Download Energy - Weebly

Unit 4: Work, Power, and Energy
I. Work
• When a force acts upon an object to cause a
displacement of the object, it is said that work
was done upon the object.
• There are three key ingredients to work: force,
displacement, and cause.
• In order for a force to qualify as having done
work on an object, there must be a displacement
and the force must cause the displacement.
• In order to work to be done, some of the force
must act in the same direction as the object
moves. If there is no movement, there is no work.
Is Work Being Done?
1) A teacher applies a force to a wall and
becomes exhausted.
2) A book falls off a table and free falls to the
ground.
3) A waiter carries a tray full of meals above his
head by one arm straight across the room at
constant speed.
4) A rocket accelerates through space.
Answers
1) No. The wall is not being displaced.
2) Yes. The force of gravity is causing the book
to free fall.
3) No. Although the waiter is exerting an
upward force on the tray and the tray is
being displaced horizontally across the room;
the force is not causing the displacement.
4) Yes. The force of the gases being expelled
from the rocket are causing the displacement
of the rocket.
Work Equation
Work = Force x Distance
W = Fd
Units of Work: N∙m
1 N∙m = 1 Joule (J)
Guided Practice
• Rita mows lawns on the weekends. If she uses
15N of force to move a lawn mower a distance
of 10 meters, how much work does Rita do?
150 J
• How much work did Joe do when he used 30N
of force to pull a table a distance of 3 meters?
90 J
Independent Practice
1) How much work is done by a force of 60N
that moves an object 6 meters?
2) In preparing for a classroom activity, two
students work together to push a desk a
distance of 4.2 meters. The combined force
they used to complete the task was 20N.
How much work did they do together?
3) Five students exert a combined force of 500N
to lift a heavy crate a distance of 2 meters off
the ground. How much work did the students
exert together in lifting this crate?
4) Nancy stores her holiday decorations in the
garage rafters. She uses a ladder to lift a 50N
box 3 meters off the ground. How much work
does she do?
5) A cat weighing 40N jumps 2 meters onto a
fence. How much work does the cat do?
Answers
1)
2)
3)
4)
5)
360 J
84 J
1000 J
150 J
80 J
II. Power
• Power: the rate at which work is done.
𝑊𝑜𝑟𝑘
Power =
𝑇𝑖𝑚𝑒
𝑊
P=
𝑡
Units of Power: Joule/second = 1 Watt (W)
• For historical reasons, the horsepower is
occasionally used to describe the power
delivered by a machine. One horsepower is
equivalent to approximately 750W.
Guided Practice
1) It takes 2 seconds to lift a crate using 750 J of
work. How much power was required to
complete this job?
375 W
2) You run up several flights of stairs in 1.5
minutes. If the work you do is equal to 900 J,
how much power do you use?
10 W
Independent Practice
1) Two students decide to go rowing on the
weekend. They row the boat for 14 minutes
and together do 168,000 J or work. How
much power did they exert?
2) How much power would a forklift need to
raise a 500N load 1.5 meters high in 10
seconds? How much power would be needed
to do the same work in 5 seconds?
3) You do 3000 J of work to slide a box across
the floor in 30 seconds. Calculate your power.
4) You apply a 250N force for 20 seconds to
slide a box 10 meters across the floor.
Calculate your power.
5) You do 800 J of work to slide a box 5 meters
across the floor. How much force did you
use? If it took 6 seconds to slide the box,
what was the power used?
Answers
1)
2)
3)
4)
5)
200W
Work = 750 J; Power = 75W; Power = 150W
100W
Work = 2500 J; Power = 125W
Force = 160N; Power = 133W
• Work and Power Practice Worksheet
• Work and Power Study Guide 1
• Formative Assessment
• More Work and Power Practice
Worksheet
• More Work and Power Study Guide
(p. 441-442: 1-4, 11, 12, 28, 29, 32)
III. Energy
• Energy: the ability to do work. In other
words, energy is transferred by a force moving
an object through a distance.
Major Forms of Energy
1) Mechanical Energy: the energy associated with the
motion and position of everyday objects.
2) Thermal Energy: the total potential and kinetic
energy of all the microscopic particles in an object.
3) Chemical Energy: the energy stored in chemical
bonds.
4) Electrical Energy: the energy associated with electric
charges.
5) Electromagnetic Energy: a form of energy that travels
through space in the form of waves.
6) Nuclear Energy: the energy stored in atomic nuclei.
• Many forms of energy can be classified into
two general types:
– Kinetic Energy
– Potential Energy (includes gravitational and
elastic)
Kinetic Energy
• Kinetic Energy: the energy of motion.
• The kinetic energy of any moving object depends
upon it mass and speed.
Kinetic Energy = ½(mass)(velocity)2
KE = ½ mv2
Units: kg∙m2/s2 = 1 Joule (J)
• Kinetic Energy is directly related to the mass and
speed. Double the mass, double the KE; double
the speed, quadruple the KE.
Potential Energy
• Potential Energy: energy that is stored as a
result of position or shape.
• There are two forms of potential energy:
gravitational potential energy and elastic
potential energy.
• Elastic Potential Energy: the potential energy
of an object that is stretched or compressed.
– Examples: rubber band, spring, basketball, shock
absorber
• Gravitational Potential Energy: potential
energy that depends upon an object’s height.
• An object’s gravitational potential energy
depends on its mass, its height, and the
acceleration due to gravity.
Potential Energy = (mass)(gravity)(height)
PE = mgh
Units: kg∙m2/s2 = 1 Joule (J)
• GPE is directly related to the mass and height
of an object. Double either the mass or height,
and you double the GPE.
