Transcript End of Chapter 7

Efficiency
Measure of how much work is put to
“good use”
Efficiency of Machines
 Law of C of E says that energy in must
equal energy out
 However, often a lot of energy is lost
 Heat, friction, sound, etc.
 Efficiency equals
 (Useful energy out) *100%
(Energy in)
 Higher the percentage….the more efficient
the machine is
Efficiency (cont.)
 Companies seek to find most efficient machines
to manufacture, transport, and develop
 Ex. Gas powered cars are not very efficient,
about 10 - 25% efficient
 Electric (hybrid) cars--much more efficient
 Get up to 3 times the mileage of some gas cars
 http://auto.howstuffworks.com/hybrid-car4.htm
 Government standards
100% Efficient
Problem
 http://www.bbc.co.uk/schools/gcsebitesize/
physics/energy/energyefficiencyrev3.shtml
Question 1
 A power plant burns 75kg of coal every second. Each kg
of coal contains 27 MJ (27 million joules) of chemical
energy.
 What is the energy output of the power station every sec?
The Solution
 = 75 x 27 million J per sec
 = 2025 million J per sec
 = 2025 million J/s or
 (2025 megaWatts)
Question 2
 The electrical power output of the power plant
is 800MW (800 million watts). But Question 1
stated that the chemical energy output of the
station was 2025 MW…..So, What has
happened to the rest of the energy?
The Answer
 Most of the rest of the energy is wasted as
heat - up the chimney of the power station, in
the cooling towers, and because of friction in
the machinery.
Question 3
 Calculate the efficiency of the power plant
as a percentage.
The Solution
 Efficiency = useful power output/total power input
 = 800,000,000 W/2025,000,000 W
 = 0.395 x 100% to create a percentage
 = 39.5%
Simple Machines
A screw applies a small force
over the long distance across
the face of all of its threads at
once to accomplish the same
work as a large force over a
small distance
 wedge, pulley, lever, ramp,
screw, wheel and axle
 Multiply force but applying
small force over greater
distance
 Amount of work done is not
increased by a machine
 By law of conservation of
energy it is impossible to
multiply energy
Mechanical Advantage
 Usefulness of machines is due to
multiplication of force, not of energy
 Often limited by how much force we
can apply, so we apply a small force
over a large distance
 One way to measure how useful a simple
machine is is by measuring its Ideal
Mechanical Advantage
By applying
a 10 N
force and
moving this
end 50 cm..
 MA = Force output / force input
You were able to apply a
100 N force on this end
and move the heavy rock
5 cm
Soo….
MA =
10/1 =
10
Incline Plane
 MA is made up by comparing the
Parallel force by the force of
gravity, on any incline the
parallel force will be much less
than the weight of the object, this
is why it is easier to walk long
distance on low incline than a
short distance and a steep incline
or climbing straight up
Schober
Brothers
embark on a
hike in
Yosemite Ntnl.
Park
…..And learn a harsh
lesson about Incline
Planes…
Idiots!
Day 1,
short
hike…little
wet….
feelin
good.
This is easy!!
Day 2 Hike -- 2600 ft of vertical climb
to top of Yosemite Falls
2600 vertical feet
Hike on Day 2-- In pain, not
even close to the top
Little higher up the mountain….
Me slumped on
a rock about to
vomit….
…………………
Will cant feel his
legs
Andy takes
picture and
laughs
Get to the top ..Exhausted…. Pass out on a
rock while squirrel eats our granola
The Point…
Activity
Difficulty Who can do it
Why??
Hike on flat ground
Easy
Anyone with 2 legs
Horizontal motion, do not need
to work against gravity
Hike up mountain
Harder
via long switchbacks
and low grade paths
Anyone with 2 legs and
are at least a little bit in
shape
(Day 1 Hike)
Working against gravity to gain PE
but spread out over a long distance.
Longer distance means shorter Force.
Also spread out over long amt. of
time
Hike up mtn. via
Very
steep steps with high Difficult
incline
Andy, 8 year old
girls,…
not Will & Jake 
(Day 2 Hike)
Working against gravity to gain
PE over a shorter distance
requires larger force applied in
short time intervals
Direct climb up
mountain face (no
path)
Extremely Only highly trained rock
Difficult
climbers
(would never even
thinking of trying it)
Essentially lifting your entire
weight straight up with every
step. Requires incredible power,
strength and endurance.
The Point…. (cont.)
 The steeper the incline plane… the higher
portion of your weight you are going to
have to lift with every step.
 Low incline, medium incline, direct climb
all require the same amount of work
because all produce same increase in PE
 Difference is in how that work is performed.
Pulleys
 Like levers, ramps, and
screws…. Sacrifices
displacement to achieve a
greater force
 By pulling a greater
displacement you have to
apply less force
 MA is shown by how many
ropes are supporting the load
in this case there are two
 http://en.wikipedia.org/wiki/Pu
lley
Another Pulley
 MA = 4
 4 ropes supporting load
 Force applied is 4 times
less than 100 N
 So rope must be pulled
with 25 N of force with a
distance 4 times greater
than the upward distance
the load moves
Levers
 Pull greater distance on long end but
achieve greater force over a small distance
on the short end