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Physics Primer
Definitions
Energy - the ability to do work
Work - the transfer of energy by applying a
force through a distance
But what is a “force”?
Position
Position - orientation and distance an object
is from some origin; measurement of
position requires a coordinate system
If the position does not change, the object is easily found
Displacement - change in position; if position is designated
with the vector r, then displacement is Dr
Velocity
Defn. - time rate of change of
displacement; is a vector
quantity; SI unit = m/s
Displacement
Dr
Average velocity =
=
Elapsed time
Dt
Instantaneous velocity = limit (average velocity)
Dt0
What is the average velocity of a dragster that takes 5.5 seconds
to go the 400 meters down the dragstrip?
Speed
Some books say that velocity is speed + direction. WRONG!
Average speed =
Distance traveled
Elapsed time
Displacement = Distance traveled
Displacement on racetrack is 0,
while distance travelled is not
Acceleration
Defn. - time rate of change of velocity;
is a vector quantity; SI unit is
m/s2
Dv
Average acceleration =
Dt
Accelerations can occur without
changing the magnitude of velocity;
Ex. Object going in circle at constant
rate
Newton’s First Law
Really, Galileo’s
“An object at rest, or in a state of constant motion,
will continue in that state unless acted upon by an
unbalanced force.”
Inverse of statement is very important: if an object is acceleration,
then a net force is operating on it, even if you cannot see the
reason for the force.
Is there a force operating in this picture,
and if so, from what direction?
Newton’s Second Law
F = ma
Relates kinematic variables to dynamic ones
Can measure accelerations  calculate forces
Note: SI unit is newtons, English is pounds
Incorrect to say that X pounds = Y kilograms
Not all forces are constant
What force is needed to accelerate a 1000 kg car to 5 m/s2?
Newton’s Third Law
“For every force, there is an equal and opposite
reaction force.”
Often misunderstood; actually means that one object acting
on a second object will have the second object act on it
Mule pulls on cart. Cart pulls back on
mule with equal and opposite force.
“Why pull?”, says mule, if force will
be negated.
Get Back To Work
Work - the transfer of energy by applying a
force through a distance
W=Fxd
DW = Fn x Dd
if F is constant
if F varies
Lifting box: F = mg
Distance lifted = h
W = mg x h = mgh
Simple Machines
Allow for the same amount of work
to be done, but with smaller forces
Trade-off of using a smaller force is
that the force is applied through a
longer distance
Box lifted straight up a height h, force supplied is F = mg
Force of gravity down inclined plane is F = mg sinq = mgh/L
Distance pushed up plane = L
Power
Power = DE = rate of energy usage
Dt
Can deliver the same amount of energy to a system using less
power, but it takes a longer amount of time
Our Western mindset usually screams for more power
Ex. SUV’s require more powerful engines; larger homes
require more powerful a.c.
How much power do you expend by climbing 3 flights of
stairs (10 m) in 10 seconds?
Potential energy
Energy stored within the force between two
objects separated by a distance; if objects
are allowed to move, force is applied through
distance = work done
TYPES OF POTENTIAL ENERGY:
Gravitational
Chemical
Nuclear
Example: Gravitational
potential energy
Potential energy due to gravity
EXAMPLES:
Water behind a dam
A rock at the top of a
steep hill
If the water or rock drops, gravity
operates over a distance, thereby doing work. This work converts
the potential energy to kinetic energy.
Kinetic energy
ENERGY OF MOTION
A moving object has momentum. If it hits
another object, it will transfer energy to it by
applying a force through a distance, i.e. work
Some of the bullet’s kinetic
energy is transferred to the
apple during the collision
Kinetic energy of falling water is
converted to motion of turbines
when water falls on them
Example
Fig. 5: Diagram of falling box
• Potential energy at highest
point is given by mgh
• As it falls, all of this potential
energy is converted to kinetic
• Right before it hits the ground,
all of that is left is kinetic
energy
• Thus, mgh = ½ mv2
Charge
Defn: Property of matter that determines how it will interact
with other matter via the electrostatic force; opposite charges
attract, like repel
All atoms are composed of subatomic
particles that contain charge; equal
amounts of + and - results in neutrality
Designation of positive and negative is
completely arbitrary
If like charges repel and unlike attract, what holds protons and
neutrons together?
Currents - Moving Charge
Oersted (1820) discovered that a current
creates a magnetic field
This means a current has a force on a
magnet
Newton’s Third Law means that a
magnet has a force on a moving charge
Direction of force is perpendicular to
velocity and magnetic field
Generator
Electric generator: rotate coils of wire perpendicular
to magnetic field
Magnet
Layers of wire coils
Electrical Power
We know that P = DE/Dt from the definition of power
The amount of energy delivered by a current is equal to the
amount of charge delivered times the voltage,
DE = Dq V
But
I = Dq/Dt
 P=IV

P = (Dq V )/ Dt
U.S. Energy Consumption
Consumption (Quadrillion Btu)
U.S. Energy Consumption
Over the last 50 years, our
consumption of energy has
increased (except for after energy
crises)
120.00
100.00
80.00
60.00
40.00
20.00
0.00
1940
1950
1960
1970
1980
1990
2000
2010
Year
Energy Consumption Per Capita
Because of more efficient
devices, our consumption per
person has stayed about the
same over the last 30 years
Consumption (million Btu)
400
350
300
250
200
150
100
50
0
1940
Source: Dept. of Energy, http://eia.doe.gov/
1950
1960
1970
1980
Year
1990
2000
2010
One Case: Crude Oil
We get energy from many different sources. One of the more
important ones we will discuss is crude oil.
Crude Oil Sources
Consumption (MBD)
12,000
10,000
8,000
U.S.
6,000
Imports
4,000
2,000
0
1970
1975
1980
1985
Year
Source: Dept. of Energy, http://eia.doe.gov/
1990
1995
2000
What are the
implications of
this graph? What
historical events
occurred during this
time that relate to
crude oil?
Import Countries
Since the mid-1970’s, we
have increased our
dependence of oil imports
on non-OPEC countries
Import Countries
7,000
Crude Oil (TBD)
6,000
5,000
4,000
Opec
3,000
Non-OPEC
2,000
Why?
1,000
0
1950
1960
1970
1980
1990
2000
2010
Year
We have increased our
reliance on oil from Central
and South America, as well
as Canada and Africa
Why?
Oil (TBD)
Oil Imports
2000
1800
1600
1400
1200
1000
800
600
400
200
0
1950
Venezuela
Mexico
1960
1970
1980
Year
1990
2000
2010