Transcript Work and Power

Work and Power
Simple Machines
Kinetic Energy
Potential Energy
Equations used so far . . .
d
v
F
t
m a
(vf-vi)
a
t
(y2-y1)
slope
(x2-x1)
W
m
g
Work

Occurs when a force causes something
to move a distance
 Motion must be in the same direction as the
force
Is this an example of work?
force
distance
yes
Is this work?
force
distance
Yes…if he
LIFTS the
barbell
Is this work?
distance?
force
No…why not?
Is this work?
force
distance
No…why not?
Work Equation
Work = Force x Distance
W = Fd
W
F
d
Units for Work
W = Fd
Or
Practice Problem
John uses 45 Newtons of force to push his
lazy dog 3.2 meters across the kitchen
floor. How much work does John do?
F = 45 N
d = 3.2 m
W=?
W = Fd
W = (45 N)(3.2 m)
W = 144 J
s.f.: 140 J
Power

The rate at which work is done
 In other words…how fast work is done
Power = Work
time
P= W
t
W
P
t
Joules = Watts (W)
second
Practice Problem
If John takes 5.0 seconds to push his dog,
what is his power output?
W = 144 J
t = 5.0 s
P=?
P = W/t
P = 144 J/5.0 s
P = 28.8 W
s.f.: 29 W
A crane uses an average force of 5200 N to lift a
girder 25 m. How much work does the crane do?
An apple weighing 1 N falls a distance of 1 m.
How much work is done on the apple by the force
of gravity?
A bicycle's brakes apply 125 N of friction force to
the wheels as the bike moves 14.0 m. How
much work do the brakes do?
A mechanic uses a hydraulic lift to raise a 1200
kg car 0.50 m off the ground. How much work
does the lift do on the car?
While rowing across the lake during a race, John
does 3960 J of work on the oars in 60.0 s. What
is his power output in watts?
Anna walks up the stairs on her way to class.
She weighs 565 N, and the stairs go up 3.25 m
vertically.


A. What is her power output if she climbs the stairs in 12.6 s?
B. What is her power output if she climbs the stairs in 10.5 s?
Assignment:
 Practice
Problems:
W
 p. 432 #1-4
1. 130000 J
2. 1 J
3. 1750 J
4. 5900 J
 p. 434 # 1-2
1.66 W
2a. 146 W
2b. 175 W
F
d
W
P
t
Review
What does the term “work”
mean in your everyday
life?
What does the term “work”
mean in Physical
Science?
 Work is the ability to
produce a force that
causes movement.
Work and Power Practice Problems:
Mixed Equations (Front only)
W = 237825 J
2. d = 0.6 m
3. F = 27 N
4. a) P = 494 watts
4. b) d = 3 m
6. W = 300 000 J
1.
Mixed Practice
(complete the back)
d
W
F
v t
m a
Velocity
Work
(vf-vi)
a
(y2-y1)
slope
(x2-x1)
Slope
F d
Force
t
W
W
Acceleration
m
g
Weight
P t
Power
Review





What does the term
“power” mean in
Physical Science?
Power is the rate (how
fast) an individual is
able to do work.
What are the two main
families of simple
machines?
1. Lever Family
2. Inclined Plane Family
HOUSEHOLD TOOLS MINI-LAB
Household Tools Mini-Lab
Purpose: To relate household tools to the six simple machines.
Tool:
Type of
machine(s)
in this tool:
Is the applied force
transferred to another part
of the tool? Which part of
the tool does the work?
Is the force that the tool exerts
on an object greater or lesser
than the force exerted on the
tool? Explain.
Simple Machines Reading Guide 13.2
Lever Family
Inclined Plane Family
The Lever Family

Lever

Pulley

Wheel & Axle
Lever Family
*Levers-First Class
In a first class lever
the fulcrum is in the
middle and the
resistance and
effort is on either
side
 Think of a *see-saw

*Levers-Second Class
In a second class
lever the fulcrum is
at the end, with the
resistance in the
middle
 Think of a
*wheelbarrow

Levers-*Third Class
In a third class lever
the fulcrum is again
at the end, but the
effort is in the
middle
 Think of a pair of
*tweezers

st
1
Class
nd
2 Class
3rd Class
– Fulcrum
–
(F)
Resistance (R)
– Effort
(E)
Levers
Levers make work easier by reducing
the amount of force necessary to move
a load.
 Mechanical Advantage of a lever is
equal to effort arm divided by the
resistance arm.
 MA =
effort

resistance
Lever Family
Pulleys
The pulley distributes the force of the load among
several ropes in the system
 Using a single pulley does not multiply the input
force, but it does change the direction of the input
force.
 Using several pulleys increases the distance the of
the input force causing a larger output force. The
mechanical advantage is equal to the number of
ropes sharing the load.
 MA = # of ropes-1 or MA = # of pulleys
 Figure 3 pg 440

