Transcript Work, Power, and Simple Machines

WORK, POWER, ENERGY AND SIMPLE
MACHINES
QOTD: Write a list of 10 examples your idea of
work.
 Demo : create a work equation using a spring
scale, string and an agenda book.

WHAT IS WORK?
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Work is done only when a force moves an object
A force acting on an object and causing it to
move a distance is work
Not every force is work..if you push against the
wall it does not move..that is not work!
Work = force X distance
Work is measured in Joules
If you pick up a bag of groceries and walk across
the room the work is picking up the groceries
not the walking.
 The
object must move some distance as a result of
your force
 The force you exert must be in the same direction
as the objects motion.

Ie: the groceries
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You walk
WHAT’S WORK?
A scientist delivers a speech to an
audience of his peers.
 A body builder lifts 350 pounds
above his head.
 A mother carries her baby from
room to room.
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A father pushes a baby in a carriage.
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A woman carries a 20 kg grocery
bag to her car?
5
WHAT’S WORK?
A scientist delivers a speech to an
audience of his peers. No
 A body builder lifts 350 pounds
above his head. Yes
 A mother carries her baby from
room to room. No

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A father pushes a baby in a carriage. Yes

A woman carries a 20 kg grocery
bag to her car? No
6
Work can be determined by calculating
 Force used x distance moved = amount of work
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Therefore what is the formula for work?
Work = force x distance
 Joule – is the SI unit for work.
 Newton = force
 Meters = distance
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Therefore if you exert:
 1 Newton of force for 1 meter of distance
= 1 joule of work or 1N/m
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Work is done when a force is exerted through a distance.
A student lifts a bag of books that weighs 135 N. If the bag
is lifted .75 m, how much work does the student do?
F = 135 N d = .75 m
W = Fd
W = (135 N)( .75 m)
W = 101.25 J
A +24 N force is applied to an object that moves 10 m
in the same direction during the time that the force is
applied. How much work is done to the object?
http://www2.franciscan.edu/academic/mathsci/
mathscienceintegation/MathScienceIntegation1011.htm#item1015
Find the equation for POWER
1. Attach a string and spring scale to a large book.
2. Pull the book .5m slowly. Use a stopwatch to determine time .
3. Record the time and distance on a data table.
4. Repeat 1- 3 – but this time pull the book faster.
5. Repeat 1-3 even faster.
Force-N
distance -m
time- s
POWER
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Power tells you how fast something is
happening..how fast the work is being done
Power = work/time or Power = Force X Distance
Time
Power is measured in watts (W)
One watt is equal to 1 joule per second of work divide
joules/seconds
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Power – the rate at which energy is transferred.
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P=W
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t
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1 Watt (W) = 1 J/s
P = power
W = work
t = time
Watts
Joules
seconds
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HOW MUCH POWER MUST A MOTOR HAVE TO OPERATE A PUMP
THAT RAISES
1500 KG OF WATER EVERY MINUTE A DISTANCE OF 12 M?
1. m = 1500 kg
t = 60 s d = 12 m
 2. Equations
P = W/t
W = Fd
F = mg
 3. Plug and chug
F = mg = (1500 kg)(9.8 m/s2) = 14,700 N
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W = Fd = (14,700 N)(12 m) = 1.76 x 105 J
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P = W/t = (1.76 x 105 J)/(60 s) = 2940 W
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http://www.physicsclassroom.com
What is Energy?
It turns out that energy is so fundamental, like space and
time, that there is no good answer to this question. However,
just like space and time, that doesn't stop us from doing very useful calculations with
Energy
We may not be able to define energy, but because it is a
conserved property of nature, it's a very useful idea.
POTENTIAL ENERGY
Potential Energy (PE):
Stored energy due to position
Examples:
rock on a cliff, battery, food,
gasoline, stretched rubber band, apple
hanging in a tree
Gravitational Potential Energy
