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Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 What is Energy? Objectives Explain the relationship between energy and work. Define potential energy and kinetic energy. Calculate kinetic energy and gravitational potential energy. Distinguish between mechanical and nonmechan- ical energy. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer In the chapter on matter, you learned that energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration below, and identify the types of energy involved. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer, continued 1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?) 2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why. 3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?) Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Energy and Work Energy is the ability to do work. When you do work on an object, you transfer energy to that object. Whenever work is done, energy is transformed or transferred to another system. Energy is measured in joules. Because energy is a measure of the ability to do work, energy and work are expressed in the same units. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Potential Energy The energy that an object has because of the position, shape, or condition of the object is called potential energy. Potential energy is stored energy. Elastic potential energy is the energy stored in any type of stretched or compressed elastic material, such as a spring or a rubber band. Gravitational potential energy is the energy stored in the gravitational field which exists between any two or more objects. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Potential Energy, continued Gravitational potential energy depends on both mass and height. Gravitational Potential Energy Equation grav. PE = mass  free-fall acceleration  height PE = mgh The height can be relative. The height used in the above equation is usually measured from the ground. However, it can be a relative height between two points, such as between two branches in a tree. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Math Skills Gravitational Potential Energy A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? 1. List the given and unknown values. Given:mass, m = 65 kg height, h = 35 m free-fall acceleration, g = 9.8 m/s 2 Unknown:gravitational potential energy, PE = ? J Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Math Skills, continued 2. Write the equation for gravitational potential energy. Section 3 What is Energy? Chapter 12 PE = mgh 3. Insert the known values into the equation, and solve. PE = (65 kg)(9.8 m/s 2 )(35 m) PE = 2.2  10 4 kgm 2 /s 2 PE = 2.2  10 4 J

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Kinetic Energy The energy of a moving object due to the object’s motion is called kinetic energy. Kinetic energy depends on mass and speed. Kinetic Energy Equation Kinetic energy depends on speed more than mass. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Math Skills Kinetic Energy What is the kinetic energy of a 44 kg cheetah running at 31 m/s? 1. List the given and unknown values. Given:mass, m = 44 kg speed, v = 31 m/s Unknown:kinetic energy, KE = ? J Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Math Skills, continued 2. Write the equation for kinetic energy. 3. Insert the known values into the equation, and solve. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Other Forms of Energy The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy. Mechanical energy is the sum of the potential energy and the kinetic energy in a system. In addition to mechanical energy, most systems contain nonmechanical energy. Nonmechanical energy does not usually affect systems on a large scale. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Other Forms of Energy, continued Atoms and molecules have kinetic energy. The kinetic energy of particles is related to heat and temperature. Chemical reactions involve potential energy. The amount of chemical energy associated with a substance depends in part on the relative positions of the atoms it contains. Living things get energy from the sun. Plants use photosynthesis to turn the energy in sunlight into chemical energy. Section 3 What is Energy? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Other Forms of Energy, continued The sun gets energy from nuclear reactions. The sun is fueled by nuclear fusion reactions in its core. Electricity is a form of energy. Electrical energy is derived from the flow of charged particles, as in a bolt of lightning or in a wire. Light can carry energy across empty space. Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves. Section 3 What is Energy? Chapter 12