Physics Unit 3

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# Physics Unit 3 - PowerPoint PPT Presentation

Physics Unit 3. Year Long Plan. First Ten Minutes (Everyday) – Revision, questions Then Learning/ Pracs Homework… 40+ Club. What you need. Textbook Student Book Scientific Calculator. Topics. Unit 3: Motion Electronics and Photonics Unit 4: Electric Power

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### Physics Unit 3

Year Long Plan
• First Ten Minutes (Everyday) – Revision, questions
• Then Learning/Pracs
• Homework…
• 40+ Club
What you need
• Textbook
• Student Book
• Scientific Calculator
Topics
• Unit 3: Motion
• Electronics and Photonics
• Unit 4: Electric Power
• Interaction of light and matter
• Detailed Study (Homework)
• …Plus an extra project…?
Project…?
• Print off News article.
Area of Study One

Motion in One and Two Dimensions

The Plan
• Review of Motion
• Projectile Motion
• Momentum
• Energy
• Circular Motion
• Gravity and Satellites
Review of Motion
• Place the following into a linked “Concept Map”. Label all the arrows that link the concepts together.
• Forces, Newton’s First Law, Newtons Second Law, Newton’s Third Law, Eqns of motion, mometum, velocity, acceleration, Impulse, Work, kinetic energy, potential energy, Types of forces, Gravity, Normal Force, Springs, Inclined Planes
Review
• CUPS 2
Scalars and Vectors
• Scalar: Physical quantity represented by only a number
• Eg Mass, temperature
• Vector: Physical quantity requiring a direction AND number (magnitude)
• Eg Force, velocity
Distance and Displacement
• Distance

Length of an object has travelled

e.g. total distance of travel

Scalar

• Displacement

Change in position of an object.

e.g. final position – initial position

Vector

Speed or Velocity?

Magnitude only

• Speed: Scalar quantity.
• Velocity: Vector quantity. Magnitude AND Direction
Velocity
• To find velocity, when travelling at a constant velocity (no acceleration) OR to find the average velocity:
Questions
• Tim travels 80m North up North Rd. He then turns and travels 60m East along east road. This travel takes 24s.
• Draw a diagram of the situation
• Calculate the total distance
• Calculate the total displacement
• Calculate the average speed
• Calculate the average velocity
Centre of mass motion
• Usain Bolt runs the 100m with a speed of 10ms-1
• Do all parts of his body move at 10ms-1?
• His arms? His legs? His Head?
• No. His arms often move faster (and slower) than 10ms-1
Centre of Mass Motion
• As physicists, we simplify the problem, and approximate Usain Bolt as a point, at his centre of mass.
• So all calculations of his speed are based on his centre of mass
Joke
• A Statistician, Engineer and Physicist go to the horse track. Each have their system for betting on the winner and they're sure of it. After the race is over, the Statistician wanders into the nearby bar, defeated. He notices the Engineer, sits down next to him, and begins lamenting: "I don't understand it. I tabulated the recent performance of all these horses, cross-referenced them with trends for others of their breed, considered seasonal variability, everything. I couldn't have lost.“ "Yeah," says the Engineer, "well, forget that. I ran simulations based on their weight, mechanical ratios, performance models, everything, and I'm no better off.“ Suddenly, they notice a commotion in the corner. The Physicist is sitting there, buying rounds and counting his winnings. The Engineer and Statistician decide they've got to know, so they shuffle over and ask him, "what's your secret, how'd you do it?“ The Physicist leans back, takes a deep breath, and begins, "Well, first I assumed all the horses were spherical and identical..."
Acceleration
• Acceleration is a measure of how much velocity changes over time
• Change in velocity:
• Acceleration:
Graphing motion
• We can graph either the displacement, velocity, acceleration as time changes
• The Gradient of a graph is the slope.
• The area under the graph is the solid area between the line and the axis
• Eg …

Area under graph

Graphing Motion
• Prac: Graphing Motion
Displacement-Time Graph

A puppy runs after a stick. It runs 10m to the stick (it takes 10s). It then waits by the stick for 10s, and finally brings the stick back to its owner over 10s.