A barbell of mass "m" is lifted vertically upwards a distance "h" by an
outside force. How much work does that outside force do on the
barbell?
Fapp
mg
W = Fdparallel
Since a = 0, Fapp = mg
W = (mg) dparallel
Since F and d are in the same direction ...and dparallel = h
W = (mg) h
W = mgh
Gravitational Potential Energy
But we know that in general,
Eo + W = Ef.
If our barbell had no energy to begin
with, Eo = 0, then W = Ef
But we just showed that we did
W=mgh to lift the barbell... so mgh=Ef
The energy of a mass is increased by
an amount mgh when it is raised by a
height "h".
Gravitational Potential Energy
The name for this form of energy is
Gravitational Potential Energy (GPE).
GPE = mgh
One important thing to note is that while changes in gravitational potential energy are
important, their absolute value is not.
Gravitational Potential Energy
You can define any height to be the zero
for height...and therefore the zero for
GPE.
0.5 m
But whichever height you choose to call
zero, changes in heights will result in
changes of GPE. For example, the floor
level can be considered zero energy or
the ladder level can be zero.
0m
0.5 m
0m
GRAVITATIONAL PE
Gravitational PE (GPE):
Energy stored by objects that are above the
earth’s surface (objects that can fall)
Depends on mass, acceleration and height
GPE increases with height
GRAVITATIONAL PE
GPE = mass  gravity  height
GPE = m g h = weight  height
GPE = m (kg)  9.8 m/s2  h (m)
j = 1 Nm
What is the change of GPE for a 5.0 kg object which is raised from the
floor to a final height of 2.0m above the floor?
answer
9
As an object falls, its GPE always _____.
A
increases
B
decreases
C
stays the same
answer
10
What is the change of GPE for a 8.0 kg object which is lowered from an
initial height of 2.0 m above the floor to a final height of 1.5m above the
floor?
answer
11
What is the change in height of a 2.0 kg object which gained 16 J of GPE?
answer
12
GPE=mg
h = GPE
h = 16/(2
h = 0.82m
KINETIC ENERGY
Kinetic Energy (KE):
Energy in the form of motion
Depends on mass and velocity of
moving object.
Object in motion has ability to do work
http://www.youtube.com/watch?featur
e=player_detailpage&v=0ASLLiuejAo
Kinetic Energy
The energy an object has by virtue of its motion
is called its kinetic energy. The symbol we will
be using for kinetic energy is KE.
Like all forms of energy, it is measured in
Joules (J).
The amount of KE an object has is given by:
KE = 1/2 mv2
KINETIC ENERGY
KE = ½ mass  velocity2
KE = m  V2
2
(j) = (kg)  (m/s)
1 j = 1 kg m/s
As an object falls, its KE always _____.
A
decreases
B
increases
C
stays the same.
answer
13
A ball falls from the top of a building to the ground below. How does
the kinetic energy (KE) compare to the potential energy (PE) at the top
of the building?
A
KE = PE
B
KE > PE
C
KE < PE
D
It is impossible to tell.
answer
14
What is the kinetic energy of a 12 kg object with a velocity of 10 m/s?
answer
15
What is the mass of an object which has 2400 J of KE when traveling at
6.0 m/s?
answer
16
17
A 3 kg object has 45 J of kinetic energy. What is its velocity?
If the speed of a car is doubled, the KE of the car is:
A
quadrupled
B
quartered
C
halved
D
doubled
answer
18
Which graph best represents the relationship between the KE and the
velocity of an object accelerating in a straight line?
C
A
KE
KE
v
v
B
D
KE
KE
v
v
answer
19
20
The data table below lists mass and speed for 4 objects. Which 2 have
the same KE?
A
B
B and D
C
A and C
D
B and C
answer
A and D
Elastic Potential Energy
Energy can be stored in a spring, this energy is called
Elastic Potential Energy.
Robert Hooke first observed the relationship between the
force necessary to compress a spring and how much the
spring was compressed.
Elastic Potential Energy
The energy imparted to the spring by this work must be stored in the Elastic Potential
Energy (EPE) of the spring:
EPE
EPE = 1/2 k x2
Like all forms of energy, it is measured in Joules (J).
Determine the elastic potential energy stored in a spring whose spring
constant is 250 N/m and which is compressed 8 cm.
EPE= 1/2 Kx2
X=distance compressed
K=spring constant
answer
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What is the spring constant of a spring that is compressed 5 cm and has
0.65 J of elastic potential energy stored in it?
answer
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EPE = 0.5 kx2
k = EPE/0.5x2
k = 0.65 / 0.5 (0.052)
k = 520 N/m
k = 1176 N/m
The same 3 kg mass compresses the same spring 2.5 cm. How much elastic
potential energy is stored in the spring?
