Chapter 8
Energy
Work = force x distance
W = Fd
-application
of a force
-movement of something
by that force
Work is measured in
Joules
Joule (J) = N*m
Power – the rate at
which work is done
measured in watts
Power = work done
time interval
Mechanical Energy
Definition: the energy due
to the position of
something or the
movement of something
Potential Energy (PE)
Definition: energy that is
stored and held in readiness
Examples: stretched rubber
band, fossil fuels, food
Gravitational PE
PE due to elevated
positions
GPE = weight X height
PE = mgh
Kinetic Energy
(KE)
Definition: energy of
motion
Examples: throwing a
ball, moving car
KE =
2
1/2mv
Work-Energy
Theorem:
Work = DE
Law of Conservation of
Energy
Energy cannot be created or
destroyed. It can be
transformed from one form
into another, but the total
amount of energy never
changes.
Machines
definition: device used to
multiply forces or simply
to change the direction of
forces
Input force: the force
you exert on the
machine
Output force: the
force exerted by the
machine
Types of
Simple
Machines
(there are 6 of them)
Lever
2. Wheel and Axle
3. Pulley
4. Inclined Plane
5. Screw
6. Wedge
1.
lever - a bar that
turns about a fixed
point;
Ex. crowbar
Fulcrum: the fixed point
on which a lever turns or
pivots
fulcrum
The lever exerts a
large force over a
short distance while
you exert a small
force over a long
distance
TYPES OF LEVERS:
- refer to drawings
and explanations of
the three different
types of levers
Wheel and Axle:
two circular objects
fastened together
and that rotate on a
common axis
-always
rotate
together
-Ex. doorknob,
steering wheel
Pulley: a wheel that has a rope
or chain passing over it; used to
change the direction of the
force that’s applied to the
object
Example: flagpole, window
blinds
Inclined Plane: a
ramp or slope that
reduces the force
you need to lift
something
- Inclined planes
decrease the
effort force
Ex. ramps
Screw: an inclined plane
wrapped around a cylinder
to make a spiral
Threads:
the ridges spiraling
around the screw
Ex.
Screw, jar lid
Wedge: an inclined
plane that moves
Ex. Axes, chisels,
knives, hatchets
Simple Machines in
Your Body…
Your
tendons and muscles
pull on your bones and make
them act as levers
incisors (front teeth) are
wedges
Compound Machines: a
combination of simple
machines that makes it
possible to do
something that one
simple machine alone
cannot do
Ex. Can opener
The handles are levers; the
crank is a wheel and axle; a
gear is then turned, which
turns another gear which
moves the blade which is a
wedge
Mechanical Advantage=
output force
input force
Tells
you how much
force is multiplied
The larger the
mechanical
advantage, the
more help the
machine provides
When we calculate
mechanical advantage we look
at ideal situations
 An
“ideal” machine would be 100%
efficient
 100% efficiency NEVER happens
in practice
Whenever work is done;
some energy will be lost
as heat
Efficiency looks at this
heat/energy loss
Efficiency =
actual mechanical advantage
theoretical mechanical
advantage
OR
Efficiency =
output work
X 100 %
input work
Even the best
designed engines
are about 35%
efficient