UNIT 2MECHANICS

CHAPTER 6

ENERGY

Chapter 6A – The Nature of Energy

• Objectives:

– Discuss the importance of energy

– State what energy can do and the units in which energy is measured

– Define mechanical work as it relates to energy

– Compare and contrast potential and kinetic energy

• Assignment: Section Review, page 133

Introduction

• Energy moves everything!– Humans, animals, plants all require energy to live

and carry out life processes– Energy drives processes in nature that recycle

water– Winds and ocean currents circulate because of

energy– Internal and external sources of energy heat the

earth

• Without energy the universe would cease to exist!– Energy holds matter together

What is Energy?

• Energy– The ability to do work

• An object moves a distance because of a force acting on it

– No one really knows what energy is, only what it does or has the potential to do

– Scalar quantity, measured in joules

– Compared to wealth• Can be exchanged for work, matter or other forms

– Work and energy are interchangeable and measured in joules

• Facet – Page 131

Potential & Kinetic Energy

• Energy can be broken into two groups:

1. Potential energy– Energy of position or energy of condition

– Gasoline

2. Kinetic energy– Energy of motion

– All moving matter regardless of size has kinetic energy

– More mass and greater speed = greater kinetic energy

Chapter 6B – Classification of Energy

• Objectives:– List the nine forms of energy given in the text– Briefly discuss the nature of each kind of energy

and state man-made and natural sources for each– Compute an object’s gravitational potential

energy (GPE), given its mass and height– Explain the relationship between matter and

energy– Given the mass of an object, compute the

equivalent mass energy according to Einstein’s special theory of relativity

• Assignment: Section Review, page 140

Identifying Forms of Energy

• Energy is classified by how it is sensed or by its principal source

• In this text (briefly discussed here, more details in future chapters)– Mechanical– Thermal– Acoustic– Electrical– Magnetic– Radiant– Chemical– Nuclear– Mass

Mechanical Energy

• The energy of a system due to its positionor its motion

• Consists of both potential and kineticenergy

Mechanical Potential Energy

• Zero Reference Position– The position from which a distance is measured

• Two types1. Gravitational Potential Energy (GPE)

– The potential energy of a system due to its weight and its height above a zero reference height

– Zero point is below the object

– Formula: GPE=wh

– w=weight h=height

– GPE changes as the height changes

– Example problem 6-1, page 134

Mechanical Potential Energy

2. Elastic Potential Energy (EPE)

• The potential energy of a system due to an elastic force acting on it and its distance from a zero reference position

• Examples

– Rubber bands

– Springs

Mechanical Kinetic Energy

• Formula: KE=1/2mv2

– m=mass of the system (kg)

– v=speed (m/s)

• Kinetic energies of large objects or single particles are easily measured

• Small molecules and substances at rest are difficult and use a different method to calculate

• Example problem 6-2, page 135

• Read Kinetic Energy vs. Momentum

Thermal Energy

• Remember the kinetic-molecular model of matter?– All matter consists of innumerable tiny particles in

constant, random motion– Every particle has kinetic energy!

• The sum of all the kinetic energies of a particle is its thermal energy– Theoretically ceases at absolute zero (-273K)– We measure the changes in thermal energy

• Heating – gaining thermal energy• Cooling – loosing thermal energy

• Principal sources: sun and earth

Acoustic Energy

• The transmission of energy through matter by particle oscillations that occur in specific directions

• Occurs in waves– Examples– Earthquakes– Explosions– Bass speakers– Submarine sonar equipment– Human vocal cords– Piccolos– Jet engines– Dog whistles– Bats– Medical ultrasound diagnostic equipment

Acoustic verses Thermal Energy

Acoustic Energy Thermal Energy

When present, always moves through matterCannot move through a vacuum

Always present in matterOnly moves at different temperatures

Causes particles to move in back and forth vibrations (periodic vibrations) when kinetic and potential energies or repeatedly exchanged

Particle motion is random in direction and duration

When transferred the distance the particle move is usually much larger than the size of the particles

Vibrations are approximately the size of the particles

Electrical Energy

• The ability to do work through the action of the electromagnetic force on and by electrical charges– Like charges repel