answer
25
LAW OF CONSERVATION OF ENERGY
The law of Conservation of Energy:
Energy cannot be created or destroyed. It may
be transformed from one form into another;
however, the total amount of energy in the
universe remains constant. (Transformers)
LAW OF CONSERVATION OF ENERGY
Energy conversions occur without a gain or loss
in energy
Energy into a system = energy out of a system
Due to friction, energy might seem to be lost,
but it has changed into thermal energy.
.
ENERGY ANALOGY
When energy is transferred, it can transform
(change form) but it still remains energy.
Analogy:
How is energy like money?
When money is transferred from one person or place
to another it can change form (transform) but it still
remains money.
ENERGY TRANSFORMATIONS (BALL)
Demonstrate: how bounce height of ball becomes lower and
lower each time it bounces. Have students infer why this
happens.
Each time the ball bounces, part of its energy is
transformed into other forms of energy, such as
thermal (heat) energy, sound energy and
vibrations in the ground. In addition, some
energy is absorbed by the ball. Therefore, it will
never bounce as high as the initial drop height.
ENERGY TRANSFORMATIONS
Ex: A light bulb is a device that transforms
electrical energy into electromagnetic (light)
energy and thermal energy
Chemical energy (coal)
heat energy (burn
to create steam)
mechanical energy
(steam is used to turn turbines)
Electromagnetic energy (generates
electricity)
heat energy (blow drier,
oven)
ROLLER COASTER
PE: 354kJ
KE: 0kJ
V: 0m/s
h=70m
PE: 0kJ
KE: 354kJ
V: 37.1m/S
PE: 0kJ
KE: 354kJ
V:
37.1m/S
Potential energy
becomes Kinetic
energy.
Kinetic
energy can
become
Potential
177kJ
energy. PE:
KE: 177kJ
h=35
V: 26.2m/S
m
Conservation of Energy
A roller coaster is at the top of a track that is 80 m high. How fast will it be going at the
bottom of the hill?
Eo + W = Ef
Eo = Ef
W=0
GPE = KE
E0 = GPE, Ef = KE
mgh = 0.5mv2
Substitute GPE and KE equations
v2 = 2gh
Solving for v yields
v2 = 2 (9.8) 80
v =39.6 m/s
answer
A student uses a spring (with a spring constant of 180 N/m) to launch a marble
vertically into the air. The mass of the marble is 0.004 kg and the spring is compressed
0.03 m. How high will the marble go?
A student uses a spring gun (with a spring constant of 120 N/m) to launch a marble
vertically into the air. The mass of the marble is 0.002 kg and the spring is compressed
0.04 m.
answer
a)How high will the marble go?
answer
A roller coaster has a velocity of 25 m/s at the bottom of the first hill.
How high was the hill?
answer
A 5 kg rock is dropped a distance of 1 m onto a spring. It compresses the spring 2 cm.
What is the spring constant?
SIMPLE AND COMPOUND MACHINES
There are six types of simple machines:
 Inclined
plane
 Wedge
 Screw
 Lever
 Pulley
 Wheel
and axle
 http://www.youtube.com/watch?feature=player_de
tailpage&v=jAPxALm9fZA
THE 6 SIMPLE MACHINES
Inclined Plane
Screw
Pulley
Lever
Wedge
Wheel and Axle
The 6 Simple Machines
Inclined Plane
Screw
Pulley
Lever
Wedge
Wheel and Axle
INCLINED PLANE, WEDGE, SCREW
A ramp is an example of an inclined plane
 Simply put in inclined plane is a flat slanted
surface
 A wedge is an inclined plane that moves and is
usually made up of 2 inclined planes
 The screw is an inclined plane wrapped around
a center bar
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INCLINED PLANES
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An inclined plane is a
flat surface that is
higher on one end
Inclined planes make
the work of moving
things easier
SCREW
The mechanical advantage of an screw can be
calculated by dividing the circumference by the pitch of
the screw.
Pitch equals 1/ number of turns per inch.
WEDGES
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Two inclined
planes joined
back to back.
Wedges are used
to split things.
LEVER AND PULLEY
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A lever is a rigid bar that pivots or moves around
a fixed point. A seesaw is an example
Fulcrum is the fixed point of a lever
A pulley is a rope, belt or chain wrapped around a
grooved wheel
A pulley can change the direction of a force or
the amount of a force
When you use a pulley you change the direction
of the force you are applying.
WHEEL AND AXLE
A wheel and axle is a simple machine made up
of two circular objects of different sizes
 The wheel is the larger object the axle is the
smaller one
 Bicycle is an example of a wheel and axle.. The
bike wheel is the large while and the sprocket
the chain wraps around is the axle