Guided Practice
1) A 0.10 kg bird is flying at a constant speed of 8.0
m/s. What is the bird’s kinetic energy?
3.2 J
2) A 70 kg man is walking at a speed of 2 m/s.
What is his kinetic energy?
140 J
3) A 1400 kg car is moving at a speed of 25 m/s.
How much kinetic energy does the car have?
437,500 J
4) A 50 kg cheetah has a kinetic energy of 18,000 J.
How fast is the cheetah running?
27 m/s
5) A diver at the top of a 10 meter high diving
platform has a mass of 50 kg. How much
potential energy does she has relative to the
ground?
4900 J
6) A ball with a mass of 2 kg is thrown vertically
into the air with an initial speed of 1 m/s. It’s
initial height is 10 meters. How much
gravitational potential energy does it possess
relative to the ground?
196 J
• Kinetic Energy and Potential Energy
Practice Worksheet
• Energy Study Guide
Energy Conversion and Conservation
• On October 9, 1992, people from Kentucky to
New York reported a bright streak of white light
shooting across the night sky. Most observers,
having seen “shooting stars” before, expected
this one to quickly burn out and disappear.
However, that did not happen. The shooting star,
or meteor, continued streaking across the sky.
After a few seconds, pieces of the meteor broke
off, creating a series of smaller streaks of light.
Eventually, the streaks disappeared from view.
Although the event was interesting, most
witnesses probably soon forgot about it.
• However, the meteor was not soon forgotten by
the owners of a red automobile in Peekskill, New
York. Unfortunately for them, a large chunk of
the meteor made it through the Earth’s
atmosphere and struck their parked car. The car
was badly damaged. Luckily, no one was in the
car at the time, so no one got hurt.
• As the Peekskill meteor traveled through the
atmosphere, some of its kinetic energy was
converted into light and heat. The light
made the meteor visible in the sky. The heat
caused a large portion of the meteor to
vaporize in the atmosphere. Upon impact,
much of the meteor’s remaining kinetic
energy went into smashing the metal body
of the car.
• The Peekskill meteor clearly shows that energy can change forms.
• Energy can be converted from one form to another.
• Energy Conversion: the process of changing energy from one form
to another.
• Examples:
– Light bulbs convert electrical energy into thermal energy and
electromagnetic energy.
– When lighting a match, your muscles use chemical energy to
move your hand to strike the match against a rough area on
the matchbox. Friction between the match and the matchbox
converts some of the match’s kinetic energy into thermal
energy. The thermal energy triggers a chemical reaction on the
match tip, releasing some of the match’s stored chemical
energy. The stored chemical energy is then converted into
thermal energy and electromagnetic energy in the flame.
Conservation of Energy
• When energy changes from one form to
another, the total energy remains unchanged
even though many energy conversions may
occur.
• The Law of Conservation of Energy: states
that energy cannot be created nor destroyed.
Energy can be converted from one form to
another. In a closed system, the energy you
begin with is the energy you end with.
• Example:
• You are pedaling a bicycle along a flat route.
When you stop pedaling, the bicycle will
eventually come to a stop. The moving bicycle
had kinetic energy. Where did it go?
• The bicycle slowed down and stopped
because of frictional forces (ground and air)
acting over a distance. The work done by the
friction changes the kinetic energy into
thermal energy.
Energy Conversions
• One of the most common energy conversions
is between potential energy and kinetic
energy.
• The gravitational potential energy of an object
is converted to the kinetic energy of motion as
the object falls.
• This is what happens when an avalanche
brings tons of snow from the top of a
mountain to the valley below.
• Energy Conversion in Pendulums:
• A pendulum consists of a weight swinging back and
forth from a rope or string.
• Pendulums were used in the first truly accurate
clocks first designed by Dutch scientist Christiaan
Huygens in 1656.
• Kinetic energy and potential energy undergo
constant conversion as a pendulum swings. At the
highest point in its swing, the pendulum is
momentarily motionless as it changes direction. At
this point, the weight at the end of the pendulum
has zero kinetic energy and maximum potential
energy.
• As the pendulum swings downward, potential
energy is converted to kinetic energy. At the
bottom of the swing, the pendulum has
maximum kinetic energy and zero potential
energy.
• Eventually frictional forces slow down the
pendulum. In a clock, a spring mechanism or
hanging weights provide energy to keep the
pendulum swinging despite the effects of
friction.
• Energy Conversion in the Pole Vault:
• In the pole vault, an athlete uses a flexible pole to propel
himself over a high bar. In order to start the jump with as
much kinetic energy as possible, the pole-vaulter sprints
down the runway as fast as he can. At the end of his
sprint, he plants the end of a long pole at the base of the
high bar and propels himself into the air. The polevaulter’s kinetic energy is partially converted into elastic
potential energy as the pole bends. The pole springs back
into shape, propelling the pole-vaulter upward, hopefully
high enough to clear the bar.
• As the pole-vaulter soars, his kinetic energy decrease
while he gains gravitational potential energy. Once the
highest point is reached, his GPE begins to convert back
to kinetic energy as the pole-vaulter falls back to the
ground.
• Energy Conversion in a Roller Coaster:
• A roller coaster goes through a series of exchanges
between potential and kinetic energy.
• Potential energy builds as the coaster climbs and
kinetic energy builds as the coaster plunges.
http://www.pbslearningmedia.org/asset/mck05_int
_rollercoaster/
• Energy Conversion Study Guide
• More Kinetic Energy and Potential
Energy Practice Worksheet
• More Energy Study Guide
(p. 469-470: 1-8, 11-13, 16, 18, 23)