Download Ch. 9 notes 2015

Physics Teacher Notes - Grothaus
Chapter 9 – Energy
Title: Energy
*** Energy can change from one form to another without a net loss
or gain ***
The idea of energy was unknown to Isaac Newton
Its existence was still being debated in the 1850’s
The concept of energy is now ingrained in every branch of science
and in every aspect of our society
Examples of where we find energy
Can we see energy?
We can only observe the effects of energy when something is happening
When energy is being transferred from one place to another
or transformed from one form to another
9.1
Work
Impulse (change in momentum) is equal to the amount of force and “how
long” the force acts: Impulse = Ft
“How long” can mean time, but can also mean distance.
Work is the product of the amount of Force and the distance through which
the object is moved
Work is done when a force acts on an object and the object moves in
the direction of the force (work is a vector)
Work = net force x distance
W = Fd
(ex: twice as much weight, or twice as far)
Weight lifter – when is work done?
Work done to lift the barbells, but as he holds it up there, he is no
longer doing work on them. Work may be done in a biological
way in the body. (heart rate pushing blood over distances, small
changes in tissues, etc.
Push a wall?
If work is being done on an object, the energy of that object changes!
Work – two categories
1. work done against another force
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 9 – Energy
Ex: archery (against the elastic bowstring), push-ups
(against your weight), push a box across the floor
(against friction)
2. change the speed of an object
Ex: changing the speed of a car
Both – transfer of energy between something and its
surroundings
W = Fd
Newtons x meters = joule
1 joule = force of 1 N exerted over 1 m
(kilo- 1000; mega- 1,000,000)
Example: Work is done lifting a barbell. How much more work is done lifting
a barbell with twice as much weight on it? How much work is done lifting a
twice-as-heavy barbell twice as far?
Review: Work is done when a force acts on an object and the object moves
in the direction of the force.
9.2
Power
Remember when we talked about velocity and acceleration. Acceleration is
how fast velocity is changing.
Like velocity, work doesn’t take into consideration the amount of time it takes
to do the work.
Carry the same load up the stairs walking and running
Same amount of work, but why are you more tired when
you run?
How fast the work is done is Power (the rate at which work is
done) (remember what rate is?)
Power equals the amount of work done divide by the time
interval during which the work is done.
A car engine that has twice as much power doesn’t produce twice as
much work or go twice as fast as the other engine, but it
can do twice the amount of work in the same amount of time or
do the same amount of work in half the time
Conceptual Physics
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Chapter 9 – Energy
Physics Teacher Notes - Grothaus
𝒑𝒑𝒑𝒑𝒑 =
𝒑𝒑𝒑𝒑 𝒑𝒑𝒑𝒑/
𝒑𝒑𝒑𝒑 𝒑𝒑𝒑𝒑𝒑𝒑𝒑𝒑
P
W
t
;
=
𝑗𝑗𝑗𝑗𝑗𝑗
/𝑗𝑗𝑗
watt 
= watt = W
Joule
sec
1 W = one joule of work is done in one second
Space shuttle uses 33,000 megawatts (MW)
of power for lift off
In the United States: horsepower (hp) for cars, kW for electricity
0.75 kW = 1 hp
134 hp energy?
Ex: To lift a quarter pound hamburger with cheese 1 meter in 1 second
requires 1 watt of power.
p. 168: 33, 37
9.3
Mechanical Energy
When you pull a bowstring, stretch a rubber band, wind a spring,
you enable these things to do work
Could be the compression of atoms in the material;
A physical separation of magnets
A rearrangement of electrical charges in the molecules of a
substance
The property of an object or system that enables it to do
work is energy
Energy is measured in joules (like Work) because when work is done,
there is a change of energy – the two are closely related (like
impulse and momentum)
Many forms
Mechanical Energy is the energy due to the position of
something or the movement of something.
