Physics ACT Review

Displacement vs. Distance

• DISTANCE – the COMPLETE length of the PATH traveled

by a moving object

• DISPLACEMENT – the length of the STRAIGHT LINE PATH

from a moving object’s ORIGIN to its FINAL POSITION

Scalar vs. Vector

• SCALAR– A measured quantity that has NO DIRECTION– Examples

• Distance, Time, Mass, Volume

• VECTOR– A measured quantity that HAS DIRECTION– SIGN SHOWS DIRECTION– Example

• Displacement

A ball rolls 5 meters north.

Distance =

Displacement =

5 m

+5 m

Examples

A cat runs 8 meters west.

Distance =

Displacement =

A bird flies 5 meters north, then 7 meters south

Distance =

Displacement =

8 m

-8 m

12 m

-2 m

Sign of displacement refers to the direction

the object is moving .

Distance vs. Time Graphs

During what time interval was the object NOT MOVING?

Distance vs. Time

0

2

4

6

8

10

12

14

16

18

0 1 2 3 4 5 6 7

Time (s)

Dis

tan

ce (

m) 2 – 3 seconds

The interval on the graph wherethe distance remains constant!

Displacement vs. Time Graphs

During what time interval(s) was the object NOT MOVING?At what distance from the origin does the object stop?During what time interval(s) was the object to the left of the origin?

Displacement vs. Time

-4

-3

-2

-1

0

1

2

3

4

5

6

0 1 2 3 4 5 6 7

Time (s)

Dis

pla

cem

en

t (m

)

When the displacement isnegative, the object has a

position to the left of the origin1 – 2 and 4 – 5 seconds

Constant displacement means thatthe object doesn’t move

The object’s final position isat +1 meter (1 meter to the

right of the origin)

Example

• A man drives his car 3 miles north, then 4 miles east.

What distance did he travel?What is his displacement from his point of origin?

3 miNorth

4 miEast

Displacement5 mi

Northeast

Distance7 mi

Velocity vs. Speed

• VELOCITY – change in DISPLACEMENT occuring over TIME– MAGNITUDE and DIRECTION

• VECTOR

• SPEED – change in DISTANCE occuring over TIME– MAGNITUDE ONLY

• SCALAR

Average Velocity

if

ifavg tt

dd

t

dv

What does this remind you of?Position vs. Time

Time

Po

sit

ion

What is happening in thisgraph?

SLOPE OF A GRAPH!

CONSTANTZERO

SLOPE

MotionlessObject

Position vs. Time

Time

Po

sit

ion

CONSTANTPOSITIVE

SLOPE

Moving withCONSTANT

positive velocity

Position vs. Time

Time

Po

sit

ion

INCREASINGSLOPE Moving with

INCREASINGvelocity

Find displacement

Using v-t GraphsVelocity vs. Time

-20

-15

-10

-5

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8 9 10 11

Time (s)

Velo

cit

y (

m/s

)

Find distancetraveled

What can we DO with a v-t graph?

Find averagevelocity

How do you use the v-t graph to findAVERAGE VELOCITY?

How do you use the v-t graph to find DISTANCE TRAVELED?

How do you use the v-t graph to find DISPLACEMENT?

Area under the graph

Area on TOP and BOTTOMboth considered POSITIVE

Area under the graph

Area on TOP = POSITIVEArea on BOTTOM = NEGATIVE

Summary

CarDistance = 100 m

Displacement = 70.7 m

Avg Speed = 2 m/sAvg Velocity = 1.4 m/s

BirdDistance = 70.7 m

Displacement = 70.7 m

Avg Speed = 1.4 m/sAvg Velocity = 1.4 m/s

Note that the bird has the same averageSPEED and VELOCITY because itsDISTANCE and DISPLACEMENT

were EQUAL!

• ACCELERATION – change in VELOCITY occuring over TIME– units are METERS PER SECOND2

– VECTOR

Positive Velocity

Positive Acceleration

Speeding up in + direction

Negative Velocity

Negative Acceleration

Speeding up in - direction

Positive Velocity

Negative Acceleration

Slowing downEventually speeds up in – direction!

Negative Velocity

Positive Acceleration

Slowing downEventually speeds up in + direction!

