chapter 8 energy. energy what is energy? you can see its effects, but it can be difficult to...
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CHAPTER 8CHAPTER 8
ENERGY
EnergyEnergyWhat is energy?
You can see its effects,
but it can be difficult
to understand.
First, let’s look at a
closely related concept:
Work
8.1 Work8.1 Work
WORK = the product of the force and the distance through which the object is moved.
To calculate WORK:To calculate WORK:
WORK = force X distance
or
WORK = Fd
Two things affect work:Two things affect work:
1. the application of force to an object
and2. movement of that object
by the force applied
Units for WORK:Units for WORK:
Units are measured in
newtons X meters Or “newton meters”
1 newton meter = 1 JOULE
Work is measured in JOULES.
8.2 Power8.2 Power
Power involves TIME
The rate at which work is done.
POWERPOWER
Power = work / time
Or
P = w/t
Work/time = Joule/second
POWERPOWER
Joule/second = watts
Power is measured in WATTS
Watts are named after
James Watt – inventor of the steam engine.
8.3 Mechanical Energy8.3 Mechanical Energy
Energy = that property of an object or a system which
enables it to do work
Energy is measured in JOULES.
MECHANICAL ENERGY is the energy due to the position
or the movement of something
Mechanical energy may be either kinetic or potential.
8.4 Potential Energy 8.4 Potential Energy (PE)(PE)
Potential energy is energy stored and held
in readiness that has the potential to do work.
For example:For example:
A ball sitting atop a hill – has energy called
“gravitational potential energy”
To calculate gravitational To calculate gravitational potential energy:potential energy:
Multiply weight X height
PE = (mass X g) X (height)
or
PE = mgh
8.5 Kinetic Energy8.5 Kinetic Energy
Kinetic energy is the energy of motion
To calculate KE:To calculate KE:
KE = ½ mv2
Where KE = kinetic energy
m = mass
v = velocity
Also…Also…
KE = FdWhere KE = kinetic energy
F = net forced = distance
Therefore:Therefore:
Fd = ½ mv2
8.5 Kinetic Energy8.5 Kinetic Energy
Work-Energy Theorem:
the theorem that states that whenever work is done, energy changes.
8.6 Conservation of Energy8.6 Conservation of Energy
How is energy transformed?
10 J of PE 8J of KE
2 J heat
10 J of PE 8J of KE
2 J heat
Some of the energy is lost as heat;
the rest is transformed into KE.
Law of Conservation of Law of Conservation of EnergyEnergy
Energy cannot be created nor destroyed; it can only
be transformed.Total energy remains
constant.
PE transformed to KEPE transformed to KE
At the top of the hill, the cart has only PE.
Towards the middle of the hill, the cart has equal amounts of PE and KE.
At the bottom of the hill, all of the PE has been transformed into KE.
On a pendulum:On a pendulum:
On a roller coaster:On a roller coaster:
Notice the total energy remains constant.
The energy is transformed from PE to
KE and back to PE.
8.7 Machines8.7 Machines
A machine is a device for multiplying forces or simply changing the direction of forces.
There are 6 types of simple There are 6 types of simple machines:machines:
1. Lever
2. Screw
3. Inclined plane
4. Pulley
5. Wheel and Axel
6. Wedge
LeversLeversA lever is a simple machine made of a bar that turns around a fixed
point
A fulcrum is the pivot point of a lever
Work input = Work outputWork input = Work output
Fdinput = Fdoutput
Where F = forceand
d = distance
Mechanical AdvantageMechanical AdvantageMechanical advantage is the ratio of output force to input
force for a machine.
OUTPUT FORCE
INPUT FORCE
Mechanical AdvantageMechanical AdvantageFigure 8.10Figure 8.10
compares the amount of force needed to the amount of force produced
EX: a girl uses a lever to lift a rock that has a weight of 80N. She applies 10N of force to do so.
The mechanical advantage = 8.
Also see the question on page 116
There are three types of levers.
Type 1Type 1: fulcrum between the : fulcrum between the force and the loadforce and the loadEX: a playground seesaw
Type 2Type 2: load is between : load is between the fulcrum and input forcethe fulcrum and input force
EX: lifting a car with a steel bar
Type 3Type 3: fulcrum at one end : fulcrum at one end and load at the otherand load at the other
EX: bicep muscles attached to bones
in forearm
PulleyPulley
A pulley is a type of lever that can be used to change direction.
http://pathfinder.esu2.k12.ne.us/java/ physics/physengl/pulleysystem.htm
input
output
input
output
8.8 Efficiency8.8 Efficiency
useful work output
Efficiency = input X 100
or
actual mechanical advantage
theoretical mechanical advantage
8.9 Energy for Life8.9 Energy for LifeThink of cells as machines.
For example:
Plant Cells Photosynthesize
Intestinal Cells Digest food
Muscle Cells Shorten
Chapter 8 Key TermsChapter 8 Key TermsEfficiencyEnergyFulcrumJouleKinetic energyLaw of
Conservation of Energy
LeverMachine
Mechanical advantage
Mechanical energyPotential energyPowerPulleyWattWorkWork-energy
Theorem