Lever Family
Wheel and Axle
A wheel and axle system makes work
easier by increasing a small amount
input force applied to the wheel.
 Mechanical Advantage is the wheel
radius divided by the axle radius.
 MA = radius wheel

radius axle
The Inclined Plane Family

Inclined plane

Wedge

Screw
Inclined Plane Family
Inclined Plane
An inclined plane makes work easier by
increasing the distance over which force is
applied. Less input force is required to lift a
load.
 Mechanical Advantage of an inclined plane is
equal to the distance of the sloped edge
divided by the height.
 MA =
slope
height

Inclined Plane Family
Wedge
A wedge is two inclined planes put back
to back.
 Mechanical Advantage of a wedge is
the slope length divided by the width of
the wedge.
 MA = slope

width
Screw
A screw is an inclined plane wrapped
around a cylinder.
 Mechanical Advantage of a screw is
length of the treads divided by the
diameter of the cylinder.
 MA = length

diameter
MA = F out
F in
WORK & MACHINES
How do machines make work
easier?
 By changing the size or direction of the
FORCE
 Machines DO NOT make work less
 Force is less, therefore distance is greater
W=F
d
Example: Lever
Less force
Greater distance
Example: Inclined Plane
Less force
Greater
distance
Work input (Win)
 The work that the operator/user does to the
machine
 Input force (Fin)—force exerted by the user
 Input distance (din)—distance covered by the user
Win = Fin x din
Work output (Wout)
 The work that the machine does to another
object
 Output force (Fout)—force exerted by the machine
 Output distance (dout)—distance covered by the
machine
Wout = Fout x dout
Hints for solving problems
 Input Force is always less than Output force
(Fin < Fout)
 Input distance is always greater than the
output distance
(din > dout)
Mechanical Advantage (MA)
 How much a machine multiplies force or
distance
 Should always be greater than 1 (or it isn’t a
very good machine!)
Equation
MA = Fout
Fin
or
MA = din
dout
Sample Problem #1
A bus driver applies a force of 55.0 N to the
steering wheel, which in turn applies 132 N of
force to the steering column. What is the
mechanical advantage of the steering wheel?
What simple machine is being used here?
If the bus driver turns the wheel
1.40 meters, how much work does
she do?
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Sample Problem #2
Two moving men are pushing a 225 N bookcase
into the back of a moving truck by exerting
75.0 N of force. The ramp is 3.00 meters long
and rises 1.00 meters to the inside of the
truck. A) How much work do the men do? B)
How much work does the ramp do? C) What
is the mechanical advantage of the ramp?
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Review
Why do we use simple
machines?
 Simple machines are
used to make work
easier.
List the two equations for
Mechanical Advantage
 MA = F out / F in
 MA = d in / d out
Thursday, October 3, 2013
BR: When you calculate Mechanical
Advantage, who is responsible for
 Input
 Output


EQ: A student helps his teacher by
lifting a heavy box, carrying it across
the room, and putting it on the lab
table. Did the student do work? When?

Agenda:
 Work and
Power Lab
Review
When you calculate Mechanical
Advantage, who is responsible for
 Input
 Output
A student helps his teacher by lifting a
heavy box, carrying it across the room,
and putting it on the lab table. Did the
student do work? When?
 Yes, only when lifting the box because
his net force and the movement were
in the same direction.
Review

Work

Power
Review

6 Simple Machines

Mechanical Advantage
Review
BR: What is in the middle
of each class of lever?
 1st
 2nd
 3rd

EQ: What are the
appropriate equations and
units for work and power?
 Work = F x d (Joules)
 Power = Work / time
(watts)

