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Chapter 11Energy and Its Conservation
Forms of Energy
How can I model the relationship between work and energy?
How can I calculate kinetic energy?
Energy Various uses of the word
energy…
Work Energy Theorem
Work causes a change in the energy of a system
So work transfers energy between a system and the external world
Energy
Different forms of energy
Can be changed from one form to another
When work is done on a system, the energy of that system increases (pushing something)
When the system does the work, the energy decreases (friction)
Model of Work-Energy Theorem If you have a job
Each time you get paid, your amount of money increases
Positive cash flow
Model of Work Energy Theorem What if you spend
your money?
Work Energy Theorem
Energy is the same as money
Energy can be saved or stored
Energy can be spent
When energy is spent, the motion of the system is changed
Kinetic Energy
Since the equation contains velocity, KE is the energy of motion
Is something is at rest, KE = 0
Kinetic Energy
Work Energy Theorem
When a ball is thrown horizontally and caught, what can you say about initial and final KE?
Homework
Worksheet
Potential Energy
How do I determine the gravitational potential energy of a system?
How do I identify and calculate elastic potential energy?
Stored Energy
Boulder on the top of a cliff
How did it get there?
What if it fell?
Stored Energy
An object can have stored energy due to its height
Does the boy or dog have more stored energy?
Gravitational Potential Energy
PE = mgy
m = mass in Kg
g = 9.8 m/s2
y = height (m)
You must start each problem by setting a reference height (where does y = 0)?
PE=?
Example
You lift a 7.30 kg bowling ball from the storage rack and hold it up to your shoulder. The storage rack is 0.610 m above the floor and your shoulder is 1.12 m above the floor.a. When the bowling ball
is at your shoulder, what is the bowling balls gravitational potential energy relative to the floor?
Example
b. When the bowling ball is at your shoulder, what is the bowling balls gravitational potential energy relative to the storage rack?
c. How much work was done by gravity as you lifted the ball from the rack to shoulder level?
Stored Energy
Gas is stored chemical energy in a tank
Compressing a spring, pulling back the string on a bow, stretched rubber bands,…
Elastic Potential Energy
PE for a Spring
When we compress a spring we create potential energy
An object hanging on a spring can have a combination of elastic and gravitational potential energy
Homework
Page 291, # 4 – 8
Page 292, # 9, 10, 13