Draw a displacement-time graph for this scenario

Displacement-Time Graphs

Gradient of the displacement-time graph is the velocity

Questions:

• Calculate the velocity over the first 10s
• Calculate the velocity over the next 10s
• Calculate the velocity over the last 10s
• Calculate the average velocity over the full 30s
Velocity-Time Graph
• Gradient of a velocity-time graph is acceleration
• Area under the velocity graph is the displacement
• Eg…
Velocity-Time Graph
• What is the initial velocity?
• What is the change in velocity over the 30s of motion
• What is the acceleration?
• Is this a constant acceleration?
• What is the total displacement between 0 and 30s?
Acceleration-Time Graph
• The area under an acceleration-time graph is the change in velocity.
Acceleration-Time Graph
• What is the acceleration at 0s?
• What is the acceleration at 10s?
• What could this graph be describing?
• Find the change in velocity between 0s and 10s
• If the initial velocity is 2ms-1, what is the velocity after 10s?
Questions
• Draw a displacement-time, velocity-time, and acceleration-time graph for the following:
• Sky diver – no air resistance
• Sky diver – air resistance
• Person walking at a constant speed along a path
• Person 20m away from their house, standing still
Equations of Motion
• Any object moving with a constant acceleration, use the equations of motion
Example
• A car accelerates from rest for 10s at an acceleration of 1.5ms-2
• What is the final speed?
• What distance does the car travel over this time
Questions
• A car, travelling at 30ms-1, accelerates to 40ms-1 in order to pass a slower car. This acceleration takes 20s. What distance does he travel during this acceleration?
• Q4 [Pg12] A car travelling at a constant speed of 80km/h passes a stationary motorcycle policeman. The policeman sets of in pursuit, accelerating uniformly to 80km/h in 10s and reaching a constant speed of 100km/h after a further 5s. At what time will the policeman catch up with the car.
• Extension question: Mr McGovern is driving down a country road at 100km/h when a beautiful duck steps out 75m in front of his car. His reaction time is 0.6s, then he applies the brakes, decelerating at 15ms-2. Will the duck live?
Vertical Motion
• Vertical motion is accelerated motion where the acceleration equals gravity (10ms-2)
Vertical Motion

How high does the tennis ball go?

• Time how long it takes to get to the top of its flight
• What was the initial velocity?
• How high did the ball go?
• After the ball left the hand, draw the forces acting on it
• When the ball was at the top of its flight, draw the forces acting on it
Question
• A footy ball is kicked vertically upwards with an initial speed of 22ms-1
• How high does it reach?
• After what time does it hit the ground again?
Forces
• When we add all the forces acting on a body, we add the forces head to tail
• eg
Forces
• The total force is found by drawing a new arrow from the tail of the first to the head of the last
Q3 2012
• A metal ring is to be held stationary by three forces. Which configuration would make the ring stationary, and why?
Newton’s Laws
• First Law: Unless acted on by a

net force, an object will continue its motion (whether that’s stationary or constant velocity

• New Name:
• Second Law: If acted on by a net force, an object will accelerate
• New Name:
Newton’s Laws
• Third Law: For every action force on object A, there is an equal and opposite reaction force on object B
• New Name:
Newton’s Laws
• Why is there misunderstanding in Newton’s Laws?
• Rules for next 6 examples.
• Quietly think of which answer you like.
• Then we will work together to decide how most people think…
• And what Newton’s Laws predict the answer should be
Newton’s Laws: Example 1
• Victoria Azerenka throws a tennis ball upwards for her serve. Consider the forces on the tennis ball after it has left the hand, but before she hits it on the way down. Is there…?
Newton’s Laws: Example 1
• A downwards force of gravity, along with a steady decreasing upwards force
• A steadily decreasing upward force from the moment it leaves her hand until it reaches its highest point, on the way down a steadily increasing downwards force of gravity
• An almost constant downwards force of gravity
Newton’s Laws: Example 1
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newton’s Laws: Example 2

An elevator is being lifted by a steel cable at a constant speed. The forces on the elevator are…

Newton’s Laws: Example 2
• The upwards force of the cable is greater than the downward force of gravity
• The upwards force of the cable is equal to the downwards force of gravity
• The upwards force of the cable is smaller than the downwards force of gravity
• None of the above: The elevator goes up simply because the cable is being shortened
Newton’s Laws: Example 2
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newton’s Laws: Example 3

A big truck and a small car collide head on.

