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?