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

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