• The truck exerts a bigger force on the car than the car onto the truck
• The car exerts a bigger force on the truck than the truck on the car
• The truck exerts a force on the car, but the car doesn’t exert one on the truck
• They exert an equal force on each other
Newton’s Laws: Example 3
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newton’s Laws: Example 4

A stationary ice hockey puck is hit. It travels in a straight line along the frictionless ice.

After leaving the hockey stick, does the puck …?

• Speed up as there is no friction
• Travel at a constant speed, and would only be stopped by the edge of the ice rink
• Slow down as the force of gravity works against it
• Slows down as it runs out of force from the hockey stick hit
Newton’s Laws: Example 4
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newtons Laws: Example 5
• The same puck is travelling at a constant speed from (a) to (b). At (b) another stick gives it a swift hit in the direction shown. What is the new direction of the puck?
Newton’s Laws: Example 5
• What is the new direction of the puck?
Newton’s Laws: Example 5
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newton’s Laws: Example 6
• Demo: A pulley is set up with a string connecting two weights of equal masses. But the masses are at different heights. What happens…?
Newton’s Laws: Example 6
• Nothing moves
• The mass on the right pulls down (and the mass on the left goes up), but at a constant speed
• The mass on the right pulls down (and the mass on the left goes up), but at an accelerated rate.
• The mass on the left pulls down (and the mass on the right goes up), but at a constant speed
• The mass on the left pulls down (and the mass on the right goes up), but at an accelerated rate.
Newton’s Laws: Example 6
• What do you think the answer is?
• What do you think most of the general population will think?
• What answer does Newton’s Laws predict?
Newton’s Third Law
• Recall: For every action force on object A, there is an equal and opposite reaction force on object B
• What is the action/reaction pair for …
• Eg
• Force on car is equal and opposite to force on truck
Newton’s Third Law
• What is the action/reaction pair for … [Hint: Draw each situation first]
• Hitting a hockey puck
• Jumping up (at the moment of jumping)
• Falling back down…
• A book on a table
Newton’s Third Law
• Common misconception…
• The action/reaction pair for gravity is NOT the normal force…
• Why? They are acting on the same body!

(Draw 1N book, 10N table)

Experiment
• Forces, Pulleys and String
Newton’s Laws
• CUPS 3, 4, 5. [Monash University]
• Reminder, print A3 sheets
Inclined Plane
• Experiment
Normal Force and Inclined Planes
• A normal force (FN or N) always acts at RIGHT ANGLES to a surface.
• Draw in the normal forces acting on the circles below:
Normal Force on an Incline
• Draw the force of gravity on the ball
• Is the normal force bigger/same/smaller than gravity?

Fg

Normal Force on an incline
• We know that the ball accelerates down the ramp. So the normal doesn’t balance out gravity!
• Draw the direction of acceleration
• Draw in the direction of the total force
Normal Force on an incline
• Show how the two forces acting on the ball add to give the total force
Normal Force on an Incline
• Draw in the right angle
• Draw in the angle of the ramp
• What is the size of the total force?
Questions from Exams
• Sample Q 3b
• Sample Q 6a & 6c
• 2012 Q 4a, b
• 2012 Q 5a – d
• 2011 Q 7, 8
Projectile Motion
• … And the effects of air resistance
Projectile Motion
• Projectile motion is made simple because we can deal separately with an object’s horizontal and vertical components of its velocity.