– Opposites attract

• Useful sources of electrical energy are mostly manmade– Electrical batteries, generators, solar cells, radio-

thermal generators are just a few examples

• Natural sources– Lightning, electric rays and eels, electrical currents

circulating in the earth’s magnetic fields in space

Magnetic Energy

• The ability of a magnetic field to do work on magnetic objects and on moving electrical charges

• All magnetic objects have both a north and south pole unlike electrical which has either positive or negative charges

• Natural sources– Rocks, the earth, large bodies in our solar system

• Man-made sources– Magnets, computer hard drives, wire conducting

electricity

Radiant Energy

• Also called electromagnetic energy

• The combined action of electrical and magnetic energies in the form of wavelike, radiant energy– Energy and magnetism usually go hand in hand

• Visible light is the most common source

• Others include– Ultraviolet light, radar, radio waves, x-rays,

microwaves, infrared light, and gamma rays

– Naturally emitted from stars, black holes, pulsars, and nebulas

• Artificial (man-made) are used for communication, illumination, imaging and medical purposes

Chemical Energy

• The potential energy stored in the chemical bonds between atoms that is released or absorbed during chemical reactions

• Depends on the kinds of atoms involved and the bonds formed

• Most energy is released as thermal, radiant, or acoustic energy

• Photosynthesis is an exception

Nuclear Energy

• The potential energy stored in an atom’s nucleus that is released or absorbed when an atom experiences nuclear fission or fusion

• Released in two ways– Fission

• Large nuclei with many protons and neutrons can be split into two or more smaller nuclei

• The sum of the masses of the smaller nuclei is less than the original

• Energy is released by breaking bonds

– Fusion• Small nuclei are “smashed” together and form larger ones• Energy is released with the formation of a new bond

• Man-made– Nuclear fission reactors for electrical power generation and

research, fission and fusion bombs

Mass Energy

• The energy equivalent to matter itself, according to the equation in Einstein’s special theory of relativity

• Largest source of energy in the universe!

• Energy obtained if you could convert all of an atom’s mass into energy

• Formula: E=mc2

• Example 6-3, page 139

Chapter 6C – Energy Conservation

• Objectives:– Show how energy can be transformed from one kind

into another– Give examples of energy conversions– Define the efficiency of a process that involves the

conversion of energy from on kind into another– State the law of energy conservation and explain the

conditions under which it is true– State the combined energy-matter law of

conservation as the first law of thermodynamics– Compare and contrast momentum and kinetic energy– Classify collisions as elastic, partially elastic, or

inelastic

• Assignment: Section Review, page 145

Energy Transformation

• Read 6.14, page 140• Conservation is never 100%, there is always

some energy that escapes the process and is unusable

• Efficiency– The process can be measure by comparing the

amount of usable energy produced with the amount available before the transformation

• Man-made energy transformations are usually 20%-40% efficient

• A standard incandescent light bulb is only 5%• A fluorescent is about 20%

First Law of Thermodynamics

• Also called the Law of Conservation of Energy– Energy can never be created or destroyed, only

changed in form

• Pendulums are a clear, simple example

• No exception has even been observed!

• Directly related to the Law of Conservation of Matter

• The total amount of matter and energy in the universe is constant

• Read 6.16, page 141

Collisions and Energy

Three kinds of collisions:1. Elastic (ball bearings, hockey pucks)

– A collision between two objects in which the momentums and kinetic energies of the colliding objects are conserved

– No real world collision is perfectly elastic

2. Partially Elastic (car crash, soccer players)

– A collision between two objects in which momentum is conserved but some of their kinetic energies is lost to other forms of energy (usually heat) during the collision

– Objects are slightly deformed but rebound– Most real world collisions are this type

3. Inelastic (hockey players)

– A collision between two objects in which the deformation is so severe that they stick together

– Total momentum is conserved, total kinetic energy is greatly reduced by the amount of energy required to deform the objects

Facet – Stop!

• Read page 144

TOMORROW!!

• Vocabulary Quiz

– Includes all vocabulary throughout the entire chapter, PowerPoints, and board; not just the box at the end.

• Complete Chapter Review in Class

• Study for Chapter 6 Test