1. kinetic energy
2. potential energy
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 9 – Energy
Mechanical Energy becomes evident only when it changes
from one form to another, or when there is motion
Mechanical Energy is relative to a frame of reference
Depends on the location we choose for our reference
Ex. A 1-N apple held 1 m above the floor has 1 J of PE
Same apple held out the window 10 m up, has 10 J
of PE
9.4
Potential Energy
Just like it sounds, potential energy is energy that is stored and held in
readiness.
There are three examples of potential energy: elastic potential energy,
chemical energy and gravitational potential energy
Elastic potential energy: A stretched or compressed spring; a bow being
drawn back; a stretched rubber band (part of a slingshot)
All have a potential to do work.
Chemical Energy: chemical energy in fuels is potential energy.
Has to do with the energy of position at the subatomic level.
Fossil fuels, electric batteries, digestion of food we
eat, conversion of solar energy to chemical energy
in plants
Gravitational Potential Energy: Work is needed to pick things up
against Earth’s gravity. (push up)
The potential energy due to elevated positions is called GPE
See picture on p. 148
Conceptual Physics
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Physics Teacher Notes - Grothaus
Chapter 9 – Energy
The amount of GPE possessed by an elevated object is equal
to the amount of work done against gravity to get it to
that elevation (only vertical distance)
Remember that W = Fd (joules)
Also remember that Weight = mg (ma, where a = g) and that weight is
a force
The upward force needed when moving at constant velocity is
the weight of the object, so the work done in lifting it through a height h
is the product mgh (weight x height)
Gravitational potential energy = weight x height
PE = mgh (units?)
Height is the distance above some reference level
Floor vs. ground (out window, neg. height)
p. 148 again. Figure 9.3
Doesn’t matter what path got it there
Hydroelectric plants use GPE
Water from an upper reservoir flows thru a long tunnel to
an electric generator
GPE of the water is changed to electrical energy
At night, when there isn’t as much demand for energy, these
plants might buy energy when it’s cheap to bring the water
back up to the top of the reservoir: pumped storage
(Drop a bowling ball on your toe – from 1 mm or from 1 m
Same mass, same weight, but more GPE – so more work)
(math practice)
9.5
Kinetic Energy
If an object is moving, then it’s capable of doing work.
Energy of motion is called Kinetic Energy (KE)
(KE comprises thermal energy, sound, and light)
Depends on the mass as well as the speed of the object
1
KE  mv 2 (show units)
2
Conceptual Physics
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Chapter 9 – Energy
Physics Teacher Notes - Grothaus
The kinetic energy of a moving object is equal to the
work required to bring it to its speed from rest, or the work the object
can do while being brought to rest.
𝑗𝑗𝑗 𝑗𝑗𝑗𝑗𝑗 𝑗 𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗 = 𝑗𝑗𝑗𝑗𝑗𝑗𝑗 𝑗𝑗𝑗𝑗𝑗𝑗
1
Fd  mv 2
2
Note what happens when you double mass –
(double mass, double KE)
Note what happens when you double speed - (double velocity,
quadruple KE)
It takes four times as much work to double the speed or an object moving
twice as fast takes four times as much work to stop it.
What happens when you triple the velocity?
Whenever work is done, energy is changed
Science of bouncing:
https://www.youtube.com/watch?v=9MwyVpVdvVA
9.7
Conservation of Energy
The law of conservation of energy states that energy cannot be created
or destroyed. It can be transformed from one form into another, but the
total amount of energy never changes.
Bow and arrow: bow’s PE transferred to arrow in KE
W = Fd Force of impact times distance travelled is not
quite equal to the KE. Some energy lost through heat
Conceptual Physics
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Physics Teacher Notes - Grothaus
Conceptual Physics
Chapter 9 – Energy
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Physics Teacher Notes - Grothaus
Chapter 9 – Energy
Bottom line: energy may change form, but total energy stays exactly the
same
Test on Friday / Monday: Look at the momentum vocabulary and the reading
guide. Train example?
Energy: vocab, reading guide, be able to solve problems (math practice from
last night), “relationship” problems. Conservation of energy problems (lab)
PE at the top = KE at the bottom!!
Conceptual Physics
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