Equations

t

vaavg

atvv if

“Acceleration is a rateof change in velocity”

“The slope of a v-tgraph tells what the

ACCELERATION ISDOING!”

“An object’s velocity at any point in time canbe found by considering:

- its starting velocity

- its acceleration

- the amount of time over which it accelerates”

What’s the hurry?

The Kinematics of Freefall

What happens as objects fall?!?• Physicists DO NOT KNOW WHY objects fall!

• But, we can describe HOW they fall

– As they fall, THEY GO FASTER

– This means that they ACCELERATE!

– They ACCELERATE at a CONSTANT RATE

“g” - The “Magic” Number

• “Little g” is a ‘shorthand’for ACCELERATION DUE TO GRAVITY

• All LARGE OBJECTS have a “little g” value!– Examples

• “g” is 1.67 m/s2 on the Moon

• “g” is 26 m/s2 on Jupiter

• “g” is 9.81 m/s2 on Earth

Over the Edge

Horizontal Projectiles

A red ball rolls off the edge of a table

What does its path look like as it falls?

Parabolic path

As the red ball rolls off the edge, a greenball is dropped from rest from the same

height at the same time.

Which one will hit the ground first?

They will hitat the SAME

TIME!!!

Push and Pull

Newton’s Laws

Newton’s First Law

An object at rest remains at rest, and an objectin motion continues in motion with constant

velocity (that is, constant speed in a straight line)unless it experiences a net external force.

Inertia

Tendency of an object to maintain its STATE OF MOTION

Proportional to MASS

Inertia

Tendency of an object to maintain its STATE OF MOTION

Proportional to MASS

Also known as the “Law of Inertia”Also known as the “Law of Inertia”

Do these guys have a lot of inertia?

LOTS OF INERTIAhard to…

GET MOVING orSTOP

MORE MASSmeans

MORE INERTIA

Force

• A push or pull on an object

• Changes STATE OF MOTION

• CONTACT FORCE– Physical interaction between objects– Normal, Tension, Friction

• FIELD FORCE– “Action over a distance”– Gravity (Weight)

A block of wood is sitting motionless on a table.What forces are acting on it?

FgWeight

FNNormal

Weight is gravitypulling towardCENTER of the

EARTH

Normal Force is aREACTION

force that any object exerts

when PUSHED ON

Net Force

• No NET FORCE if – MOTIONLESS – MOVING WITH CONSTANT VELOCITY

• Unbalanced force CHANGE IN MOTION

• Changing motion ACCELERATION

Force Acceleration

• How much acceleration?

• Depends on:– AMOUNT OF FORCE

• MORE FORCE = MORE ACCELERATION

– MASS OF OBJECT• MORE MASS = LESS ACCELERATION

Newton’s Second Law

“The acceleration of an object is directly proportional tothe net external force acting on the object and inversely

proportional to the mass of the object.”

m

Fa net maF

Unit of force is the NEWTON (N)

Free Body Diagrams

• VECTOR diagrams!

• Shows ALL FORCES acting on an object

• Must be properly LABELED

Fg

FN

MotionlessEquilibrium

Newton’s Third Law“For every action, there is an equal and

opposite reaction”

Fg

FN

Fg

FT

Third Law Examples

• A firefighter directs a stream of water from a hose to the east. In what direction is the force on the hose?

• A man getting out of a rowboat jumps north onto the dock. What happens to the boat?

There will be a force on the hose to the WEST

The boat will move to the SOUTH

Riding the Surf

Wave Properties

Definitions

PULSE – a single disturbance in a medium

WAVE – a regularly repeating pulse in a medium that transmits energy without transmitting mass

MEDIUM – a continuous collection of particles

Examples: AIR

WATER

METAL

Transverse Waves

Direction of wave travel is PERPENDICULAR to the motion of the medium

Example:Ocean waves

Transverse Waves

CREST OR PEAK – the highest point on a waveTROUGH – the lowest point on a waveAMPLITUDE – the amount that a wave rises or falls

Tells how much ENERGY the wave contains

WAVELENGTH – the distance from one crest to another

Transverse Waves

• The particles in a transverse wave only move UP and DOWN

• ENERGY is transferred but the particles DO NOT MOVE in the direction of wave travel

