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## Physics ACT Review

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**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**Examples**Sign of displacement refers to the direction the object is moving . A bird flies 5 meters north, then 7 meters south Distance = Displacement = A ball rolls 5 meters north. Distance = Displacement = A cat runs 8 meters west. Distance = Displacement = 5 m 8 m 12 m +5 m -8 m -2 m**Distance vs. Time Graphs**2 – 3 seconds The interval on the graph where the distance remains constant! During what time interval was the object NOT MOVING?**Displacement vs. Time Graphs**The object’s final position is at +1 meter (1 meter to the right 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 During what time interval(s) was the object to the left of the origin? During what time interval(s) was the object NOT MOVING? At what distance from the origin does the object stop?**Example**4 mi East • A man drives his car 3 miles north, then 4 miles east. Distance 7 mi 3 mi North Displacement 5 mi Northeast What distance did he travel? What is his displacement from his point of origin?**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? SLOPE OF A GRAPH! What is happening in this graph? INCREASING SLOPE Moving with CONSTANT positive velocity Moving with INCREASING velocity Motionless Object CONSTANT POSITIVE SLOPE CONSTANT ZERO SLOPE**Using v-t Graphs**What can we DO with a v-t graph? Find average velocity Find distance traveled 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 How do you use the v-t graph to find DISTANCE TRAVELED? How do you use the v-t graph to find DISPLACEMENT? How do you use the v-t graph to find AVERAGE VELOCITY?**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 SECOND2 • VECTOR Negative Velocity Negative Velocity Positive Velocity Positive Velocity Negative Acceleration Negative Acceleration Positive Acceleration Positive Acceleration Slowing down Eventually speeds up in + direction! Slowing down Eventually speeds up in – direction! Speeding up in + direction Speeding up in - direction**Equations**• “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” “Acceleration is a rate of change in velocity” “The slope of a v-t graph tells what the ACCELERATION IS DOING!”**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 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. Also known as the “Law of Inertia” Inertia Tendency of an object tomaintain its STATE OF MOTION Proportional to MASS**Do these guys have a lot of inertia?**MORE MASS means MORE INERTIA LOTS OF INERTIA hard to… GET MOVING or STOP**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? Normal Force is a REACTION force that any object exerts when PUSHED ON Normal FN Weight is gravity pulling toward CENTER of the EARTH Fg Weight**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 theNEWTON (N)**Free Body Diagrams**VECTOR diagrams! Shows ALL FORCES acting on an object Must be properly LABELED Motionless Equilibrium FN Fg**Newton’s Third Law**“For every action, there is an equal and opposite reaction” FT FN Fg Fg**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**MEDIUM – a continuous collection of particles Examples: AIR WATER METAL PULSE – a single disturbance in a medium WAVE – a regularly repeating pulse in a medium that transmits energy without transmitting mass**Transverse Waves**Direction of wave travel is PERPENDICULAR to the motion of the medium Example: Ocean waves**Transverse Waves**WAVELENGTH – the distance from one crest to another AMPLITUDE – the amount that a wave rises or falls Tells how much ENERGY the wave contains CREST OR PEAK – the highest point on a wave TROUGH – the lowest point on a wave**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**WAVELENGTH – the distance from one compression to another AMPLITUDE – the size of a compression The ENERGY contained in the wave COMPRESSION – area where particles in the medium are densely populated RAREFACTION – area where particles in the medium are sparsely populated**Longitudinal Waves**• The particles in a longitudinal wave only move SIDE to SIDE • ENERGY is transferred and particles MOVE BACK and FORTHin 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 WORK (results in force) Kinetic Energy (KE) energy of a MOVING object 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**FA**FA FA FA How can we manipulate energy in electric fields? Apply FORCE to push like charges TOGETHER + + 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 Current can only be sustained if there is aPOTENTIALDIFFERENCEor VOLTAGE between two points! The unit of current is the AMPERE or AMP 1 ampere = 1 COULOMB PER SECOND 1A = 1C/s**Resistance**Resistance: is a measurement of how strongly an object will oppose current An object’s resistance depends on FOUR factors: Resistivity Length Is specific to a material – the higher it is, the more natural resistance the material has. How long is the object? The longer it is, the more it will oppose current flow. Cross-sectional Area Temperature How big is the object across? The wider it is, the more current it will allow to pass. If the object is warm, the molecules inside will be bouncing around more – opposing current.**Magnetism**• Magnetism is aFIELD FORCE • Acts over a DISTANCEwithoutCONTACT • Magnets produceFIELDSaround them thatinfluence some types of metal • IRON, NICKEL, and COBALT • Closely related toELECTRICITY**Light is a WAVE**Electromagnetic Spectrum and EM Waves