1. Work and Energy
Chapter 4, Sect 2 - 3

2. 1
When something is able to change it surroundings or itself, it has energy. Energy is the ability to cause change. Without energy, nothing would ever change.

3. 2
Because energy can be described as the ability to do work, energy can be measured with the same units as work. Energy, like work, can be measured in joules (J).

4. 3
The moving tennis racket has energy. That racket causes change when it deforms the tennis ball and changes the tennis ball’s motion. The racket also does work on the tennis ball, applying force to that ball through a distance. When this happens, the racket transfers energy to the ball. The tennis racket and tennis ball are systems. A system is anything around which you can imagine a boundary.

5. 4
Turn on an electric light, and a dark room becomes bright. Turn on a portable music player, and sound comes through your headphones. In both situations, a change occurs. Energy has many different forms. Automobiles make use of the chemical energy of gasoline. Many household appliances require electrical energy to function. Radiant energy from the Sun warms Earth. A hammer used to drive a nail is mechanical energy. In short, energy plays a role in every activity you do.

6. kinetic energy (joules) = ½ mass (kg) X [speed (m/s)]25
When you think of energy, you might think of objects in motion. Objects in motion can collide with other objects and cause change. Therefore, objects in motion have energy. Kinetic energy is energy due to motion. A car moving along a highway and a ballet dancer leaping through the air have kinetic energy. The kinetic energy from an object’s motion depends on that object’s mass and speed.
kinetic energy (joules) = ½ mass (kg) X [speed (m/s)]2
KE = ½ mv2

7. 6
Energy does not always involve motion. Even motionless objects can have energy. Potential energy is energy that is stored due to the interactions between objects. One example is the energy stored between an apple hanging on a tree and Earth. Energy is stored between the apple and Earth because of the gravitational force between the apple and Earth. Another example is the energy stored between objects that are connected by a compressed spring or a stretched rubber band.

8. 7
If you stretch a rubber band and let it go, it sails across the room. As it flies through the air, it has kinetic energy due to its motion. Where did this kinetic energy come from? Just as there is potential energy due to gravitational forces, there is also potential energy due to the elastic forces between the particles that make up a stretched rubber band. The energy of a stretched rubber band or a compressed spring is called elastic potential energy. Elastic potential energy is energy that is stored by compressing or stretching an object

9. 8
The food that you eat and the gasoline in cars also have stored energy. This stored energy is due to the chemical bonds between atoms. Chemical potential energy is energy due to chemical bonds. You might notice chemical potential energy when you burn a substance. When an object is burned, chemical potential energy becomes thermal energy and radiant energy.

10. 9
Gravitational potential energy is energy due to the gravitational forces between objects. Gravitational potential energy is often shorted to GPE

11. 10
Consider the blue vase above. Together, the blue vase and Earth have potential energy. Any system that has objects that are attracted to each other through gravity has gravitational potential energy. An apple and Earth have gravitational potential energy. The solar system also has gravitational potential energy.
GPE = mgh
GPE = gravitational potential energy
m = mass
g = gravity (9.8 N/kg)
h = height

12. 11
You might have heard about energy conservation or have been asked to conserve energy. These ideas are related to using energy sources, such as coal and oil, wisely. The law of conservation of energy, on the other hand, is a universal principle that states that total energy remains constant.

13. 12
The law of conservation of energy states that energy cannot be created or destroyed. Energy can only be converted from one form to another or transferred from one place to another.

14. 13
Suppose you are riding on a roller coaster. As your height above the ground changes, gravitational potential energy changes. As your speed changes, kinetic energy changes. Think about the motion of the roller-coaster cars. When the cars are high above the ground, GPE is large and kinetic energy is small. When the cars are low, GPE is small and kinetic energy is large. Energy is changing back and forth between GPE and kinetic energy.

15. 14
Look at the apple tree. An apple-Earth system has gravitational potential energy. The apple-Earth system does not have kinetic energy while the apple is hanging from the tree because the apple is not moving. However, when the apple falls, it gets closer to Earth, so the GPE of the apple-Earth system decreases. This potential energy is transformed into kinetic energy as the apple’s speed increases. If potential energy is being converted into kinetic energy, then the mechanical energy of the apple- Earth system doesn’t change as the apple falls. The potential energy that the apple-Earth system loses is gained back as kinetic energy. The form of mechanical energy changes, but the total amount of mechanical energy remains the same.

16. 15
Energy transformations also occur during projectile motion when an object moves in a curved path. Consider the ball-Earth system. When the ball leaves the bat, the ball is moving fast, so the system’s kinetic energy is relatively large. The ball’s speed decreases as it rises, so the system’s kinetic energy decreases. However, the system’s gravitational potential energy increases as the ball goes higher. At the top of the ball’s path, the system’s GPE is larger and kinetic energy is smaller. Then, as the baseball falls, the system’s GPE decreases as its kinetic energy increases. However, the mechanical energy of the ball-earth system remains constant as the ball rises and falls.

17. 16
The mechanical energy transformations for a swing are similar to the mechanical energy transformations for a roller coaster. The ride starts with a push, which transfers kinetic energy to the rider. As the swing rises, the rider loses speed but gains height. In energy terms, kinetic energy changes to GPE. At the top of the rider’s path, GPE is at its greatest. Then, as the swing moves back downward, gravitational potential energy changes back to kinetic energy. At the bottom of each swing, the kinetic energy is at its maximum and the GPE is at its minimum. As the rider swings back and forth, energy is continually transformed between kinetic energy and GPE

18. 17
Energy transformations can also involve electrical energy. Think about all the electrical devices that you use every day. Electric stoves and toasters transform electrical energy into thermal energy. Televisions transform electrical energy into radiant energy and sound energy. The electric motor in a washing machine transforms electrical energy into mechanical energy. Lightbulbs transform electrical energy into radiant energy and thermal energy.

19. 18
Fuel stores energy in the form of chemical potential energy. For example, most cars run on gasoline, which has chemical potential energy. A car engine transforms this chemical potential energy into thermal energy and then into mechanical energy for the car’s motion. A car engine also gets very hot when it is used. This is evidence that much of the thermal energy is never converted to mechanical energy.

20. power (watts) = energy (joules)19
Think about the energy that your body extracts from food every day. You probably get enough energy from food in one day to jump nearly 10 km into the air! If this is true, then why can’t you do this? You might have enough energy, but you don’t have enough power. Power is the rate at which energy is converted. Power can be found using the following equation:
power (watts) = energy (joules)
time (seconds)
P = E t

21. 20
You transfer energy from your surroundings to your body when you eat. The chemical potential energy of food supplies the cells in your body with energy that they need to function. Energy from food is often measured in Calories (C).

22. 21
You have probably seen descriptions of Calories per serving on food packages, such as cereal boxes or milk cartons. One Calorie is equal to 4,000 joules

23. 22
Every gram of fat in a food supplies a person with about 10 C (40,000 J) of energy. Carbohydrates and proteins each supply about 5 C (20,000 J) of energy per gram. Everything your body does requires energy. The number of Calories that you need for different activities depends on your weight, your body type, and your degree of physical activity.

24. 23 Calories Used in 1 Hour Activity Small framed body Medium Large
Sleeping 48 56 64
Sitting 72 84 96
Eating 98
112
Standing
123
Walking
180
210
240
Playing tennis
380
420
460
Bicycling
500
600
700
Running
850
1,000

25. Why can’t you ever trust an atom?
____ ____ ____ ____ ____ ____ ____ ____ ____ ____
____ ____ ____ ____ ____ ____ ____ ____ ____ ____

26. My physics teacher said I have potential…
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___
___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___ ___