45o

Projectile Motion
• Find the vertical and horizontal components of the golf balls velocity if it had an initial velocity of 20ms-1, and angled at 20o to the horizontal.

a) Draw the diagram

b) Work out vertical component of velocity

c) Work out horizontal component of velocity

a)

b)

c)

d) Of course!

Projectile Motion
• Find the vertical and horizontal component of a mortar round if it is fired at an angle of 75o to the horizon at a speed of 140ms-1
Projectile Motion

Why did we split up the velocity into horizontal and vertical components?

• Because gravity only acts on the vertical component
• Therefore horizontal component stays at constant velocity
Projectile Motion
• Horizontal Component of Velocity: Use
• Vertical Component of Velocity: Use the equations of motion
Vertical Drop

0s

1s

2s

3s

4s

Projectile

0s

1s

2s

3s

4s

Compare the two

0s

1s

2s

3s

4s

Projectile Motion
• Horizontal Component of Velocity: Use
• Vertical Component of Velocity: Use the equations of motion
Projectile Motion
• Golf ball hit had an initial velocity of 20ms-1, and angled at 20o to the horizontal. How far does it go before hitting the ground? (Assume no air resistance)

20o

Projectile Motion
• Use y-component of velocity. Need to use an equation of motion
• Work out v first
Projectile Motion
• Now use t with the horizontal component of velocity to work out the horizontal distance
• Don’t give up your day job if that is as far as you can hit!
Projectile Motion
• Summary:
• Split initial velocity into horizontal and vertical components
• Horizontal component uses v=x/t
• Vertical component uses equations of motion
Question
• Golf ball hit had an initial velocity of 50ms-1, and angled at 35oto the horizontal. How far does it go before hitting the ground? (Assume no air resistance)

35o

Questions
• Sample – Q5 a & b
• 2011 – Q 12 and 13
2011 Q12 and 13
• Domenic fires a toy cannon, and the projectile leaves the barrel with a velocity of 24 ms-1at an angle of 37o to the horizontal as shown. Ignore air resistance
2011 Q12 and Q13
• How long does it take for the projectile to move from point A to point B?
• What is the maximum height of the trajectory?
Projectiles and Air resistance
• When you account for air resistance in projectile motion, how does it change the trajectory?
• Demo
• Draw trajectory
Happy Gilmour Golf
• After the stunning success of your last project: Angry Shapes, the game designer has come back to you with a fresh project: Happy Gilmour Golf
Momentum
• What is momentum?
• Mass x velocity
• Units: kgms-1
• What is it good for?
• Analysing collisions or when velocity has changed
Conservation of momentum
• The reason it is so good for analysing collisions is that…
• In a closed system, TOTAL MOMENTUM is CONSERVED
• Conserved = Stays the same.
Concept Question Example
• The man jumps from his boat to the shore.
Momentum
• Write down what YOU think happens to the boat.
• Why..?
Demo
• Demo of what just happened
Momentum
• Before the man jumps, what is his momentum? (Use a word to describe it)
• Before the man jumps, what is the boat’s momentum? (Use a word to describe it)
• Before the man jumps, what is the combined TOTAL momentum of the boat and man?
Momentum
• When the man is in the air, what is his momentum? (Use a word to describe it)
• When the man is in the air, what is the momentum of the boat?
• When the man is in the air, what is the combined TOTAL momentum of the boat and the man?
Conservation of Momentum
• When driving his Lamborghini home from school, Mr. McGovern doesn’t notice the car that has stopped in front him, and collides with this car. Before the collision, Mr. McGovern was travelling at 15ms-1. After the collision, the two cars stick together as shown in the figure below, and move with the same speeds. Mr. McGovern’s car has a mass of 2000kg, and the other car has a mass of 1500kg.
• Figure 1: Before collision only Mr. McGovern’s car is moving. After collision both cars stick together and move away at the same speed.
Momentum
• What is the total speed of the two car wreck after the collision.
• First: Plan how you will do this
• Then, do it!
Questions
• Sample Q1a
• 2012 Q2
How do we relate momentum to other things in motion?
• When is momentum useful for calculating stuff?
• Collisions
• When the momentum is changing.
• What needs to happen for the momentum of something to change?
• A force!
How do we relate momentum to other things in motion?
• So… momentum not helpful when it stays the same
• But its helpful when we have a collision, or a change in momentum
• How are force and change in momentum related?

