Energy: Basics
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 Dr
Velocity
Defn. - time rate of change ofdisplacement; is a vector quantity; SI unit = m/s
Average velocity = = Displacement DrElapsed time Dt
Instantaneous velocity = limit (average velocity)Dt0
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 = DvDt
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
DW = Fn x Dd 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 sinq = mgh/L
Distance pushed up plane = L
PowerPower = = rate of energy usage DE
Dt
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
Energy needs in the
modern world
How do our current uses of energy
compare with those in the “old days”?
THEN:Chemical energy
in livestock (sugar, fat)
NOW:Chemical energy
in gasoline
AGRICULTURE
THEN:Chemical energy
in humans (sugar, fat)
NOW:Fossil fuels, electricity from
chemical energy in coal
INDUSTRY
THEN:Chemical energy
in biomass
NOW:Electricity from chemical
energy in coal
LIGHT
THEN:Chemical energy in biomass (wood)
NOW:Fossil fuels, electricity from
chemical energy in coal
HEAT & COOKING
THEN:Chemical energy
in humans (sugar, fat)
NOW:Chemical energy in
fossil fuels
LANDSCAPING
THEN:Chemical energy
in humans or animals
NOW:Chemical energy in
fossil fuels
TRANSPORTATION
THEN:Chemical energy
in humans
NOW:Electricity from chemical
energy in coal
EDUCATION
We now use energy from fossil fuels
instead of energy from humans,
animals or biomass
MORAL:
U.S. Energy ConsumptionOver 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
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 CountriesSince the mid-1970’s, we have increased our dependence of oil imports on non-OPEC countries
We have increased our reliance on oil from Northand South America
Why?
Why?