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CHAPTER 4: ENERGY

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SECTION 1: The Nature of Energy

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WARM-UP: What is energy? List some different types of energy.

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LEARNING GOALS Distinguish between kinetic and potential energy. Calculate kinetic and gravitational potential energy. Describe different forms of potential energy.

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WHAT IS ENERGY? Changes are constantly occurring in the world. Examples: light bulbs heating the air, sunlight on window, you breathing

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WHAT IS ENERGY? Every change requires energy. Energy: ability to cause change

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FORMS OF ENERGY Electromagnetic energy: energy that travels in waves Example: UV, Infrared, Radio Mechanical energy: energy in physical movement or position Example: moving car, flying baseball

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FORMS OF ENERGY Electrical energy: energy carried by moving electrons Example: any kind of electricity Thermal energy: energy related to temperature Example: cup of hot tea

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UNITS OF ENERGY Joule (J) – SI unit of energy Other units: calorie (cal) Calorie (Cal) Kilowatt Hour (kWh)

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KINETIC ENERGY Kinetic Energy: the energy that a moving object has because of its motion. Depends on the mass and speed of the object.

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KINETIC ENERGY KE = ½ mv 2 KE = kinetic energy in joules (J) m = mass in kg v = velocity in m/s

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KINETIC ENERGY KE = ½ mv 2 m = 2(KE/v 2 ) v = √2(KE/m)

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EXAMPLE A jogger with the mass of 60 kg is moving at a speed of 3 m/s. What is the jogger’s kinetic energy?

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EXAMPLE A sprinter has a mass of 80.0 kg and a kinetic energy of 4,000 J. What is the sprinter’s speed?

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EXAMPLE A baseball is moving at a speed of 40 m/s and has 120 J of kinetic energy. What is the mass of the baseball?

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POTENTIAL ENERGY Potential Energy: stored energy due to an object’s position. Three types: elastic chemical gravitational

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POTENTIAL ENERGY Elastic Potential Energy: energy stored by something that can stretch or compress Ex: rubber bands, springs

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POTENTIAL ENERGY Chemical Potential Energy: energy stored in chemical bonds Ex: gasoline, food, batteries

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POTENTIAL ENERGY Gravitational Potential Energy: energy stored in objects due to their position above the Earth’s surface Ex: a boulder on a cliff

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POTENTIAL ENERGY GPE = mgh GPE = gravitational potential energy (J) m = mass in kg g = 9.8 m/s 2 h = height in m

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POTENTIAL ENERGY GPE = mgh m = GPE/(gh) h = GPE/(mg) g = GPE/(mh)

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EXAMPLE What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

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EXAMPLE How high above the ground is a baseball with a mass of 0.15 kg and a GPE of 73.5 J?

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EXAMPLE A rock climber is 200 m above the ground and has a GPE of 117,600 J. What is the rock climber’s mass?

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CHANGING ENERGY If all of the object on the shelves have the same mass: Which has the most potential energy? Which will be moving faster if they were to all fall?

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CHANGING ENERGY As an object begins to fall, it has both GPE and KE. As the object gets closer to the ground, it has less GPE and more KE.

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EXAMPLE An 80 kg diver jumps from a 10 m platform. What is the GPE of the diver at the top of the platform? What is the GPE of the diver after falling 5 m?

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CHECK-IN: Explain how the kinetic energy of a truck could be increased without increasing the truck’s speed.

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