energy energy. introduction energy is always present, but never visible! we see the evidence of...

EnergyEnergy

ENERGY

IntroductionIntroductionEnergy is always present, but

never visible!We see the evidence of energy

◦Pushing a wheelchair◦Jumping◦Eating

Movement, sound, heat, light all provide evidence of energy

Trebuchet Video: Consider Trebuchet Video: Consider these questionsthese questions

How is a trebuchet constructed? Which simple machine do you see? What is the purpose of the

counterweight?What type of energy does the

trebuchet have when stationary? What happens when the firing pin is

released? How would you differentiate

between potential and kinetic energy?

http://www.teachersdomain.org/asset/hew06_vid_trebuchet/

Work- Energy CorrelationWork- Energy CorrelationIn order for work to be done, a

force must be applied to an object and cause it to move in the same direction as the force.

Work transfers energy from one object to another

What is energy?

What is energy?What is energy?

Energy is the ability of an object to produce a change in itself or the environment. It is the ability to do work.

Types of EnergyTypes of Energy

Mechanical Energy(ME) - enables an object to do work

A. Kinetic Energy (KE) B. Potential Energy(PE)

Kinetic EnergyKinetic EnergyKinetic energy is defined as the

energy of a moving objectExamples:

◦Thrown football◦Waterfall◦Rock falling from a cliff

Potential EnergyPotential EnergyPotential energy is defined as the

energy in matter due to its position or the arrangement of its parts

Often referred to as stored energyThe more potential an object has,

the more potential energy is hasHas many different forms including

◦Gravitational potential energy◦Elastic potential energy◦Chemical potential energy◦Electrical potential energy

Toy observationsToy observations1. Describe your toy. 2. Take a few minutes to determine how

your toy works. Make a short statement about how the toy works.

3. State how you think your toy uses energy.

4. How many forms of energy does your toy display?

5. Name the types of energy you observe. 6. Switch toys with another table and

repeat the above observations.

Gravitational Potential Gravitational Potential EnergyEnergyWhen something is lifted or

suspended in air, work is done on the object against the pull of gravity

This work is converted to a form of potential energy, called gravitational potential energy

Once the object falls the potential energy is converted to kinetic energy

Elastic Potential EnergyElastic Potential EnergyOccurs when an object resists

being pulled out of shapeExamples: a stretched rubber

band, a spring, trampoline, our skin

The elastic potential energy in a rubber band can be used to do work- such as a toy plane . . . A rubber band untwists and causes a propeller to spin

Chemical Potential EnergyChemical Potential EnergyChemical potential energy is the

energy stored in molecules (including in us)

Examples: food, gasolineWhen gasoline is burned through

combustion, the arrangement of molecules changes, and energy is released

The released energy is used to do work

Electric Potential EnergyElectric Potential EnergyElectrical potential energy is the

result of energy from a battery, a power plant, a hydroelectric dam, solar cells, or a windmill

Electric potential energy can be converted to sound, light, motion, etc.

Work and EnergyWork and EnergyWork and Energy are relatedWhen work is waiting to be done,

we call the energy potentialWhen work is being done, we call

the energy kinetic

Rubber BandRubber BandIn order to make a rubber band fly,

you must do work on itWhile it is stretched, the rubber band

has potential energyOnce you release it, it has kinetic

energyIf you pull it back a greater distance

(therefore doing more work) the potential is greater

The result is that the rubber band will go faster and further (more potential energy leads to greater kinetic energy, thus more work can be done)

Situations to considerSituations to consider

Is work done in the following situations? ◦Pushing against a wall?

No, there is no displacement, no work, so kinetic energy does not change

◦Pushing at constant speed? No, work done by the applied force is equal to

work done by the frictional force, so kinetic energy does not change

◦Free fall Gravity is the only force acting, so there is a net

force and work is done . . . Kinetic energy changes

Kinetic EnergyKinetic Energy

Because energy is a property of matter, it can be quantifiedThe equation for kinetic energy is:

Where “m” is the mass and “v” is the velocity of the object

The unit for energy is joules

2

2

1mvKE

Example 1Example 1What is the kinetic energy of a 45

kg object moving at 13 m/s?

smv

kgm

/13

45

2

)/13)(45(2

1smkgKE

JKE 5.3802

Example 2Example 2The kinetic energy of a boat is

calculated at 52,000 J. If the boat has a mass of 39,000 kg, with what velocity is it moving?

JKE

kgm

000,52

000,39

2

2

1mvKE

2))(000,39(2

1000,52 vkgJ 2

000,39

)2(000,52v

kg

J

smv /63.1

Potential EnergyPotential EnergyRemember that potential energy

is the energy of positionTo quantify potential energy use

the following:

Where “m” is mass, “g” is gravity and “h” is the height in meters

mghPE

Example 3Example 3A 3.8 kg object is lifted to a

height of 3 meters. What is the potential energy of the object?

mh

smg

kgm

3

/8.9

8.32

)3)(/8.9)(8.3( 2 msmkgPE

JPE 7.111

mghPE

Example 4Example 4A 30 kg child climbs 15 m up a

tree. When he stops to look around, what is the child’s potential energy?

2/8.9

15

30

smg

mh

kgm

mghPE

)15)(/8.9)(30( 2 msmkgPE

JPE 4410

State whether each of the following State whether each of the following has kinetic energy, potential has kinetic energy, potential energy, or both. energy, or both.

1. If an object is at rest, it certainly does NOT possess this form of energy.

2. Depends upon object mass and object height.3. The energy an object possesses due to its motion.4. The amount is expressed using the unit joule

(abbreviated J).5. The energy stored in an object due to its position

(or height).6. The amount depends upon the arbitrarily

assigned zero level.7. Depends upon object mass and object speed.8. If an object is at rest on the ground (zero height),

it certainly does not possess this form of energy.

Work-Energy TheoremWork-Energy Theorem

Work is the transfer of energy from one object to another, or . .

The change in KINETIC ENERGY!

KEKEKEW if

Total EnergyTotal EnergyAt any point, the total energy of

an object

ME = KE + PE

Law of Conservation of Law of Conservation of EnergyEnergyEnergy cannot be created or destroyed.

The total energy in a system before an interaction equals the energy after an interaction.

This is true for a closed, isolated system (no external forces present)

Law of Conservation of Law of Conservation of EnergyEnergy

ffii PEKEPEKE

Example 1Example 1

A 12 kg rock is at the edge of a 95 m cliff.a. What is the rock’s initial PE and KE? b. If the rock falls to the ground, what is its final PE and KE just before it hits? c. What is the rock’s velocity just before it hits the ground?

Example 2Example 2

A 33 kg cart rests at the top of a hill. a.If the cart has a PE of 4800J, what is the height of the hill?b.When the cart has reached point B (h = 5.0 m), what is its KE?

Example ProblemExample Problem

A greyhound at a race track can run at a speed of 16.0 m/s. What is the KE of the 20.0 kg greyhound as it crosses the finish line?

2)/0.16)(20(2

1smkgKE

JKE 560,2

Example Problem 2Example Problem 2

Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s.

JkgKE 653,104)3.18)(625(2

1 2

Example 3Example 3

Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of 12 000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed?

2))(40(2

1000,12 vkgJ

2)40(000,24 vkgJ

2600 v smv /5.24

Example ProblemExample Problem

Legend has it that Isaac Newton “discovered” gravity when an apple fell from a tree and hit him in the head. If a 0.20 kg apple fell 7.0 m before hitting Newton, what was its change in PE during the fall?