The Story of Energy Forms and Functions

What are 5 things

E helps us do?

Batteries store

energy!

This car

uses a

lot of

energy

We get

our

energy

from

FOOD!

Even this

sleeping

puppy is

using stored

energy.

Energy… O…is the ability to make things move

All Energy

potential kinetic

with an example of each

Potential Energy

Stored-up energy, or energy held in readiness

“potential"

simply means

the energy has

the ability to do

something

useful later on

When a roller coaster slows to a stop at the top of a hill, it has potential energy because of where it is (position in space). It has the potential to move because it is above the ground and has somewhere to go.

For example:

OSubstances like wood, coal, oil, and gasoline have stored energy because of their chemistry – they can burn

OStored energy is potential energy

Potential

forms of energy

Chemical: E stored in bonds of molecules

Nuclear: E stored in nucleus of atoms

Mechanical: E stored in objects by applying

force

Gravitational: E of place or position

Gravitational Potential Energy

o If an object is elevated and has the force of gravity acting on it, the object has PE.

o P.E. = mass x height x accel. due to gravity

PE = mgh Height (m)

Mass (kg) Potential Energy

(joules)

Acceleration

of gravity (9.8

m/sec2)

Ug = mgh

There are a lot of advantages of knowing some

physics but this one is definitely not one of them.

try a calculation

What is the potential energy of a 50 kg

gorilla on top of a skyscraper if he is 480 m

above the street below?

480 m

50 kg PE = mgh

PE = (50 kg)(9.8 m/s2)(480 m)

PE = 235 kJ

O A cart with a mass of 102 kg is pushed up a ramp.

O The top of the ramp is 4 meters high.

O How much potential energy is gained by the cart?

O If an average student can do 50 J of work per second, how

much time does it take to get up the ramp?

Ug = (102 kg)(9.8 m/s2)(4 m) = 3,998 J

3,998 ÷ 50 = 80 seconds to push the cart up

the ramp.

Kinetic Energy

The energy of motion

O If an object is moving, it has KE.

Examples of Kinetic Energy:

Kinetic

forms of energy

Radiant: E traveling in waves

Thermal: or heat, the internal energy in

substances

Motion: movement of a substance from one place

to another

Sound: movement of E through substances in

longitudinal waves

Electrical: movement of electrons

KE = 1 mv2

2 Speed (m/sec)

Mass (kg) Kinetic Energy

(joules)

Energy is measured in the same units as work because

energy is transferred during the action of work.

O The kinetic energy of a moving object

depends on two things: mass and speed.

What is the kinetic energy of a 5-g bullet

traveling at 200 m/s?

What is the kinetic energy of a 1000-kg car

traveling at 14.1 m/s?

5 g

200

m/s K = 100 J

K = 99.4 J

2 21 12 2

(0.005 kg)(200 m/s)K mv

2 21 12 2

(1000 kg)(14.1 m/s)K mv

Try a couple:

O Kinetic energy becomes important in calculating things like braking distance

Law of Conservation of Energy

O Potential Energy can be changed into Kinetic Energy

O Also Kinetic Energy can be changed into Potential Energy

Law of Conservation of Energy

O As energy takes different forms and changes things by

doing work, nature keeps perfect track of the total.

O No new energy is created and no existing

energy is destroyed but it can change form.

O A falling object converts gravitational

potential energy into kinetic energy

O friction converts kinetic energy into

vibrational (thermal) energy

O makes things hot (rub your hands together)

O irretrievable energy

for example…

more Energy transformation examples:

ball drop

Energy Story

How do your

calculations and graphs

demonstrate

conservation of Energy?

Why does the ball

eventually stop

bouncing?

Perpetual Motion O Why won’t a pendulum swing forever?

O It’s hard to design a system free of energy transformations that don’t include loss of heat to the environment

O The pendulum slows down by several mechanisms

O Friction at the contact point: requires force to oppose; force acts through distance work is done

O Air resistance: must push through air with a force (through a distance) work is done

O Gets some air swirling: puts kinetic energy into air

O Perpetual motion means no loss of energy O solar system orbits come very close

Energy Exchange O Though the total energy of a system is constant,

the form of the energy can change

O A simple example is that of a simple pendulum, in

which a continual exchange goes on between kinetic

and potential energy

pivot

height reference

h KE = 0; PE = mgh KE = 0; PE = mgh

PE = 0; KE = mgh

Trapeze

Anna the Russian Barre

bar

As Anna jumps and lands on the bar, her energy changes forms

multiple times, but her total energy never changes. This is because

energy can change forms, but cannot be created or destroyed—in other

words total energy in a system is conserved.

