Download Work Energy Heat

1/4 Bell Work
A survey of 50 editors showed that 20 could write with
their left hand and 10 could write with either hand.
 How many could write with their right hand?
1/4 Schedule
 Notes Ch 13.1 “Work, Power, and Machines”
 Simple Machines Station 1
 Make sure surface is level.
 Keep packet for later.
 Start CR 13.1
Notes:
1. Ch 13 “Work” packet – TBA
2. Concept CR 13.1 due WED
Unit: Work,
Energy, and
Heat
Ch 13-14 Objectives:
1. Describe how changes in form relate to
conservation of energy.
2. Calculate work, energy, heat and temperature.
3. Explain transfer of energy and heat.
Ch 13.1 “Work, Power,
and Machines”
Objectives:
〉 Calculate work.
〉 Describe the relationship between work and
power.
〉 Explain how machines make work easier.
Work, work, work?
1. Which of the following is an example of work:
bowling or reading?
2. Are the following work?
a. A man pushes against a brick wall, which doesn’t
move. Is this an example of work?
b. A student carries her books to class. Is this an
example of work?
Work, work, work?
c. A woman raises and lowers dumbbells at the gym. Is
this an example of work?
d. A book falls off a table and lands on the floor. Is this
an example of work?
What Is Work?
Work Definition
 The transfer of energy to an object by a force that
causes the object to move in the direction of the force
 Work is zero when an object is not moving.
 Work is measured in joules (J):
1 N • m = 1 J = 1 kg • m2/s2
What Is Work?
Work Definition
 Calculated by multiplying the force by the
distance over which the force is applied.
 work = force x distance, or W = Fd
 The force must be applied in the direction of the
object’s motion.
Math Skills
Work
Imagine a father playing with his daughter by lifting her
repeatedly in the air. How much work does he do with each
lift if he lifts her 2.0 m and exerts an average force of 190
N?
1. List the given and unknown values.
Given:
force, F = 190 N
distance, d = 2.0 m
Unknown:
work, W = ? J
Math Skills, continued
2. Write the equation for work.
work = force  distance
W=f d
3. Insert the known values into the equation, and solve.
W = 190 N  2.0 m = 380 N•m
W = 380 J
Power
Work vs Power
〉 Power is the rate at which work is done, or how
much work is done in a given amount of time.
 Power is measured in watts (W):
1 W = 1 J/s
work
W
power =
, or P =
time
t
Math Skills
Power
Lifting an elevator 18 m takes 100 kJ. If doing it takes 20 s, what
is the average power of the elevator during the process?
1. List the given and unknown values.
Given:
work, W = 100 kJ = 1  105 J
time, t = 20 s
Distance is not needed.
Unknown:
power, P = ? W
Math Skills, continued
2. Write the equation for power.
work
power =
time
W
P=
t
3. Insert the known values into the equation, and solve.
1´ 105 J
P=
= 5 ´ 103 J/s
20 s
P = 5 ´ 103 W = 5 kW
1/5 Bell Work
Knowledge is knowing a tomato is a fruit. Wisdom is not
putting it in a fruit salad.
- Peter Kay, English comedian
 What do you think Peter Kay means?
1/5 Schedule
 Notes Ch 13.1 “Work, Power, and Machines” – 13.2
“Simple Machines”
 Simple Machines Station 2
 Keep packet for later.
 CR 13.1-13.2
Notes:
1. Ch 13 “Work” packet – TBA
2. Concept CR 13.1 due WED
Machines and
Mechanical Advantage
Machines make work easier.

changing the size of an input force, the direction of the
force, or both.
Machines and
Mechanical Advantage
 Look at the sodas containers below.
 What tools could you use?
 Why do these tools make it easier to open the sodas?
Machines and Mechanical
Advantage
Mechanical advantage is a useful ratio.
 mechanical advantage: expresses how much machines
multiply force or distance
 AKA: How much easier is the task?
output force
input distance
mechanical advantage =
=
input force output distance
Math Skills
Mechanical Advantage
Calculate the mechanical advantage of a ramp that is 5.0
m long and 1.5 m high.
1. List the given and unknown values.
Given:
input distance = 5.0 m
output distance = 1.5 m
Unknown:
mechanical advantage = ?
Math Skills, continued
2. Write the equation for mechanical advantage.
We need only the distance part of the full equation:
input distance
mechanical advantage =
output distance
3. Insert the known values into the equation, and solve.
mechanical advantage =
5.0 m
= 3.3
1.5 m
Ch 13.2 “Simple
Machines”
Objectives:
〉 Identify the 6 types of simple machines and
give examples.
〉 Describe and locate the two different parts of
all levers.
〉 Explain how using an inclined plane changes
the amount of force needed to move an object.
Open Door, Open Mind
Doors are actually machines,
specifically levers.
