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Energy
KEY CONCEPTS
 How are energy and work related?
 What factors does the kinetic energy of an object
depend on?
 How is gravitational potential energy determined?
 What are the major forms of energy?
 ENERGY is the ability to do work
 What is work?
Work is a transfer of energy by a force moving an
object through a distance
 Energy and work both measured in JOULES (J);
1 J = 1 N.m
Most forms of energy can be classified into two general
types:
1. KINETIC ENERGY
2. POTENTIAL ENERGY
 KINETIC ENERGY-energy an object has because of
its motion
KE = ½ mv2
where KE is kinetic energy (in J), m is mass (in kg) and
v is velocity (in m/s)
 SEE “Math Skills-Calculating Kinetic Energy” p.
448
 POTENTIAL ENERGY-energy something has because of
its position or shape; two types
A. GRAVITATIONAL POTENTIAL
ENERGY-potential energy that
depends on an object’s height
PE = mgh
where PE is potential energy (in J), m is mass (in kg), g is
acceleration due to gravity (9.8 m/s2) and h is height above
a reference point (in m)
 What is the potential energy of a 10 kg rock 5 m above the
ground?
B. ELASTIC POTENTIAL ENERGYpotential energy an object has because it is
stretched or compressed
SEE FIG. 5, P. 450
 All energy can be considered to be KE, PE or energy in
fields such as those produced by electromagnetic
waves
The major forms of energy are
 1. MECHANICAL-associated with motion and
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position of everyday objects
2. THERMAL-total of potential and kinetic energy of
all microscopic particles in an object
3. CHEMICAL-energy stored in chemical bonds
4. ELECTRICAL-energy associated with electric
charges (+ and -)
5. ELECTROMAGNETIC-form of energy that travels
through space in the form of waves (light, X-rays, etc.)
6. NUCLEAR-energy stored in atomic nuclei
 SECTION 15.1 ASSESSMENT
KEY CONCEPTS
 Can energy be converted from one form into another?
 What is the law of conservation of energy?
 What energy conversion takes place as an object falls
toward Earth?
 How are energy and mass related?
 An ENERGY CONVERSION is the process of
changing energy from one form to another
 What type of energy conversions take place when a
match is lit? When a battery-powered radio is turned
on?
 The LAW OF CONSERVATION OF ENERGY states
that energy cannot be created or destroyed; the total
amount of energy present before a change is equal to
the total amount after (in a closed system)
 Work done by friction changes kinetic energy into
thermal energy.
 Friction reduces efficiency.
 Falling objects PE is converted into thermal energy
because of air resistance.
 Sometimes small amounts of friction can be ignored.
 Gravitational PE is converted to KE as an object falls
 Ex. pendulum (SEE FIG. 13, P. 456), pole-vaulter (SEE
FIG. 14., P. 457)
 Mechanical energy = KE + PE
For a closed, frictionless system
(KE + PE)i = (KE + PE)f
 SEE “Math Skills-Conservation of Mechanical
Energy”, “Math Practice” p. 458
 Einstein’s famous equation
E = mc2
where E is energy (in J), m is mass (in kg) and c is the
speed of light (3 X 108 m/s), says that energy and mass
are equivalent, and can be converted into each other
 When do mass-energy conversions happen?
 SEE SECTION 15.2 ASSESSMENT