1. Section 1 What is Energy? Chapter 19 Energy and Work: Working Together In science, energy is the ability to do work. Work is done when a force causes an object to move in the direction of the force. A tennis player does work on a racket by exerting a force on it. The racket does work on the ball.

2. Section 1 What is Energy? Chapter 19 Energy and Work: Working Together, continued When one object does work on another, energy is transferred from the first object to the second. This energy allows the second object to do work. So, work is a transfer of energy.

3. Section 1 What is Energy? Chapter 19 Energy and Work: Working Together, continued Like work, energy is expressed in units of joules (J). There are many different kinds of energy, and all can be measured and compared.

4. Section 1 What is Energy? Chapter 19 Kinetic Energy Kinetic energy is the energy of motion. All moving objects have kinetic energy. Kinetic energy can be used to do work. For example, kinetic energy allows a hammer to work on a nail.

5. Section 1 What is Energy? Chapter 19 Kinetic Energy, continued Mass and Speed The amount of kinetic energy that a moving object has depends on the object’s mass and speed. The faster an object is moving, the more kinetic energy it has. The more mass an object has, the greater its kinetic energy.

6. Section 1 What is Energy? Chapter 19 Kinetic Energy, continued Speed has a greater effect on kinetic energy than mass does. A truck with twice the mass of a car but moving at the same speed has twice the amount of kinetic energy. But if the speed of the car doubles, its kinetic energy increases by four times.

7. Section 1 What is Energy? Chapter 19 Potential Energy Potential energy is the energy an object has because of its position. For example, a stretched bow has potential energy because work has been done to change its shape. The energy of that work is turned into potential energy.

8. Section 1 What is Energy? Chapter 19 Potential Energy, continued Gravitational Potential Energy When you lift an object, you are doing work on it. You use a force that is working against the force of gravity. So, when you lift an object, you transfer energy to it and give it gravitational potential energy.

9. Potential Energy Section 1 What is Energy? Chapter 19

10. Section 1 What is Energy? Chapter 19 Potential Energy, continued Books on a shelf have gravitational potential energy. The amount of gravitational potential energy an object has depends on its weight and height. Higher and heavier objects have greater gravitational potential energy.

11. Section 3 Energy Conversion and Transfer Chapter 19 Kinetic and Potential Energy Conversions An energy conversion is a change from one form of energy to another. Any form of energy can change into any other form. Often, one form of energy changes into more than one other form.

12. Section 3 Energy Conversion and Transfer Chapter 19 Kinetic and Potential Energy Conversions, continued As a skateboarder moves at high speed up a half pipe, he has a lot of kinetic energy. At the top of the half pipe, he hangs in the air. All his kinetic energy has changed to potential energy. As he zips down the side, his potential energy has become kinetic again.

13. Section 3 Energy Conversion and Transfer Chapter 19

14. Section 3 Energy Conversion and Transfer Chapter 19 Energy Conversions and Friction Roller coasters have a mechanism that pulls the cars up to the top of the first hill. Then the cars are released. Potential energy is turned into kinetic energy, and back into potential energy as they go up and down hills. But the cars never return to their original height.

15. Section 3 Energy Conversion and Transfer Chapter 19 Energy Conversions and Friction, continued Energy is not lost—it is converted to other forms. Friction and Thermal Energy On a roller coaster, there is friction between the cars’ wheels and the track, and between the cars and the air around them. Friction is a force that opposes motion between two surfaces that are touching.

16. Section 3 Energy Conversion and Transfer Chapter 19 Energy Conversions and Friction, continued Because of friction, some energy is always converted to thermal energy. As a result, not all of the potential energy of the cars changes into kinetic energy as the cars go down the first hill. Likewise, not all of the kinetic energy of the cars changes back into potential energy.

17. Section 3 Energy Conversion and Transfer Chapter 19

18. Section 3 Energy Conversion and Transfer Chapter 19 Energy in a Closed System A closed system is a group of objects that transfer energy only to each other. The closed system of a roller coaster consists of the track, the cars, and the air around them. On a roller coaster, some mechanical energy (the sum of kinetic and potential energy) is always converted into thermal energy because of friction.

19. Section 3 Energy Conversion and Transfer Chapter 19 Energy in a Closed System, continued Sound energy also comes from the energy conversions of a roller coaster. If you combine the kinetic energy at the bottom of the first hill, thermal energy due to friction, and sound energy, the total amount is the same as the original amount of potential energy. So energy is conserved and not lost.

20. Section 3 Energy Conversion and Transfer Chapter 19 Energy in a Closed System, continued The law of conservation of energy states that energy cannot be created or destroyed. The total amount of energy in a closed system is always the same. Energy can change from one form to another, but all the different forms always equal the same total.

21. Section 3 Energy Conversion and Transfer Chapter 19