???

Change in momentum

Force

Impulse!

How do we relate momentum to other things in motion?
• Change in momentum (lamborgini) = final momentum – initial momentum.
• Change in momentum = Impulse = F x t
Question
• Sample Q6 b
Energy
• Types of energy? Two types:
• Kinetic
• Potential (stored)
• Kinetic
• Electricity
• Sound
• Elastic
• Gravitational
• Heat
• Chemical
• Elastic
• Nuclear
• Light energy
• Wave
Energy
• Place them into kinetic energy or potential energy
Energy
• Kinetic Energy =
• Gravitational potential energy = mgh
Conservation of Energy
• Energy can never be created or destroyed, only moved from one form to another.

A

B

Energy

A steel ball rolls along a smooth, hard, level surface with a certain speed.

It then smoothly rolls up and over the hill shown below.

How does its speed at point B after it rolls over the hill

compare to its speed at point A before it rolls over the hill?

a. Its speed is significantly less at point B than at point A.

b. Its speed is very nearly the same at point B as at point A.

c. Its speed is slightly greater at point B than at point A.

d. Its speed is much greater at point B than at point A.

e. The information is insufficient to answer the question.

Work
• Work is defined as: how much an object energy has changed by.
• Eg. A 1kg brick is lifted 1m vertically and placed on a table. How much work has been done?
Work
• The work can also be found from the area under a Force-Distance graph
• This is especially useful if the force is not a constant force.
Questions
• 2012 1a & b
• 2011 Q14 & 15
Elastic and inelastic collisions
• Elastic collision means that kinetic energy is conserved
• Inelastic collision means that kinetic energy is not conserved.
• Hang on a minute! How can that be?
• The energy is still conserved, but it is transferred to some wasteful form like sound or heat
Questions
• Sample Q 1b
Springs
• Demo: What direction is the force from the spring after you extend it and compress it?
• Does the force get bigger or smaller the more you compress or extend it?
• What could the equation that relates force and extension be?
Springs: Hooke’s Law
• k = spring constant (depends on the spring).
• Eg, would this spring have a big spring constant or a little spring constant?
Springs
• Springs can also be used on the horizontal …
Springs and elastic potential energy
• Springs store energy with the equation
Springs
• Demo: Sonic Ranger
• Draw graphs of …
• Displacement, velocity, acceleration, kinetic energy, elastic potential energy, gravitational potential energy, total energy
Questions
• Sample Question 3a -b
• 2011 Question 16-20
Circular Motion
• Things that travel in a circular motion…
• Bucket on the end of a string.
• Hammer throw
• Cyclist in a velodrome
• Car going around a corner
Circular Motion
• Speed = ??
• Speed = distance/time
• Speed = (2πr)/time
Circular motion
• What about the car’s velocity?
• Velocity is changing, because as it goes around the circle, its direction changes!
• A changing velocity means …
• Acceleration!
• Acceleration means there must be a force!
Centripetal acceleration
• Acceleration in a circle is called “centripetal acceleration”
• NOT “centrifugal acceleration”
• Substitute in the formula for the speed of an object in circular motion

Velocity

Centripetal Acceleration
• What is the direction of the acceleration?

Velocity

Velocity

Velocity

Centripetal Acceleration
• Centripetal Acceleration is always towards the centre
Question.
• A car is travelling around a circular track, and a driver drops his apple core out the window. Litterer! Which direction does it travel as it falls?
Forces that cause circular motion
• Tension force
• Gravity
• Friction
• What direction must these forces be acting in?
• F = ma
• So, in the same direction as the centripetal acceleration
• Called the centripetal force (NOT centrifugal!)
Questions
• Sample Q 2
Example: Circular Motion

Mass = 150g

• Calculate the radius of the ball’s path
• Draw all forces acting on the ball
• What is the net force? What is this called
• Calculate the tension force in the string
• How fast is the ball travelling?
2: Draw the forces on the ball

Mass = 150g

• Gravity and Tension
Circular Motion
• How do we add forces?

