Chapter 15 – Energy

Work is done when a force moves an object through a distance

Energy – The ability to do work

In other words, energy is transferred by moving an object through a distance.

So, work is a transfer of energy

15.1 Energy and Its Forms

Think about the energy needed to carry your backpack up a flight of stairs

- You do work by lifting the backpack against gravity (requires energy)

- The energy to do this comes from your muscles

- Your muscles get the energy from the food we eat

- The energy in the plants you eat, or animals you eat that ate the plants, comes from the sun

15.1 Energy and Its Forms

Recall, work is done when a force moves an object through a distance

15.1 Energy and Its Forms

What is Newton’s 2nd Law? F ma

A kinematics equation: v v a df i2 2 2

Solve for a: av v

df i

2 2

2 Substitute a:

Rearrange:

Fd m v m vf i 1

2

1

22 2

Work

Kinetic Energy

Applying a force to an object through some distance sets that object into motion

What is meant by kinetic energy?

An object in motion can apply a force to something and in turn do work to that object (it has energy– the ability to do work)

The moving bowling ball can do work on the pins

Kinetic Energy – energy an object has because it is in motion

KE mv1

22

The unit of KE is the joule

15.1 Energy and Its Forms

- Doubling the mass the kinetic energy

K inetic Energy KE m v( ) 1

22

- Doubling the speed the kinetic energy

15.1 Energy and Its Forms

Doubles

Quadruples

Page 448

- Read Math Skills

- Do Math Practice

15.1 Energy and Its Forms

Potential Energy – Energy that is stored as a result of position or shape

15.1 Energy and Its Forms

Positive work must be done to raise the hammer from the ground.

When raised to a height and dropped, gravity can do work on the hammer

As it falls, the hammer will gain kinetic energy, and be able to do work on the pile

The hammer, when raised has the potential to do work

15.1 Energy and Its Forms

15.1 Energy and Its Forms

Gravitational Potential Energy – Potential energy that depends on an object’s height

- That work becomes Gravitational Potential Energy

- The baby works to get to the top of the diving board (F x d)

- The work done is dependent on the mass of the baby and the height the baby climbed

15.1 Energy and Its Forms

Gravitational Potential Energy depends on mass, height, and the acceleration due to gravity (g)

- mass (m) is in kilograms

G ravita tiona l Po ten tia l Energy PE mgh( )

- g is 9.8 m/s2

- height (h) is in meters

- always measured relative to an arbitrary reference level

15.1 Energy and Its Forms

Elastic Potential Energy – The potential energy of an object that is stretched or compressed

- Something is considered elastic if it springs back to its original shape after being stretched or compressed

15.1 Energy and Its Forms

There are 6 major forms of energy

- Mechanical Energy

- Thermal Energy

- Chemical Energy

- Electrical Energy

- Electromagnetic Energy

- Nuclear Energy

15.1 Energy and Its Forms

Mechanical Energy – the energy associated with motion and the position of everyday objects.

It is the sum of an object’s potential and kinetic energy

15.1 Energy and Its Forms

Thermal Energy – The total potential and kinetic energy of all the atoms and molecules in an object

Recall the kinetic theory of matter – atoms and molecules are always in motion

15.1 Energy and Its Forms

Chemical Energy – The energy stored in chemical bonds

When bonds are broken, the released energy can do work

15.1 Energy and Its Forms

Electrical Energy – energy associated with electric charge

Electric charges can exert forces that do work

15.1 Energy and Its Forms

Electromagnetic Energy – a form of energy that travels through space in the form of waves

The sun radiates electromagnetic waves

15.1 Energy and Its Forms

Nuclear Energy – The energy stored in atomic nuclei

Fission and Fusion are two processes that release nuclear energy

15.1 Energy and Its Forms

Energy conversion – energy can be converted from one form into another

15.2 Energy Conversion and Conservation

Gravitational potential energy to kinetic energy

15.2 Energy Conversion and Conservation

Energy conversion – energy can be converted from one form into another

Elastic potential energy to kinetic energy

15.2 Energy Conversion and Conservation

Potential energy to electric energy and electromagnetic energy

Energy conversion – energy can be converted from one form into another

Law of Conservation of Energy – energy cannot be created or destroyed

15.2 Energy Conversion and Conservation

Energy Conversions

Pendulums

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

Energy Conversions

Pole Vault

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

Energy Conversions

Mechanical Energy – The sum of an object’s total KE and total PE

Mechanical Energy = KE + PE

15.2 Energy Conversion and Conservation

Energy Conversions Calculations

Law of Conservation of Mechanical Energy

(ignore friction!)

(KE + PE)beginning = (KE + PE)end

15.2 Energy Conversion and Conservation

15.2 Energy Conversion and Conservation

If you throw a baseball straight up with a speed of 5 m/s, what will be the speed of the ball when it comes back to your hand. Ignore air resistance?

Example 1

A 1000 kg car is coasting at 10 m/s toward a hill that is 10 m high. Will the car make it to the top of the hill if the driver does not step on the gas pedal?

Example 2

Page 458 Math Skills and Math Practice

Read Page 459 on E = mc2

Questions to answer from Roller Coasters video

1. How does the roller coaster get to the top of the first hill?

2. How does the roller coaster continue to move after the first hill?

3. When does the roller coaster have the most potential energy?

4. What happen’s to the roller coaster’s potential energy as it goes down a hill?

5. Why are the frames of modern roller coasters made out of steel, instead of the wooden frames that were once used?

15.2 Energy Conversion and Conservation

15.2 Energy Resources

Energy Conversions

The gravitational potential energy of an object is converted to kinetic energy of motion as the object falls (roller coaster)

The elastic potential energy of a spring is converted to kinetic energy of motion as the spring is restored (screen door)

15.2 Energy Conversion and Conservation

HEAT

HEAT

• Conduction- transfer of thermal energy with no overall transfer of matter

• Occurs within a material or between materials that are touching

• Due to particle collisions

• NEWTON’S CRADLE

CONDUCTION

Conduction in gases

• Slower than in liquids or solids.

• Why?

• Gas particles are further apart and therefore collide less frequently

Why are metals good condcutors?

• Some electrons move freely in metals

• These collide with each other and other atoms to transfer thermal energy

Thermal Conductors

• Conduct thermal energy well

• What are pots and pans made of?

• Why do tile floors feel colder than carpet or wood?

Thermal Insulators

• Conduct thermal energy poorly

• Air- very good insulator- double paned windows

• What are coolers made of?

CONVECTION

• Transfer of energy when particles of fluids move from one place to another

• We all know that hot air……

• RISES

• Why?

• Becomes less dense as particles gain KE and spread out.

• Therefore, cool air sinks

CONVECTION

Convection Currents

• The cycling of hot air rising and cool air sinking

• Important in ocean currents, weather systems, movements of hot rocks in the Earth

RADIATION

• Transfer of energy by waves moving through space

• What do you feel when you stand by a hot stove without touching it?

• What do you feel when you go outside on a sunny day?

• When temperature increases rate of radiation increases.