When you exert a force on it
(input), that force is exerted
along the entire door
(output).
Open Door, Open Mind
1. There is always one point along the lever that remains
still while the rest moves. This point is the fulcrum.
Where is the fulcrum of a door?
2. You can push at any point along the width of a door
and it will open. Which position requires the least force:
near the hinges, in the middle, or the side farthest from
the hinges? (Hint: Which feels easiest?)
3. If you are trying to prop the door open with a small,
light doorstop, where would you place the doorstop?
What Are Simple
Machines?
6 Simple Machines
Types:
〉 Lever
〉 Pulley
〉
Wheel and axle
〉
Inclined plane
〉 Wedge
〉
Screw.
What Are Simple
Machines?
Simple machines are divided into two families: the lever
family and inclined plane family.
Lever family:
 simple lever
 pulley
 wheel and axle
Inclined plane family:
– simple inclined plane
– wedge
– screw
The Lever Family
Levers
 Parts: 1. Rigid arm 2. Pivot point/Fulcrum
 Three classes
 1st: Fulcrum between input and output force, EX see
saw
 2nd: Fulcrum at end, smaller input = bigger output, EX
wheelbarrow  multiply force
 3rd: Fulcrum at end, big input = smaller output, EX
muscles  multiply distance
The Lever Family,
continued
Fulcrum located in middle.
Force applied to end.
Force applied middle..
The Lever Family,
continued
Pulleys are modified levers.
 The middle of a pulley is like the fulcrum.
 The rest of the pulley behaves like a first-class lever.
 A wheel and axle is a lever or pulley connected to a shaft.
 Screwdrivers and cranks are common wheel-and-axel
machines.
The Mechanical Advantage
of Pulleys
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1/6 Bell Work
Pete Platform and Gary Gladhand met at a club to discuss
the overthrow of their party leader. They each ordered
Pepsi on the rocks. Gary downed his and ordered
another. He gulped down the second, but decided to wait
on the third. Meanwhile, Pete, who was sipping his drink
suddenly dropped over dead. Both men were set up for
assassination.
 Where was the poison? Why did Gary survive, but
Pete died?
1/6 Schedule
 Notes Ch 13.2 “Simple Machines”
 Simple Machines Station 3
 Write procedure 3a
 Keep packet
 CR 13.1-13.2
Notes:
1. Ch 13 “Work” packet – TBA
2. Concept CR 13.1-13.2 THURSDAY
The Inclined Plane
Family
Incline planes and Force
〉 Pushing an object up an inclined plane
requires less input force than lifting the same
object does.
The Inclined Plane Family
 A wedge is a modified
inclined plane.
 A screw is an inclined plane
wrapped around a cylinder.
Compound Machines
Simple + Simple = Compound Machine
〉 EX: scissors use two first-class levers joined at a
common fulcrum; each lever arm has a wedge
that cuts into the paper.
Compound Machines
 More Examples and Results
Fulcrum-> Load-> Effort
Magnifies force to crack nut
Fulcrum-> Effort-> Load
Magnifies movement, low force
1/7 Bell Work
If a block weighs 8 kg plus half a block, how much does a
block and a half weigh?
1/7 Schedule
 Notes Ch 13.3 “What is Energy?”
 Simple Machines Lab 3 and 5
 Cross off Station 4
 Station 5: draw and label machines
 Turn in
 Practice with Simple Machines at
 http://www.msichicago.org/play/simplemachines/
Notes:
1. Ch 13 “Work” packet – MONDAY
2. Simple Machines Lab – FRIDAY
Ch 13.3 “What is
Energy?”
Objectives:
〉 Describe the relationship between work and energy.
〉 Explain the relationship between potential and kinetic
energy.
〉 Define nonmechanical energy.
Energy Transfer
Energy is always conserved. It just changes from one form to
another. For example, sunlight is the ultimate source of
energy on Earth. Look at the illustration below, and identify
the types of energy involved.
Energy Transfer
1.
How does sunlight provide the energy the girl needs
to swing the bat? (Hint: What do you need to have
energy?)
2.
When the girl hits the ball, she exerts a force on it.
Does she do work on the ball in the scientific sense of
the term? Explain your answer.
3.
After the girl hits the ball, the ball moves very fast and
has energy. When the ball hits the fielder’s glove, it
stops moving. What happens to the energy the ball
once had? (Hint: What do you hear and feel as you
catch the ball?)
Energy and Work
Relating Energy and Work
〉 Whenever work is done, energy is transformed or
transferred.
〉 Energy: the capacity to do work
 Energy is also measured in joules (J).
Potential Energy
Potential energy: the energy that an object has
because of the position, shape, or condition of the
object
〉 Potential energy (PE) is sometimes called energy
of position because it results from the relative
positions of objects in a system.