FT

60o

Fg

Circular Motion

FT

60o

2.94N

FG

1.47N

Circular Motion
• Blah

FT

60o

2.94N

FG

1.47N

Recall… Ball on a ramp
• Which direction is the acceleration?
• Which direction is the net force?
• What forces add together to give the net force?
What about a bike on a velodrome wall?
• Draw the direction of the total force (the centripetal force)
• Draw the forces that make up this total force
Circular Motion
• What is the difference between this and the ball that rolls down the ramp?
• On the ramp, the total force (and acceleration) is down the ramp. On the velodrome, the total force (and acceleration) is towards the centre
Circular Motion
• If on the velodrome, the bike wasn’t moving… what would happen?
• Acceleration would be the same as the ball on the ramp and they would roll down the incline!
Circular motion in a vertical plane
• Imagine you are in the roller coaster car below, and it travels with a constant speed of 8ms-1along the track
Circular Motion in a vertical plane
• Describe what you feel as you get to point A

A

Circular Motion in a vertical plane
• Describe what you feel as you get to point B

B

Circular Motion in a vertical plane
• Lets do the maths…
• Find the Normal forces on an 80kg man in the coaster at point A and at point B
• The track can be broken into two circular sections, with radii = 10m

B

10m

10m

A

Circular Motion in a vertical plane
• At point A
• – This is the total force on the man
• Direction=upwards
• What are the two forces that act on the man in the coaster at A?
• Gravity and the Normal Force.
Circular Motion in a vertical plane
• Together these two forces add up to 512N
• What is the normal force equal to?
• 1296N

512N

Circular Motion in a vertical plane
• At point B
• – This is the total force on the man.
• Direction = Downwards.
• So centripetal force is the same, but in different direction
Circular Motion in a vertical plane
• Together gravity and the normal force add up to 512N
• What is the normal force equal to?
• 272N

512N

Circular Motion in a vertical plane
• So compare the normal forces acting on the man at the two points
• How does this compare to what we feel?

B

272N

1296N

A

Circular Motion in a vertical plane
• We feel the roller coaster pushing with a bigger force at point A
• At point B, it pushes with a smaller force, we feel more “weightless”

B

272N

1296N

A

Circular Motion in a vertical plane
• Demo: water in the bucket…
• How to keep the water in the bucket?...
• What is the minimum speed…?
Circular Motion in a vertical plane
• In order for the water to stay in the bucket (or the people to stay in a roller coaster…), the centripetal acceleration must be equal to, or greater than the acceleration due to gravity
• So
Circular Motion in a vertical plane
• Why?
• If they have a centripetal acceleration greater than gravity, they move around the roller coaster faster than they “fall”
• Eraser example
Questions
• Sample Q 7
• 2011 Q 4, 5, 6
• 2011 Q 9, 10, 11
Does the earth’s gravity extend as far as …?
• Do this as a Think pair share…
• To a person standing on earth’s surface?
• A person who jumped in the air?
• An aeroplane in the sky?
• Satellites orbiting the earth?
• The moon?
• How come weightless in satellites/space ships?
Law of gravity
• Newton isn’t famous for “discovering gravity”, but for correctly figuring out that the thing that pulls us (and apples) to the surface of the earth, is the same thing that keeps the planets orbiting the sun (and the moon around the earth)
• G = 6.67x10-11
Law of Gravity
• If I have a mass of 80kg, what is the force of gravity on me?
• Using the earth’s mass of 6x1024kg and radius of 6.4x106m
Newton’s Law of Gravity
• If
• And
• What does
• at the earth’s surface
Gravitational fields
• Using Newton’s law, what happens to earth’s gravitational field as you move further away from it?
• It gets smaller
Gravitational Fields
• A field is a series of arrows, which show the direction of a certain force
Gravitational Fields
• The arrows are the same distance apart and direction because…
• Distance apart = strength of field
• At the earths surface gravity is the same.
Gravitational Fields
• When we zoom back…
Gravitational Fields
• As we move out, the arrows are further apart = less gravity
Gravitational Fields
• Using and
• [Mass earth: 6x1024kg ; Radius:6400km]
• At…
• 400km (ISS), g = …
• 36000km (comm sat), g = …
• Something to think about …