At the top of the jump, Anna's

energy is entirely in the form of

gravitational potential energy, P.

P depends on Anna's height, h,

above the ground, the

acceleration due to gravity, g,

and her mass, m:

P = mgh

As she begins to fall back down,

her velocity increases as her height

decreases.

P decreases, but her energy of

movement, kinetic energy, K,

increases.

K depends only on Anna's

mass, m, and velocity, v:

K = ½mv²

When Anna lands on the bar, her

kinetic energy is transferred to

bending the bar, and now takes

the form of elastic energy, U.

U depends on how deep the bar's

bend is, d, and its "springiness", a

constant k.

U = ½kd²

Even at the bottom of the bar's

bend, Anna still has a tiny bit of

potential energy.

solo Trapeze

Regina you can trapeze

Swinging back and forth, the solo trapeze is a giant pendulum. The

time it takes to swing forward, then back to where it started is called

the period. This time has very little to do with the height of the swing.

It depends mainly on the length of the pendulum, the longer the

pendulum, the longer the period.

The length of the pendulum, L1,

L2, or L3 is always the distance

from the pivot point near the

ceiling to Regina's center of

mass, m.

Which of these positions, if held

for the entire swing, would take

the longest time to go back and

forth?

A coffee mug is dropped from your hand and

shatters on the floor.

1. Define the scenario (start and stop point)

2. Draw a diagram of the scenario snapshot.

3. Describe the scenario in words.

4. Use energy cubes or energy theatre to work through the

scenario and map the energy transformations.

5. Finalize your drawing.

6. Show the energy conversions and conservation of energy.

O potential energy turns

into kinetic energy

O kinetic energy of the mug

goes into: O ripping the mug apart

O sending the pieces flying

O sound

O heating the floor and pieces through friction as the pieces slide to a stop

O In the end, the room is slightly warmer

Nerf Gun O Pulling back the arming mechanism puts

potential energy into the system (spring E)

O Pulling the trigger releases the PE and transforms it to mechanical E (Kinetic)

O Air exerts force on the nerf bullet pushing it out of the gun (some E loss to friction – thermal)

O Air resistance and gravity slow the bullet in flight (more E loss to atmosphere)

O Once all PE is expended bullet falls to the ground.

O In the end, all E loss is heat (irretrievable)

47

Kinetic Energy

O Kinetic energy for a mass in motion is

K = ½mv2 Example: How much energy does a 0.1 kg ball have

traveling at 5 m/s?

K = 0.5(0.1)(5)2

= 0.25 J of KE

48

Potential Energy

O potential energy for a mass is

PE = mgh Where g = 9.8 m/s2

Problem: How much energy does a 3 kg rock have

teetering on a cliff 35 m high?

PE = 3(9.8)(35)

= 1029 J of PE

If the rock from the previous question

falls off the cliff, how fast is it

traveling when it hits the ground?

PE = KE

mgh = ½ mv2

v = √2gh

= √2(35)(9.8)

= 26.2 m/s

35 m

A diver of mass m drops

from a board 10.0 m

above the water

surface.

Find his speed when he

hits the water. (Neglect

air resistance.)

mgh = 1/2mv2

v = √2(9.8)(10) = 14 m/s

Find the diver’s speed

when he is 5.00 m

above the water

surface.

At 5 m, KE = ½ PE 1/2mgh = 1/2mv2

v = √(9.8)(10) = 9.9 m/s

What is the kinetic energy of a 0.38 kg

soccer ball that is traveling at a speed of

120 m/s?

KE = 1/2mv2

= 1/2(0.38)(120) = 22.8 J

What is the mass of a baseball that has a kinetic energy of 105 J and is traveling at 10 m/s?

KE = ½ mv2

m = 2 x KE/v2

= 2(105)/102

= 2.1 kg