Potential Energy, continued
Major Types of Potential Energy
 Elastic Potential Energy: any object that is stretched or
compressed to increase or decrease the distance between its
parts.
 EX: stretched bungee cords, compressed springs
 Gravitational Potential Energy: any system of two or more
objects separated by a vertical distance.
 EX: a roller coaster at the top of a hill, skydiving
Potential Energy, continued
 Gravitational potential energy depends on both mass and
height.
 grav. PE = mass  free-fall acceleration  height, or
PE = mgh
 The height can be relative…. “with respect to _________”
 EX floor, ceiling, Earth, cliff…
Math Skills
Gravitational Potential Energy
A 65 kg rock climber ascends a cliff. What is the climber’s
gravitational potential energy at a point 35 m above the base of
the cliff ?
1. List the given and unknown values.
Given:
mass, m = 65 kg
height, h = 35 m
free-fall acceleration, g = 9.8 m/s2
Unknown:
gravitational potential energy, PE = ? J
Math Skills, continued
2. Write the equation for gravitational potential energy.
PE = mgh
3. Insert the known values into the equation, and solve.
PE = (65 kg)(9.8 m/s2)(35 m)
PE = 2.2  104 kg•m2/s2
PE = 2.2  104 J
1/8 Bell Work
Approximately how far would a tire with a 2 foot
diameter roll in 700 revolutions?
Hint: Use circle equations in planner.
1/8 Schedule
 Notes Ch 13.3 “What is Energy?”
 Simple Machines Lab 3 and 5
 Cross off Station 4
 Station 5: draw and label machines
 Turn in
 Practice with Simple Machines at
 http://www.msichicago.org/play/simplemachines/
Notes:
1. Ch 13 “Work” packet – MONDAY
2. Simple Machines Lab – FRIDAY
1/8 Schedule
 Finish notes Ch 13.3 “What is Energy?”
 Turn in Simple Machines Lab
 Kinetic and Potential Energy Practice Wksht due
MONDAY
Notes:
1. Simple Machines Lab – TODAY
2. Ch 13 “Work” packet – MONDAY
3. Kinetic and Potential Energy Prac Wksht due MONDAY
More Potential Energy
Problems
Put these in your notebook.
PE = mgh
1. The world record for pole vaulting is 6.15 m. If the
pole vaulter’s gravitational potential is 4,942 J, what is
his mass?
2. What is the gravitational potential energy associated
with a 75 kg tourist at the top floor of the Sears Tower
in Chicago, with respect to the street 436 m below?
Kinetic Energy
Kinetic Energy Factors
〉 Kinetic energy depends on both the mass and the
speed of an object.
 KE = ½  mass  speed squared, or KE= ½mv2
 Does kinetic energy depend more on mass or velocity?
Kinetic Energy, continued
• Atoms and molecules have
kinetic energy.
Math Skills
Kinetic Energy
What is the kinetic energy of a 44 kg cheetah running at 31
m/s?
1. List the given and unknown values.
Given:
mass, m = 44 kg
speed, v = 31 m/s
Unknown:
kinetic energy, KE = ? J
Math Skills, continued
2. Write the equation for kinetic energy.
kinetic energy =
KE =
1
´ mass ´ speed squared
2
1
mv 2
2
3. Insert the known values into the equation, and solve.
KE = ½ (44 kg)(31 m/s)2 = 2.1 × 104 kg•m2/s2
KE = 2.1 × 104 J
More Kinetic Energy
Problems
KE= ½mv2
1. A cheetah can run briefly with a speed of 31 m/s.
Suppose a cheetah with a mass of 47 kg runs at this
speed. What is the cheetah’s kinetic energy?
2. The kinetic energy of a golf ball is measured to be
143.3 J. If the golf ball has a mass of about 47 g, what
is its speed? (Remember to convert g to kg.)
1/11 Bell Work
 Create an equation whose answer is 11 that also
includes fractions.
EX: (24/3) + 3 = 11
1/11 Schedule
 Finish notes Ch 13.3- start 13.4 “Converting”
 Work Time
 Kinetic and Potential Energy Practice Wksht due
TODAY
 Con Rev Ch 14 now due TUESDAY
Starting Solar Oven Project!
Bring in cardboard, foil, newspaper,
etc. for WEDNESDAY
Notes:
1. Simple Machines Lab – LATE
2. Kinetic and Potential Energy Prac Wksht due TODAY
3. Ch 13 “Work” packet – TUESDAY
Other Forms of Energy
Nonmechanical energy
〉 Energy at the level of the atom.
 mechanical energy: the amount of work an object can do
because of the object’s kinetic and potential energies
 In most cases, nonmechanical forms of energy are just
special forms of either kinetic or potential energy.
Other Forms of Energy, continued
Chemical reactions involve potential energy.
