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?