Download - Physics Primer
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 objectis 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 r
Velocity
Defn. - time rate of change ofdisplacement; is a vector quantity; SI unit = m/s
Average velocity = = Displacement rElapsed time t
Instantaneous velocity = limit (average velocity)t0
What is the average velocity of a dragster that takes 5.5 secondsto go the 400 meters down the dragstrip?
SpeedSome 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
AccelerationDefn. - time rate of change of velocity;is a vector quantity; SI unit ism/s2
Accelerations can occur without changing the magnitude of velocity;Ex. Object going in circle at constantrate
Average acceleration = vt
Newton’s First Law“An object at rest, or in a state of constant motion, will continue in that state unless acted upon by an unbalanced force.”
Really, Galileo’s
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 LawF = ma
Relates kinematic variables to dynamic ones
Can measure accelerations calculate forces
Note: SI unit is newtons, English is poundsIncorrect to say that X pounds = Y kilograms
What force is needed to accelerate a 1000 kg car to 5 m/s2?
Not all forces are constant
Newton’s Third Law“For every force, there is an equal and opposite reaction force.”
Often misunderstood; actually means that one object actingon a second object will have the second object act on it
Mule pulls on cart. Cart pulls back onmule with equal and opposite force.“Why pull?”, says mule, if force willbe negated.
Get Back To WorkWork - the transfer of energy by applying a force through a distance
W = F x d if F is constant
W = Fn x d if F varies
Lifting box: F = mg
Distance lifted = h
W = mg x h = mgh
Simple MachinesAllow for the same amount of workto be done, but with smaller forces
Trade-off of using a smaller force isthat 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 sin = mgh/L
Distance pushed up plane = L
PowerPower = = rate of energy usage E
t
How much power do you expend by climbing 3 flights of stairs (10 m) in 10 seconds?
Can deliver the same amount of energy to a system using lesspower, but it takes a longer amount of time
Our Western mindset usually screams for more powerEx. SUV’s require more powerful engines; larger homes require more powerful a.c.
Potential energyEnergy stored within the force between two objects separated by a distance; if objects are allowed to move, force is applied through distance = work doneTYPES OF POTENTIAL ENERGY:
Gravitational
Chemical
Nuclear
Potential energy due to gravity
Water behind a damA rock at the top of a steep hill
EXAMPLES:
If the water or rock drops, gravity operates over a distance, thereby doing work. This work converts the potential energy to kinetic energy.
Example: Gravitational potential energy
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
ENERGY OF MOTION
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
Kinetic energy
Example• 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 mv2Fig. 5: Diagram of falling box
ChargeDefn: 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 subatomicparticles that contain charge; equal amounts of + and - results in neutrality
Designation of positive and negative iscompletely arbitrary
If like charges repel and unlike attract, what holds protons andneutrons together?
Currents - Moving Charge
Oersted (1820) discovered that a currentcreates a magnetic field
This means a current has a force on a magnet
Newton’s Third Law means that amagnet has a force on a moving charge
Direction of force is perpendicular tovelocity and magnetic field
Generator
Magnet
Electric generator: rotate coils of wire perpendicularto magnetic field
Layers of wire coils
Electrical Power
P = I V
We know that P = E/t from the definition of power
The amount of energy delivered by a current is equal to the amount of charge delivered times the voltage,
E = q V P = (q V )/ t
But
Iq/t
U.S. Energy ConsumptionU.S. Energy Consumption
0.00
20.00
40.00
60.00
80.00
100.00
120.00
1940 1950 1960 1970 1980 1990 2000 2010
Year
Con
sum
ptio
n (Q
uadr
illio
n B
tu)
Energy Consumption Per Capita
0
50
100
150
200
250
300
350
400
1940 1950 1960 1970 1980 1990 2000 2010
Year
Con
sum
ptio
n (m
illio
n B
tu)
Over the last 50 years, our consumption of energy has increased (except for after energycrises)
Because of more efficient devices, our consumption perperson has stayed about thesame over the last 30 years
Source: Dept. of Energy, http://eia.doe.gov/
One Case: Crude Oil
Crude Oil Sources
0
2,000
4,000
6,000
8,000
10,000
12,000
1970 1975 1980 1985 1990 1995 2000
Year
Con
sum
ptio
n (M
BD
)
U.S.Imports
Source: Dept. of Energy, http://eia.doe.gov/
We get energy from many different sources. One of the moreimportant ones we will discuss is crude oil.
What are the implications of this graph? Whathistorical eventsoccurred during thistime that relate tocrude oil?
Import CountriesImport Countries
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
1950 1960 1970 1980 1990 2000 2010
Year
Cru
de O
il (T
BD
)
OpecNon-OPEC
Since the mid-1970’s, we have increased our dependence of oil imports on non-OPEC countries
Oil Imports
0200400600800
100012001400160018002000
1950 1960 1970 1980 1990 2000 2010
Year
Oil
(TBD
)
Venezuela
Mexico
We have increased our reliance on oil from Centraland South America, as wellas Canada and Africa
Why?
Why?