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Energy

- Chapter 8 Physics Matters
- (By James Trefil and Robert Hazen)

Some initial questions

What does it mean to do work?

Can you do a work out without ever actually doing

any work?

Why is it dangerous to speed/tailgate when

driving a car?

Other questions we will eventually be able to

answer!

Why does the roller coaster work without a motor

on the car?

How come a ball never bounces as high as it was

dropped?

Work

Work is done whenever a force is exerted on an

object over a distance

- It is the transfer of energy that results in

moving an object over a distance.

Work Force x Distance

N

m

Newton-meters are known as Joules (J)

Work

How much work do you to to lift a 5 kg book from

the floor onto a table if the table is 3m high?

How far will you lift a 20N bag if you do 100 J

of work on it?

Work and Energy

- When you do work you transfer energy or use

energy. - Energy is a systems ability to do work
- Since work is measured in Joules and work is a

transfer of energy, energy must also be measured

in Joules.

There are many different forms of energy, what

are they?

Is this Work?

Power

Power is the rate at which energy is used, or the

rate at which work is done.

- The faster energy is used or transferred, or

the faster work is done on an object, the more

power that is expended by the machine doing the

work.

Power

For what everyday things do we tend to pay

attention to power?

One major type of thing are the bulbs that light

our rooms. How do we differentiate between

different types of bulbs?

Watts! Wattage is a unit of measurement that

tells us how much energy the bulb will use in a

certain amount of time.

Power is measured in Watts (joules per

second). - So, a more powerful bulb will use

stored energy faster then a weaker bulb with

the same amount of stored energy.

Horsepower

The English system form of power was based on how

much a single horse can pull.

This was known as horsepower!

1 horsepower ? 746 Watts

A space shuttle has over 37 million horsepower!

Power

How powerful is an elevator that can lift a mass

of 400kg up a building of height 100 m in 20

seconds?

Question

Lindsey wants to hike up this 400 m mountain and

she has a mass of 48 kg. If she takes the path

that you see and hikes a distance of 620 m, how

much work does she do against gravity to get up

the mountain?

Does Andi, who has the same mass as Lindsey, do

more or less work if instead of hiking, she rock

climbs up a sheer cliff strait up to the

pinnacle?

Question Continued

Who is more powerful if Lindsey takes 3 hours to

hike the path, but Andi, being a professional

rock climber (maybe I should have said Maggie),

takes only an hour to scale the cliff?

Hmmmmmm

When both Lindsey and Andi each reach the top of

the mountain, do they have any energy left?

Yes! But what type of energy to do they have?

Potential Energy

- Basically it is energy stored in a system

- Both girls have what is called Gravitational

Potential Energy. - Since gravity could exert a force on either of

them to make them fall to the ground from the top

of the mountain, there is potential for work to

be done on them, and hence, potential energy.

Gravitational Potential Energy

Which is mass x acceleration due to gravity x

height

Potential Energy

How much potential energy does each girl have?

Gravitational Potential Energy

- Independent of Path to get there

Kinetic Energy

If Philip is waiting at the top of the mountain

to show Lindsey the awesome new set of wings he

built, and he has a mass of 55 kg, how much

potential energy does he have?

Can he convert his potential energy into anything?

Yes! If he jumps off the cliff, gravity will do

work to pull him toward the ground, converting

his potential energy into kinetic energy.

Kinetic Energy

Kinetic Energy is the energy an object has

because it is moving.

It is similar to momentum as they both result

from the movement of masses, but they differ in

the fact that KE is a scalar quantity. Also,

as velocity increase, momentum also increases

linearly, but KE increases as a squared function

(if v goes up by 2, KE goes up by 4)

Kinetic Energy

How much Kinetic Energy will Philip have as he

hits the ground, if he jumps of the mountain at

the sheer cliff face, and his new wings fail

completely?

Notice any Coincidences?

Conservation of Energy

Just like with momentum, the total energy of a

system is never lost It is conserved!

This means, that in the case of Philip jumping

off a mountain, he converted his Potential Energy

at the top of the mountain completely into

Kinetic Energy by the time he reached the ground.

If energy is always conserved, why wont our

experiments always work?

Conservation of Energy

If this cliff is 150 m high, and the person in

the picture has a mass of 65 kg, how much PE does

he have at the top of the cliff? How much KE

does he have at the top of the cliff?

How much KE does he have as he hits the water?

How much PE does he have?

How much PE and KE does he have half way down the

cliff?

Conservation of Energy

What is his velocity as he hits the water?

Conservation Equation

The total energy in a system is equal to the sum

of the potential energy and the kinetic energy at

any given time.

Conservation of Energy

This means that when something falls from a

height, its initial potential energy is equal to

its final kinetic energy.

- As an object falls, it is speeding up but losing

height. Therefore, it is gaining kinetic energy

at the same rate that it is losing potential

energy.

- In fact, gravity is doing work on the object to

convert its PE into KE!

Conservation of Energy

There are two more important things to

realize. For a given system

AND

When PE is at its maximum, KE is at its minimum,

and when KE is at its maximum, PE is at its

minimum.

The Work-Energy Theorem

The work done on an object is equal to the

combination of the changes in both kinetic and

potential energy.

Spring Potential

Just like how increasing the height of an object

will store potential energy in the object based

on how much gravity might be able to move it,

elastic things can also store potential energy.

This is known as Elastic Potential Energy.