Transcript Ch5 Work - Abilene ISD

Ch. 5-1 Work
Ch 5-1, page 168 - 171
Journal page 39
You (have/have not) done work on a chair you
hold at arm’s length all day.
 A force that causes ______ of an object in
the direction (parallel) of the force does
____ on the object.
 Write the formula for work.
 Work is not done on an object unless…
 Work is done only when components…
 Why is no work done on the chair in the
above example?
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If you are pushing a crate along the ground,
some of the force is directed downward. Only
the ______ component of your applied force
does work on the crate.
[Sketch Fig 5-2, page 169.]
Write the formula for work when force is applied
at an angle to the direction of motion.
If Θ = 0o, cos 0o = … and all of the force is
directed…
If Θ = 90o, all force is applied straight up or
down, and then cos 90o = 0 and Wx = _____.
Write the units for work expressed two ways.
The sign of work is important
Journal pg ___, book pg. 170 - 171
Work is a ___________ quantity though it
can be positive or negative.
 Work is positive when…
 Work is negative when…
 Give examples of both + and – work.
 If the work done changes the object’s
speed, a + sign tells you… A – sign
means…
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Energy
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Journal page __, cont, book page 172 - …
Kinetic Energy is …
– If a constant net force is applied to an object
we know the object is…
– This is Newton’s _______ ______.
Kinetic energy depends on…
 Write the formula for kinetic energy.
 KE is a (vector or scalar) quantity.
 What are the units for KE?
 Work Sample Problem 5B, page 173
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Work-Energy Theorem
Practice 5C
page 174-176, J. p 42
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Define the work-kinetic energy theorem
and write the formula for the work-kinetic
energy theorem in words and variables.
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Work Practice 5C problem #1, 2, 4 on
page 176.
Potential Energy
Journal p. 43, book page 177 - 180
Give two examples of objects with PE.
 Potential energy is _________ energy.
 potential energy—(define)
 gravitational potential energy—(define)
 Write the formula (variables & words) for GPE.
 The unit for PE is…
 Since GPE is a result of an object’s _____,
it must be measured relative to some
_____ level.
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Elastic potential energy
Journal pg 43, cont. book page 178
Define elastic PE…
 Write the formula (variables & words) for
elastic PE.
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Conservation of Energy
Journal pg 47, book page 181—
When we say energy is conserved, we
mean…
 Saying energy is conserved does NOT
mean…
 An example of a conserved quantity is…
 Describe three types of energy involved in
the swing of a pendulum of a clock.
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In this animation, you see a mass attached to the end of a string which forms a
pendulum. The pendulum begins with only gravitational potential energy (GPE) since it
is not moving yet. After being released, GPE is turned into kinetic energy (KE). Notice
that no matter where the pendulum is, the sum of the GPE and KE is always equal to
the original amount of energy the system started with. This demonstrates the Law of
Conservation of Energy.
Questions:
Where does the pendulum have the highest velocity?
How does the original height of the pendulum compare to its final height?
This simulation is assuming that there is no air friction and no friction in the pivot point
of the rope.
from http://regentsprep.org/Regents/physics/phys02/pend/default.htm
Mechanical Energy
1
Journal pg 48, cont., book page 182—
Mechanical energy is often…
 Write the formula (both words & variables) for
mechanical energy.
 Write an expanded formula for ME when
KE and PE are involved (under yellow box).
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“Practice 5E,” #2, page 185.
Energy conservation occurs even
when acceleration varies
The diagram below depicts the motion of Li Ping Phar (esteemed Chinese ski
jumper) as she glides down the hill and makes one of her record-setting jumps.
The total mechanical energy of Li Ping Phar is the sum of the potential and kinetic energies. The
two forms of energy sum up to 50 000 Joules. Notice also that the total mechanical energy of Li
Ping Phar is a constant value throughout her motion. There are conditions under which the total
mechanical energy will be a constant value and conditions under which it will be a changing
value. This is the subject of Lesson 2 - the work-energy relationship. For now, merely remember
that total mechanical energy is the energy possessed by an object due to either its motion or its
stored energy of position. The total amount of mechanical energy is merely the sum of these two
forms of energy. And finally, an object with mechanical energy is able to do work on another
object.
From the Physics Classroom, KE
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Mechanical energy (is/is not) conserved in
the presence of friction because…
Rate of Energy Transfer
Journal page 49, book page 187 - 188
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The rate at which work is done is called _____.
Power is (define)…
Write the formula in words and variables.
Write an alternative formula for power and show
how it is derived.
The SI unit of power is the _____ which is equal
to…
– One horsepower is equal to _______ W which
= _______ kW (about (what fraction) of a kW)
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Machines with different power ratings do…
– so what about that stage curtain?