SP3. Students will relate
transformations and flow of
energy within a system.
a. Identify energy transformations
within a system (exmp. Lighting of
a match.)
Key Concepts
Vocabulary
• There are many
different forms of
energy
• Energy can change from
one form to another
• Energy
• Law of conservation of
energy
POTENTIAL
ENERGY
NUCLEAR
LIGHT
ENERGY
KINETIC
ENERGY
CHEMICAL
SOUND
HEAT
ELECTRICAL
Energy may change from one form
to another, but total amount of
energy in a system never
changes.
Examples of Energy Transformations
• Burning a match
– Chemical energy
light, heat
• Bouncing a ball
– Potential energy  kinetic energy, sound, and
heat
Examples
• Photosynthesis
– Light  chemical
energy
What energy transformations take place in
the following scenario?
A local farmer raises cattle
for beef. On Friday
night, you go out to your
favorite restaurant and
eat a hamburger.
Heat
Nuclear
Light
Heat
Chemical
Heat
Chemical
Kinetic
Kinetic
Review
What forms of
energy do you
encounter each day?
What energy
transformations take
place regularly in
your everyday life?
Group Potential Energy-Calculate the gravitational
potential energy of the following:
A 1,200 kg car at the top of a hill that is 42 m high.
A 65 kg climber on top of Mount Everest (8,800 m high).
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Group Potential Energy-Calculate the gravitational potential
energy of the following:
A 0.52 kg bird flying at an altitude of 550 m.
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Group Kinetic Energy-Calculate the kinetic energy in joules of a
1,500 kg car that is moving at a speed of 15 m/s.
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Review
Kinetic energy is displayed while an object is __ ____.
IN MOTION
List the equation for calculating Kinetic Energy and provide
the appropriate triangle.
KE = ½ mv 2
Review
Potential energy is displayed due to an object’s
____.
POSITION
List the equation for calculating Potential Energy
and provide the appropriate triangle.
PE = mgh
Review
Answer the following based on yesterday’s lab
When you drop a ball, where does it have
the greatest amount of . . .
• Potential energy?
At the top just before it is released
• Kinetic energy?
At the bottom just as it hits the ground
List the equation for calculating velocity using
Kinetic Energy.
. KE .
V = (1/2)m
Review
A skydiver prepares to jump out of a plane. At what point will
she have the greatest kinetic energy? potential energy?
The greatest kinetic energy occurs at the lowest point-just
before she reaches the ground.
The greatest potential energy occurs just at the highest
point-just before she begins to jump out of the plane
Describe the energy transformation-Ms. Berrie turns her
projector on and shows the movie “Osmosis Jones” to her
students.
Electrical  light + sound
Energy
Problems
Energy TestCalculating Velocity
Additional Group Practice
You drop a 2 kg watermelon from a 5m tall ladder.
How fast is the melon traveling when it strikes the
ground?
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Additional Group Practice
You drop a 0.250 kg baseball from a height of 6 m.
How fast is the ball traveling when it strikes the
ground?
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Additional Group Practice
After 10 s of free-fall, a 70 kg skydiver has 30.5 J of
kinetic energy. How fast is he traveling towards earth
at that instant?
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Additional Group Practice
After 120 s of free-fall, the same 70 kg skydiver has
220 J of kinetic energy. How fast is he traveling
towards earth?
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Individual Practice
 How
fast is a 0.35 kg ball traveling as it hits the
ground if it is dropped a total distance of 2 m?
A
30 kg cannon ball is shot from a cannon and
reaches a maximum of 410 J of kinetic energy.
How fast is it travelling at that particular
moment?
A
welder working on a new office building drops
his construction hat as he reaches the top of the
50 m building. If the mass of the hat is 1.5 kg,
how fast is the hat travelling when it hits the
ground?
Review
BR: Based on the law of conservation of energy, what do you
know about the total potential energy at the top of a fall and
the total kinetic energy at the bottom of a fall?
The total amount of energy remains the same.
PE top = KE bottom
EQ: Describe the following energy transformation:
Mrs. Molyson drinks a cup of Gatorade then runs a 5k.
Chemical  Kinetic
Roller Coaster Review Part I
Review
Based on the law of conservation of energy, what do you know
about the total potential energy at the top of a fall and the total
kinetic energy at the bottom of a fall?
The total amount of energy remains the same.
PE top = KE bottom
Describe the following energy transformation:
Mrs. Molyson drinks a cup of Gatorade then runs a 5k.
Chemical  Kinetic
Study Guide: Work, Power,
Machines, Energy
Equations
Work
Power
Mechanical Advantage
Kinetic Energy
Potential Energy
Review
List the 3 requirements for work.
Force
2. Distance
3. F & d must be in the same direction.
1.
Review
Where would you expect the greatest KE? greatest PE?
KE ___________
PE ___________
A measure of the amount of KE in a material is ____
HEAT / TEMPERATURE