• More ENERGY means more AMPLITUDE

Longitudinal WavesDirection of wave travel is PARALLEL to the

motion of the medium

Example: Sound waves

Longitudinal Waves

COMPRESSION – area where particles in the mediumare densely populated

RAREFACTION – area where particles in the mediumare sparsely populated

WAVELENGTH – the distance from one compression toanother

AMPLITUDE – the size of a compression

The ENERGY contained in the wave

Longitudinal Waves

• The particles in a longitudinal wave only move SIDE to SIDE

• ENERGY is transferred and particles MOVE BACK and FORTH in the direction of wave travel

• More ENERGY means more AMPLITUDE

How does it do that?!?

Introduction to Energy, Work, and Power

Where does FORCE come from?

Potential Energy (PE)stored in a device or ‘field’

Gravitational PESpring PE

Electrical PotentialChemical Energy

Nuclear Bonding Energy

Kinetic Energy (KE)energy of a

MOVING object

WORK (resultsin force)

Thermal (Internal) Energy (Q)“Waste Energy” or Heat

lost during any energy transfer

Definitions

• Energy– the ability to do WORK

• Work– Release of ENERGY in MOVING an object.– FORCE exerted through a DISTANCE

• Power– WORK done in a certain amount of TIME

W = F·d

P = W/t

Units

• Unit of energy JOULE (J)

• Work is “change” in energy JOULE (J)

• Power is ENERGY / TIME WATT (W)

Law of Conservation of Energy

• The energy of a closed system will always remain constant – energy cannot be created or destroyed.

• Seems to be violated by “waste” energy

• We must INCLUDE waste energy!

ETOT = KE + PE + Q

Electricity Comes Alive

Electrical Current

How can we manipulate energy in electric fields?

+ +

Apply FORCE to push like charges TOGETHER

FA FA

+ -FA FA

Apply FORCE to push unlike charges APART

How much electrical PE?• Electrical P.E. is also called VOLTAGE or

POTENTIAL DIFFERENCE

• Unit for electrical potential VOLT

• 1 Volt = 1 Joule/Coulomb

q

WV Amount of work done in moving

a charge and the amount of chargemoved

CurrentCurrent: is the rate at which charge flows

through a given point

The unit of current is the AMPERE or AMP

1 ampere = 1 COULOMB PER SECOND

1A = 1C/s

Current can only be sustained if there is a POTENTIAL DIFFERENCE or VOLTAGE between two points!

ResistanceResistance: is a measurement of how strongly

an object will oppose current

An object’s resistance depends on FOUR factors:

Resistivity Length

Cross-sectional Area Temperature

Is specific to a material – the higher it is, the more naturalresistance the material has.

How big is the object across?The wider it is, the more current

it will allow to pass.

How long is the object? The longer it is, the more it will

oppose current flow.

If the object is warm, the molecules inside will be bouncing around more – opposing current.

Magnetism

• Magnetism is a FIELD FORCE– Acts over a DISTANCE without CONTACT

• Magnets produce FIELDS around them that influence some types of metal– IRON, NICKEL, and COBALT

• Closely related to ELECTRICITY

Light is a WAVE

Electromagnetic Spectrum and EM Waves

EM Waves• Light is part of the ELECTROMAGNETIC

SPECTRUM

• EM WAVES are radiated by ALL objects at the SPEED OF LIGHT

• c = 3 x 108 m/s (in vacuum)

Example #1

What is the wavelength of a radio wave travelingthrough outer space with a frequency of 5 x 104 Hz?

v = fλ

3 x 108 m/s = (5 x 104 Hz) λ λ = 600 m

EM Wave Speed

• Depends on the MEDIUM OF TRAVEL

• The “speed” of a medium depends on its density and is described by its INDEX OF REFRACTION (n)

• HIGH index = SLOW medium

v

cn

Index of Refraction = speed of light in vacuum speed of light in medium

Example #2What is the speed of light in lucite?

n = 1.50

n = c / v1.50 = (3 x 108 m/s) / v

v = 2 x 108 m/s

EM Waves

Humans 310KOuter space 3K

Wein’s Law

ALL objects emit EM radiation with a frequency related to their temperature

Sun 5630K