1. UNIT 2 MECHANICS
CHAPTER 6
ENERGY

2. Chapter 6A – The Nature of Energy
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

3. 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

4. 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

5. Identifying Forms of Energy
Chapter 6B – Classification of Energy
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

6. Mechanical Energy
The energy of a system due to its position or its motion
Consists of both potential and kinetic energy

7. Mechanical Potential Energy
Zero Reference Position
The position from which a distance is measured
Two types
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 113

8. Mechanical Potential Energy
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

9. 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 114

10. 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 (0K)
We measure the changes in thermal energy
Heating – gaining thermal energy
Cooling – loosing thermal energy
Principal sources: sun and earth

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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 118

18. Energy Transformation
Chapter 6C – Energy Conservation
Energy Transformation
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%

19. 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

20. Collisions and Energy Three kinds of collisions:
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