8.7ms-2

0.22ms-2

So how do satellites orbit the earth?
• There is still gravity there…
• Newton’s thought experiment
• [Draw on board]
Satellites
• So, satellites go around the earth, in circular motion
• What force keeps them in motion?
• Gravity
• What equations can we remember from circular motion?
• Therefore,
Satellites
• What do we know about the force of gravity?
• Equate the two
• What cancels?
Satellites
• Whoop de doo basil – what does it all mean?
• It doesn’t matter what the mass of the satellite is!
• If this wasn’t the case it would be a wee problem… think space walks …
Kepler’s Law
• In a circular motion,
• So equation from last page becomes
• Working on the board
• Or
• This is similar to , but we have the time period instead of the velocity
Satellites
• Eg, sometimes you might want to know the time period of a satellite (how many hours it takes to go around the earth) and sometimes you want to know what the orbiting speed is.
Geostationary Satellites
• A geostationary satellite is one that has an orbiting period exactly equal to one day (24 hours in earth’s case)
Example Questions
• Calculate the orbiting radius of a geostationary orbit
• Calculate the orbiting time of the ISS [400km height]
Question 2012 Q8 a
• Note… Tricky little question…
Looking at Satellites
• And iridium flares
• Who has seen a satellite going across?
• http://www.heavens-above.com
• http://spotthestation.nasa.gov/
Apparent weight and weightlessness
• ISS orbits at 400km (g=8.7ms-2)
• But …
How can they be weightless?
• If a space ship travelled into deep space, where g=0ms-2 , then the astronauts would be truly weightless
• Astronauts in “near earth” orbits “appear weightless”
When else do you feel reduced weight or weightlessness?
• Getting to the top of a lift
• Starting the lift (going down)
• Driving over the top of a small hill
• Falling
Apparent Weight
• Our apparent weight is equal to the normal force acting on us
• At the bottom of a roller coaster we feel heavy (large normal force)
• At the top we feel light (low normal force)
Apparent Weightlessness
• When a space shuttle is orbiting the earth, the force of gravity is = to the centripetal force
• There is no normal force!
• Astronauts are basically in a continual free fall around the earth
• Therefore appear to be weightless!
• But why don’t they crash into earth?
• They are moving sideways as well
Apparent Weightlessness
• Its like the astronauts are at the top of a roller coaster loop-to-loop, with a slow enough force that they feel weightless. But their entire orbit around the earth feels like this!
Questions
• Sample Q 4 a-c
• 2012 Q8, b
Revised Concept Map
• Make a concept map with Forces in the middle.
• And do some questions for revision…
Some questions (from exams)
• Finish remaining questions from Sample, 2012, and 2011 in the “Motion” section
• 2012: Q1a-d, 6a-b, 7a-c
• 2011: 1,2,3, 21-23
Topics
• Review of electronics
• Voltage dividers and thermistors
• Diodes
• Amplification
• Photonics systems and modulation
Electronics Review
• Symbols
• Voltage
• Current
• Resistance
• Ohms Law
• Power
• Series
• Parallel
Voltage
• The amount of energy supplied by the battery per coulomb.
• It is effectively “used up” by components of a circuit
• Measured in Volts
• A voltmeter must be in parallel with a component
• Also called “potential difference”
Current
• How many coulombs per second.
• Total current depends on the components of the circuit: They “draw” current out of the battery
• Measured in Amps
• Ammeter must be in series
• Current flows from + to – in a circuit. Or from high voltage to low voltage (although the electrons flow the opposite way)
Prac:
• Revision of Electronics 1
Resistance
• All electrical components have a resistance
• for an Ohmic resistor
Ohm’s Law
• An Ohmic resistor has constant resistance over it when different voltages are applied over it
• Has a straight line graph for V-I
Power
• P=VI
• Can be calculated for each electrical component (power used)
• Or calculated for battery (power supplied)
• Measured in Watts (W)
Series Circuits
• This is an example of light bulbs in series…
Series
• In a series part of the circuit…
• Current doesn’t change
• Voltage is used up
• eg
Series
• Eg, Find the total resistance of these bulbs