 The amount of chemical energy associated with a substance
depends in part on the relative positions of the atoms it
contains.
Living things get energy from the sun.
 Plants use photosynthesis to turn the energy in sunlight into
chemical energy.
The sun gets energy from nuclear reactions.
 The sun is fueled by nuclear fusion reactions in its core.
Fusion
Other Forms of Energy, continued
 Energy field storage.
 Electric fields result in electrical energy based on the
location of charged particles.
 When electrons move from an area of higher electric
potential to an area of lower electric potential, they gain
energy.
Electric field lines
Other Forms of Energy, continued
 Light carries energy across empty space.
 Light energy travels from the sun to Earth in the form
of electromagnetic waves.
 Electromagnetic waves are made of electric and magnetic
fields.
1/12 Bell Work
 If there are 6.02 x 1023 atoms in a mole, how many
moles are there in 18 x 1023 atoms of iron?
1/12 Schedule
 Finish notes 13.4 “Converting”
 Work Time
 Kinetic and Potential Energy Practice Wksht LATE
 Con Rev Ch 14 now due TODAY
Starting Solar Oven Project!
Bring in cardboard, foil, newspaper,
etc. for WEDNESDAY
Notes:
1. Simple Machines Lab – LATE
2. Kinetic and Potential Energy Prac Wksht LATE
3. Ch 13 “Work” packet – TODAY
Ch 13.4 “Conservation of
Energy”
Objectives
〉 Describe how energy changes.
〉 Define and apply the Law of Conservation of
Energy.
Converting Energy
You and your sled have gravitational potential energy when
you get to the top of a snowy hill. Then you slide down,
speeding up as you go.
Converting Energy
1. When does the sled have the most potential energy?
Least potential energy?
2. Where does the sled have the most and least kinetic
energy?
3. After the sled reaches the bottom of the hill, it coasts
across level ground and eventually stops. What
happened to the energy the sled had?
Energy Transformations
Potential can become kinetic energy.
 EX: As a roller coaster car goes down a hill, PE
changes to KE.
Kinetic can become potential energy.
 Example: The KE of a roller coaster car at the bottom
of a hill can do work to carry it up another hill.
The Law of Conservation of Energy
Law of Conservation of Energy: Saving/Keeping
Energy
 Energy cannot be created or destroyed.
 Total amount of energy in the universe never changes.
 Energy may change from one form to another.
 Light, heat, sound, …
The Law of Conservation
of Energy
Energy does not appear or disappear.
 Whenever total energy in a system increases, it enters
the system from an external source.
Thermodynamics describes energy conservation.
 1st Law of Thermodynamics = A net change in energy
equals the energy transferred as work and as heat.
 aka “No free lunch.”
The Law of Conservation
of Energy
Different Types of Systems
•
open system: energy and matter are exchanged with the
surroundings
•
closed system: energy but not matter is exchanged
•
isolated system: neither energy nor matter is exchanged
 Most real-world systems are open.
Connecting Potential and
Kinetic Energy
 Assuming that energy is conserved and there is no
friction, at the bottom of the hill the cart’s potential
energy is completely converted to kinetic energy. Use
the formulas to calculate the cart’s speed at the bottom
of the hill.
 What are the equations for potential and kinetic
energy?
?
m
Review Conservation of
Energy
If energy was conserved Ep = Ek. Otherwise it was “lost”.
1. Use the car’s potential energy at the top of the ramp.
Ep = mgh
2. It should equal the car’s kinetic energy when it hit.
Ek = ½*mv2
3. Solve for v.
Stop Notes
 Stop here for today.
Efficiency of Machines
Not all work by a machine is useful.
 because of friction, work output < work input
 Efficiency is the ratio of useful work out to work in.
useful work output
efficiency =
work input
Math Skills
Efficiency
A sailor uses a rope and an old, squeaky pulley to raise
a sail that weighs 140 N. He finds that he must do 180
J of work on the rope to raise the sail by 1 m. (He does
140 J of work on the sail.) What is the efficiency of the
pulley? Express your answer as a percentage.
1. List the given and unknown values.
Given:
work input = 180 J
useful work output = 140 J
Unknown: efficiency = ? %
Math Skills, continued
2. Write the equation for efficiency.
useful work output
efficiency =
work input
3. Insert the known values into the equation, and solve.
140 J
efficiency =
= 0.78
180 J
To express this as a percentage, multiply by 100 and add
the percent sign, “%.”
efficiency = 0.78 ´ 100% = 78%
STOP NOTES
Review Conservation of
Energy
Assume these were the numbers from the lab. Double
check your numbers are in a similar range.
 m = 55 g
 h1 = .31 m
 h2 = .91 m
 d = .95 m
1. Find the time the car was in midair.
 t = √(2h2/g) = ?
2. Find the car’s velocity when it hit.