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Types of Energy and the law
of conservation of energy
p. 236-241
Energy Transformations
see p.236 chart
 Any
form of energy can be transferred
from one place to another, or
transformed from one form to another.
 Whenever anything happens, energy is
transferred or transformed.
 Energy cannot be created or destroyed,
and therefore, the total amount of
energy in the system remains the
same.
Discussion: Forms and Types of Energy
Section 5.3
Can you think of types of energy that each image represents?
Section 5.3
Forms and Types of Energy
sound energy
elastic energy
thermal energy
electrical energy
gravitational energy
9.7 Conservation of Energy
The law of conservation of energy
states that energy cannot be
created or destroyed. It can be
transformed from one form into
another, but the total amount of
energy never changes.
Efficiency of Energy Transformations
chemical → kinetic
radiant → chemical
electrical → thermal
chemical → kinetic
Conservation of Energy
A system is a set of elements
that influence one another and
on which external influences act.
Systems can be large (the
universe) or small (an ant), and
may contain many elements or
as few as two.
 When
the energy interactions of a
group of objects need to be
analyzed, we often assume that
these objects are isolated from all
other objects in the universe.
 Such a group is called an isolated
system.
 In an isolated system, the total
amount of energy never changes.
Conservation of Energy
Potential
energy will
become the
kinetic energy
of the arrow.
Conservation of Energy
As you draw back the arrow in a bow,
you do work stretching the bow.
• The bow then has potential
energy.
• When released, the arrow has
kinetic energy equal to this
potential energy.
• It delivers this energy to its target.
Conservation of Energy
Part of the PE of the wound spring changes into KE.
The remaining PE goes into heating the machinery
and the surroundings due to friction. No energy is
lost.
Conservation of Energy
When the woman leaps
from the burning
building, the sum of her
PE and KE remains
constant at each
successive position all
the way down to the
ground.
Conservation of Energy
The water behind a dam has potential
energy that is used to power a generating
plant below the dam.
• The generating plant transforms the
energy of falling water into electrical
energy.
• Electrical energy travels through wires
to homes where it is used for lighting,
heating, cooking, and operating electric
toothbrushes.
Mechanical Energy

Mechanical energy (Em) is the sum of the
kinetic energy, and the potential energy of a
system:
Quantifying Energy Transformations
Phase 1: Before the dive p.238
Since motionless diver kinetic energy is zero Ek=1/2mv2
 Gravitational potential energy is Eg=mgh (calculate…)


Total mechanical energy: Em=Ek+Ep (calculate…)
Phase 2: At half way point

Once diver leaves platform he will accelerate
down at 9.8m/s2; half way the diver is 5m above
water
 Eg=mgh
Ek=1/2mv2
Em=Ek+Ep
 To find V: v2=vi2 +2a d (calculate)
 Now sub into Ek=1/2mv2
Phase 3: At the water’s surface
Ek=1/2mv2
Em=Ek+Ep
 When diver reaches after his height above water
is 0m, so Eg=0 (calculate to show this)

Eg=mgh
 Ek=1/2mv2 to
find v: v2=vi2 +2a d (calculate)
Sample problem
A toy-car track is set up on a tabletop as shown
below.
 The mass of the car is 25 g.
 Calculate the speed of the car at the bottom of
the ramp, assuming friction is negligible.

Homework
P. 241 practice #1
 P. 241 questions #1-4

p. 241 question #3