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ENERGY
Energy
Energy Defined
• Energy is the ability of an
object to produce a
change in itself or the
environment.
Energy Transfer
• Work is done
when energy
is transferred
from one form
of energy to
another.
The Units for Measuring
Energy
•Are the same as the units for
measuring WORK
•JOULEs in the SI system
•FOOT-POUNDS in the
British System
TYPES OF
ENERGY
Mechanical Energy
• Energy is
the ability
to do work
Mechanical Energy(ME) enables an object to do work
A. Kinetic Energy (Ek) energy of motion
B. Potential Energy(Ep) stored energy
Gravitational Potential
Energy
• An object lifted
against the force
of gravity has
stored energy.
• UNIT= JOULE
Gravitational Potential
Energy
• Only changes in potential
energy can be measured
Ep = mgh
m= mass
g= gravity
h= height
• As an object, such
as a boulder,
begins to move, it
gains velocity.
• Gaining velocity
increases the
object’s kinetic
energy.
Example 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 Ep during
the fall?
Kinetic Energy
1
2
Ek  mv
2
Unit: Joules
Kinetic Energy Examples
Football linebacker
Pitched baseball
4500 J (110 kg
150 J (148 g @45
@ 9.0 m/s)
m/s)
Example 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?
Work
• Work is the transfer of energy from
one object to another, or . . .
• The change in KINETIC
ENERGY!
W  KE f  KEi  KE
Ek and Ep
are
constantly
changing.
Law of Conservation
of Energy
• Energy cannot be created or
destroyed.
• ENERGY is merely
transformed .
APPARENT LOSS OF
ENERGY
• FRICTION
WASTES
ENERGY BY
CHANGING
USEFUL
ENERGY INTO
HEAT
Total Energy
• At any point, the total energy
of an object
Et = Ek + Ep + HEAT
Potential Energy to Kinetic
Energy
Law of Conservation of
Energy
KEi  PEi  KE f  PE f
ELASTIC POTENTIAL
ENERGY
Elastic Potential
Energy
•Sling shots
•Trampolines
•Rubber
bands
•Rubber
• balls
A Jack in the box has stored
energy in a compressed spring.
k=
force to
stretch
divided by
distance
stretched
The
Elastic
Constant
ELASTIC POTENTIAL
ENERGY
•EEP=
1/2
2
kd
Elastic Potential Energy
• If a spring of stiffness
constant (k) is
stretched an amount
(d), the elastic
potential energy stored
in the spring is equal
to 1/2 the the spring
constant times the
square of the distance
stretched.
An automobile stores chemical
energy in the gasoline.
Thermal Energy
• The total energy of
vibrating atoms and
molecules that make
up a substance.
• Temperature is the
average KINETIC
ENERGY of a
substance.