MECHANICAL ADVANTAGE Demo: Use a ramp and 4 books and a spring scale
and measure distance to move the 200g mass up
vertically and horizontally on a ramp
Create a data table use books as height w/ 200g
hanging mass
1st Write a hypothesis –more –less- the same-work
2nd calculate the work for 1. vertically-straight up
2. up the ramp
THERE ARE 2 TYPES OF MECHANICAL ADVANTAGE.
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IMA – Ideal mechanical
advantage.
This is the number of
times a machine is
designed to multiply your
effort force.
It is based on
measurements of the
machine.
Ignores friction
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AMA – Actual mechanical
advantage
This is the number of
times the machine actually
multiplies your effort
force..
Includes the effects of
friction
IMA is always greater than AMA.
MECHANICAL ADVANTAGE
Mechanical Advantage – when you increase
distance you decrease force but the work
remains the same.
 Machines –

Multiply force
 redirect force- ie: pull down rope –lifts sail

 work

force
equation-
x
distance = work
Machines do not increase the amount of work.
They spread out the distance so you don’t have
to use the same amount of force to receive the
same amount of work.
 Prove it:
 Work 32 J
=
work 32 J
 Force x distance
force x distance
 8N x 4 m
4Nx8m

Ideal Mechanical advantage = ratio between
output force and input force or output distance
and input distance without friction
 If you have force information use:
 Output force /Input force = MA

If you have distance information use:
 Input distance/output distance = MA
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Mechanical advantage – multiplying force if you
need 3200 N to lift a piano then use a ramp to
exert 1600 N of force.
OF 3200N = 2 the ramp doubled your
 IF 1600N
force.
 Your output force is 2x your input force.
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MA- is 2 no units
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Mechanical Advantage – multiplying distance you use a ramp that is 6 meters long to raise a
piano 3 meters
ID- 6 meters
=2
the ramp doubled
 OD 3 meters
the distance
 mechanical advantage of two
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Write a paragraph on what you now know and
did it differ from what you knew before,

Mechanical advantage to machines problem
set /answers

http://library.thinkquest.org/CR0210120/Mec
hanical%20Advantage.html
MACHINES
An instrument that makes work easier is
called a machine
 Machines do not have to be complex
electrical or gas powered deviced. Even
simple objects can be a machine.
 A pair of pliers would make it easier to
take out a bolt so the pliers would be a
machine

MACHINES CONT.
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There are two types of work involved in using a
machines:
 Work
that goes into the machine (input)
 Work done by the machine (output)
Work that comes out of the machine is NEVER
greater than the force that is applied to the
machine or work that goes into the machine
MACHINES CONT.
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Machines make work easier because they
change either the size or the direction of the
force put into the machine.
Machines multiply either the force or distance to
make work easier, but never both!
The comparison of the work output to the work
input is called efficiency.
The closer the amount of output is to the
amount of input the more efficient the machine
is.
EFFICIENCY CONT.
Efficiency is measured in percent and is
never more than 100%. This is because
the output can never be more than the
input
 The lower the friction of the machine the
more efficient it will be. Keeping a car
engine oiled makes it work better and
more efficient

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Efficiency – a measure of how much work that
is put into a machine is changed to useful
work; answer will be a percentage.

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efficiency = Wout x 100%
Win
Win = work put into the machine
Wout = work put out by the machine
EFFICIENCY
What are some factors that may make a
machine inefficient?

A wooden ramp is used to push a box into the
back of a truck. Mary must do 800 J of work to
move the box. If there was no friction, she
would only have to do 700 J of work. What is
the efficiency of the machine

A rusty pulley is used to raise a pail 5 m off the
ground. If the pulley was perfect, only 5000 J
of work would have to be used. Because the
pulley is rusty, 6500 J of work must be done.
What is the efficiency?

If a machine could do 40 J of work but is only
75% efficient, what is the amount of work the
machine actually does?
A windmill has an efficiency of 47%. If the
wind does 250 J of work on the blades of the
windmill, how much work output can the windmill
do?
Fr = resistance/output force

dr = resistance/output distance
Win = Fede
Fe = effort/input force

de = effort/input distance

For an ideal machine:

Win = Wout

Fede = Frdr

Wout = Frdr

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A worker applies an effort force of 20 N to
pry open a window with a resistance force of
500 N. Find the mechanical advantage of
thecrowbar.

Fe = 20 N
Fr = 500 N
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MA = Fr = 500 N
Fe 20 N
MA = 25
MA = ?

Find the effort force needed to lift a 2000 N
rock, using a jack with a mechanical
advantage of 10.
 Fr
= 2000 N
MA = 10

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
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
MA = Fr / Fe
Fe = Fr / MA
Fe = (2000 N)/(10)
Fe = 200 N
Fe = ?
SOURCES USED
www.phs.d211.org/Science/okeefenm/Okeefe
/Okeefe/PhySci233/EnergyMachines/Mechani
cal%20Advantage.ppt –
 www.cwcboe.org/gcms/teachers/apanagiotaki
s/Notes/Work%20&%20Power/Mechanical%2
0Advantage%20and%20Efficiency.ppt - Similar
pages
 education.jlab.org/jsat/powerpoint/0708_simp
le_machines_8.ppt 