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? 2 – 3 seconds The interval on the graph where the 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? When the displacement is negative, the object has a position to the left of the origin 1 – 2 and 4 – 5 seconds Constant displacement means that the object doesn’t move The object’s final position is at +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 mi North 4 mi East Displacement 5 mi Northeast Distance 7 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 What does this remind you of? What is happening in this graph? SLOPE OF A GRAPH! CONSTANT ZERO SLOPE Motionless Object CONSTANT POSITIVE SLOPE Moving with CONSTANT positive velocity INCREASING SLOPE Moving with INCREASING velocity

Find displacement Using v-t Graphs Find distance traveled What can we DO with a v-t graph? Find average velocity How do you use the v-t graph to find AVERAGE 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 BOTTOM both considered POSITIVE Area under the graph Area on TOP = POSITIVE Area on BOTTOM = NEGATIVE

Summary Car Distance = 100 m Displacement = 70.7 m Avg Speed = 2 m/s Avg Velocity = 1.4 m/s Bird Distance = 70.7 m Displacement = 70.7 m Avg Speed = 1.4 m/s Avg Velocity = 1.4 m/s Note that the bird has the same average SPEED and VELOCITY because its DISTANCE and DISPLACEMENT were EQUAL!

ACCELERATION –change in VELOCITY occuring over TIME –units are METERS PER SECOND 2 –VECTOR Positive Velocity Positive Acceleration Speeding up in + direction Negative Velocity Negative Acceleration Speeding up in - direction Positive Velocity Negative Acceleration Slowing down Eventually speeds up in – direction! Negative Velocity Positive Acceleration Slowing down Eventually speeds up in + direction!

Equations “Acceleration is a rate of change in velocity” “The slope of a v-t graph tells what the ACCELERATION IS DOING!” “An object’s velocity at any point in time can be 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/s 2 on the Moon “g” is 26 m/s 2 on Jupiter “g” is 9.81 m/s 2 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 green ball is dropped from rest from the same height at the same time. Which one will hit the ground first? They will hit at the SAME TIME!!!

Push and Pull Newton’s Laws

Newton’s First Law An object at rest remains at rest, and an object in 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”

Do these guys have a lot of inertia? LOTS OF INERTIA hard to… GET MOVING or STOP MORE MASS means 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? FgFg Weight FNFN Normal Weight is gravity pulling toward CENTER of the EARTH Normal Force is a REACTION 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 to the net external force acting on the object and inversely proportional to the mass of the object.” Unit of force is the NEWTON (N)

Free Body Diagrams VECTOR diagrams! Shows ALL FORCES acting on an object Must be properly LABELED FgFg FNFN Motionless Equilibrium

Newton’s Third Law “For every action, there is an equal and opposite reaction” FgFg FNFN FgFg FTFT

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 wave AMPLITUDE – 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 Waves Direction of wave travel is PARALLEL to the motion of the medium Example:Sound waves

Longitudinal Waves COMPRESSION – area where particles in the medium are densely populated RAREFACTION – area where particles in the medium are sparsely populated WAVELENGTH – the distance from one compression to another 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 PE Spring PE Electrical Potential Chemical Energy Nuclear Bonding Energy Kinetic Energy (KE) energy of a MOVING object WORK (results in 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! E TOT = KE + PE + Q

Electricity Comes Alive Electrical Current

How can we manipulate energy in electric fields? ++ Apply FORCE to push like charges TOGETHER FAFA FAFA +- FAFA FAFA 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 Amount of work done in moving a charge and the amount of charge moved

Current Current: 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!

Resistance Resistance: is a measurement of how strongly an object will oppose current An object’s resistance depends on FOUR factors: ResistivityLength Cross-sectional AreaTemperature Is specific to a material – the higher it is, the more natural resistance 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 –A–Acts over a DISTANCE without CONTACT Magnets produce FIELDS around them that influence some types of metal –I–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 10 8 m/s (in vacuum) Example #1 What is the wavelength of a radio wave traveling through outer space with a frequency of 5 x 10 4 Hz? v = fλ 3 x 10 8 m/s = (5 x 10 4 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 Index of Refraction = speed of light in vacuum speed of light in medium Example #2 What is the speed of light in lucite? n = 1.50 n = c / v 1.50 = (3 x 10 8 m/s) / v v = 2 x 10 8 m/s

EM Waves Humans  310K Outer space  3K Wein’s Law ALL objects emit EM radiation with a frequency related to their temperature Sun  5630K