1. Section 2: Describing Energy
Energy is the ability to cause change. K
What I Know W
What I Want to Find Out L
What I Learned

2. ENERGY UNIT
5(A) Recognize and demonstrate that objects and substances in motion have kinetic energy such as vibration of atoms, water flowing down a stream moving pebbles, and bowling balls knocking down pins.
4(A) Describe and calculate an object’s motion in terms of position, displacement, speed, and acceleration.
5(B) Demonstrate common forms of potential energy, including gravitational, elastic, and chemical, such as a ball on an inclined plane, springs, and batteries.
2(D) Organize, analyze, evaluate, make inferences, and predict trends from data.
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Describing Energy

3. Essential Questions How can you calculate kinetic energy?
What are some different forms of potential energy?
How can you calculate gravitational potential energy?
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Describing Energy

4. VOCABULARY New Energy work system kinetic energy potential energy
elastic potential energy
chemical potential energy
gravitational potential energy
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Describing Energy

5. WHAT IS WORK? Work is force applied through a distance.
Work requires both force and motion.
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Work and Machines

6. Doing Work Conditions that must be satisfied for work to be done:
applied force must make the object move.
movement must be in the same direction as the applied force.
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Work and Machines

7. Force and Direction of Motion
When you lift a stack of books, your arms apply a force upward and the books move upward.
Because the force and distance are in the same direction, your arms have done work on the books.
When you carry books while walking, you might think that your arms are doing work. However, in this case, the force exerted by your arms does no work on the books.
The force exerted by your arms on the books is upward, but the books are moving horizontally.
The force you exert is at right angles to the direction the books are moving.
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Work and Machines

8. CALCULATING WORK amount of force exerted
Depends on
amount of force exerted
distance over which the force is applied.
One joule is about the amount of work required to lift a baseball a vertical distance of 0.7 m.
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Work and Machines

9. CHANGE REQUIRES ENERGY
Energy is the ability to cause change.
Anything that causes change must have energy.
Energy has several different forms.
1. Electrical,
2. Chemical,
3. Radiant,
4. Thermal
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Describing Energy

10. WORK TRANSFERS ENERGY
Energy can also be described as the ability to do work.
Therefore, energy can be measured with the same units as work.
Energy, like work, can be measured in joules.
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Describing Energy

11. SYSTEMS A system is anything that you can imagine a boundary around.
It is useful to think of systems when describing energy.
A system can be a single object, such as a baseball, or a group of objects, such as the solar system.
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Describing Energy

12. KINETIC ENERGY the energy a moving object has because of its motion.
depends on the object’s mass and its speed
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Describing Energy

13. SOLVE FOR KINETIC ENERGY
Use with Example Problem 4.
Problem
A jogger with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy from this forward motion?
SOLVE FOR THE UNKNOWN
Set Up the Problem
KE = mv2
Solve the Problem
KE = (60.0 kg)(3.0 m/s)2
KE = (60.0 kg)(9.0 m2/s2)
KE = 270 J
Response
ANALYZE THE PROBLEM
KNOWN
mass: m = 60.0 kg
speed: v = 3.0 m/s
UNKNOWN
kinetic energy: KE
EVALUATE THE ANSWER
Check the last step by estimating. Round 9.0 m2/s2 upward to 10 m2/s2. Then, 1 2 (60.0 kg)(10 m2/s2) = 300 J. This is close to 270 J, so the final calculation was reasonable.
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Describing Energy

14. POTENTIAL ENERGY Stored energy due to the interactions between objects
A hanging apple in a tree has stored energy.
If the apple stays in the tree, the energy will remain stored.
If the apple falls, that stored energy is converted to kinetic energy.
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Describing Energy

15. ELASTIC POTENTIAL ENERGY
energy stored by something that can stretch or compress, such as a rubber band or spring.
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?
The stretched rubber band had energy stored as elastic potential energy.
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Describing Energy

16. CHEMICAL POTENTIAL ENERGY
Energy stored due to chemical bonds
Energy is stored when the bonds that hold the atoms together are formed.
When methane burns, it combines with oxygen to form carbon dioxide and water. In this chemical reaction, chemical potential energy is converted to other forms of energy.
Describing Energy
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17. GRAVITATIONAL POTENTIAL ENERGY
Energy due to gravitational forces between objects.
Together, an object near Earth and Earth itself have gravitational potential energy.
Gravitational potential energy can be calculated from the following equation:
Near Earth’s surface, gravity is 9.8 N/kg.
Like all forms of energy, gravitational potential energy can be measured in joules.
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Describing Energy

18. CHANGING GPE
According to the equation for gravitational potential energy, the GPE of an Earth system can be increased by increasing the object’s height.
Gravitational potential energy also increases if the mass of the object increases.
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Describing Energy

19. SOLVE FOR GRAVITATIONAL POTENTIAL ENERGY
Use with Example Problem 5.
Problem
A 4.0-kg ceiling fan is placed 2.5 m above the floor. What is the gravitational potential energy of the Earth-ceiling fan system relative to the floor?
SOLVE FOR THE UNKNOWN
Set Up the Problem
GPE = mgh
Solve the Problem
GPE = (4.0 kg)(9.8 N/kg)(2.5 m) =
98 J • m = 98 J
Response
ANALYZE THE PROBLEM
EVALUATE THE ANSWER
Round 9.8 N/kg to 10 N/kg. Then, GPE = (4.0 kg)(10 N/kg)(2.5 m) = 100 J. This is very close to the answer above. Therefore, that answer is reasonable.
KNOWN
mass: m = 4.0 kg
gravity: g = 9.8 N/kg
height: h = 2.5 m
UNKNOWN
gravitational potential energy: GPE
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Describing Energy

20. Review Essential Questions Vocabulary
How can you calculate kinetic energy?
What are some different forms of potential energy?
How can you calculate gravitational potential energy?
Vocabulary
energy
system
kinetic energy
potential energy
elastic potential energy
chemical potential energy
gravitational potential energy
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Describing Energy