Download PHY 102 S08

No Lab Activity this week
Chapter 3 Mallard HW quiz due by midnight
Wednesday 02/20
Lunar eclipse Wednesday 02/20 – total eclipse 9 – 10 pm
Chapter 3 quiz in class Thursday 02/21
Tuesday, February 19
Spring 2008
Chapter 3
Energy
...continuation...
Great Idea:
The many different forms of energy are
interchangeable, and the total amount of
energy in an isolated system is conserved.
Interchangeability of Energy
Fundamental property of the universe:
every form of energy can be converted to
every other form of energy.
Interchangeability of Energy
Figure 3-4 Energy changes during a bungee jump
G = EP
K = EK
E = elastic energy
T = ET thermal energy
Energy in the Earth-Sun System
100%
Earth’s atmosphere
thermal energy reradiated
solar energy absorbed
incoming solar energy
incident solar energy
35%
~10%
solar energy reflected
solar energy reflected
solar energy consumed
Energy Consumption on Earth
Conversion of solar energy into chemical
potential energy through the trophic levels
Release of Energy Back to Space
Use of stored energy to: generate electricity, obtain
fuel, accomplish work
Eventually, stored energy is released as heat, or
thermal energy, back into space
Law of Conservation of Energy
First law of thermodynamics
While the energy in an isolated system
may change from one form to another,
the total amount of energy does not
change – it is conserved
Any change in the total energy of a system
arises from transfer of energy between the
system and its surroundings.
Our Energy Future
Current world energy
consumption:
• ~ 4.7 × 1020 J/yr
• largely based on
fossil fuels (oil, coal,
and gas).
Advantages: cheap, high-grade
Disadvantages: nonrenewable, human
and environmental health impacts
http://en.wikipedia.org/wiki/World_energy_resources_and_consumption
Alternate Energy Sources
Current world energy
consumption:
• ~ 4.7 × 1020 J/yr
Solar energy reaching
Earth:
Advantages: renewable,
(relatively) clean
Disadvantages: expensive, lowefficiency, storage, lack of
infrastructure
• ~ 2 × 1024 J/yr
solar panels – Berwyn, IL
Ford Lock and Dam, MN
Twin Groves Wind Farm, central IL
In-class exercises
Concept Question 1
Equal forces are used to move blocks A and
B across the floor. Block A has twice the
mass of block B, but block B moves twice
the distance moved by block A. Which
block, if either, has the greater amount
of work done on it?
Concept Question 2
A string is used to pull a wooden block across the
floor. The string makes an angle to the
horizontal as shown in the diagram.
a.
Does the force applied via the string do
work on the block?
b.
Is the total force involved in doing work or
just a portion of the total force?
Concept Question 3
In the situation pictured below, if there is a
frictional force opposing the motion of
the block, does this frictional force do
work on the block?
Concept Question 4
If there is just one force acting on an
object, does its work necessarily
result in an increase in the object’s
kinetic energy?
Concept Question 5
Two balls of the same mass are accelerated
by different net forces such that one ball
gains a velocity twice that of the other
ball in the process.
Is the work done by the net force acting
on the faster-moving ball twice that done
on the slower-moving ball?
Sample Problem 1
A woman does 160 J of work to move a
table 4 m across the floor.
What is the magnitude of the force
that the woman applied to the table
if this force is applied in the
horizontal direction?
Sample Problem 2
A rope applies a horizontal force of 180
N to pull a crate a distance of 2 m
across the floor. A frictional force of
120 N opposes the motion.
a.
What is the work done by the force applied
by the rope?
b.
What is the work done by the frictional
force?
c.
What is the total work done on the crate?
Sample Problem 3
A net force of 60 N accelerates a 4-kg
mass from rest over a distance of
10 m.
a. What is the work done by this net
force?
b. What is the increase in kinetic energy
of the mass?
c. What is the final velocity of the
mass?
Sample Problem 4
At the low point in its swing, a
pendulum bob with a mass of
0.2 kg has a velocity of 5 m/s.
a. What is its kinetic energy at the low
point?
b. Ignoring air resistance, how high will
the bob swing above the low point
before reversing direction?