the gravitational potential energy of a 93.0 kg sky diver who is 550 m above the ground. 2. What is the kinetic energy of a 0.02 kg bullet traveling 300 m/s? 3. If a 2.0 kg rock has a gravitational potential energy of 850 J, how high above the ground is it sitting? 4. If the 2.0 kg rock in problem #3 above falls off the cliff it is sitting on has a kinetic energy of 850 J, what is the speed it is traveling before it hits the ground? 5. What is the speed of a 0.15 kg snowball moving through the air when it has 49 J of energy? 28

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Energy Homework 4 Write the Q and A 1.Can an object have potential energy and kinetic energy at the same time? Why or why not? 3. How can you increase the kinetic energy of an object without increasing its speed? 4. How do mass and height affect gravitational potential energy? 5. What are the energy transformations that take place in: a. photosynthesisb. computerc. walking 6. True/False: Any two objects on the same shelf of a cupboard have the same potential energy ? 29

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