100Ω

100Ω

100Ω

Series
• Find the total resistance of these sets of bulbs

50Ω

30Ω

100Ω

50Ω

30Ω

20Ω

Parallel Circuits
• This is an example of bulbs in parallel…
Parallel
• In a parallel part of the circuit…
• Current splits up (but not necessarily in half)
• Voltage is the same in each arm of the parallel
• eg
Parallel
• Find the total resistance

6Ω

3Ω

Parallel
• Find the total resistance

12Ω

6Ω

12Ω

1Ω

Combining Series and Parallel…
• Eg

4Ω

4Ω

4Ω

4Ω

4Ω

4Ω

Prac
• Revision Prac.
• Similar to the question in sample.
• Set up, which bulb is the brightest for maybe three different circuits
Combining series and parallel
• Sample Q9a-c [Together]
• 2012 A2 Q1
• 2011 A2 Q1-4
Voltage Dividers
• In the following circuit, what would the voltage be, measured over…
• Bulb A
• Bulb B?

12V

100Ω

100Ω

A

B

Voltage Dividers
• Yes, in a series circuit, the voltage is divided between the components
Voltage Dividers
• What about in the following circuit. What is the voltage over
• Bulb A
• Bulb B

12V

100Ω

50Ω

A

B

Voltage Dividers
• Bulb A
• Bulb B

12V

100Ω

300Ω

A

B

Voltage Dividers
• An now…
• Bulb A
• Bulb B

12V

100Ω

40Ω

A

B

Voltage Divider Prac
• Make a voltage divider
• Make one with a variable resistor
Voltage Dividers
• What is a general rule for how the voltage is divided in a series circuit?
Voltage Dividers
• What are they good for?
• Demo: Variable resistor
• Room in the book to draw the circuit diagram.
Voltage Divider
• If we swap the variable resistor from before with a thermistor or LDR, we can get a cool “control circuit”
Voltage Divider
• Thermistor: Is a resistor, whose resistance changes depending on its temperature
• Symbol:
• Graph (Board)
Voltage Divider
• LDR: Light Dependant Resistor.
• A resistor whose resistance depends on the amount of light falling on it.
• Symbol:
Voltage Divider
• Using a thermistor (or LDR), we can make a control circuit to control a fan (or air conditioner):
• Circuit diagram
• When the temperature rises, the resistance of the thermistor decreases.
Voltage Divider
• When the temperature rises, the resistance of the thermistor decreases.
• The voltage increases in the output.
• When the voltage in the output reads a certain amount, the fan circuit will turn on!
Voltage Divider Questions
• Samp. Q 13
• 2011 – A2 – Q 5 & 6
Diodes
• Quick Prac – Diodes. Increase voltage. Reverse Bias. Make graph
Diodes
• Diodes are a non-ohmic device
• They only allow current to flow in one direction.
• This is called “forward bias”
• A diode connect in “reverse bias” will allow no current to flow
• Diode symbol:
Diodes
• Threshold Voltage. After a diode reaches its threshold voltage, it conducts like a wire: resistance free
• Note: You cannot connect a diode to a circuit without a resistor! It will short circuit and explode…
Example Questions
• Consider the following circuit. The diode has a threshold voltage of 3V.

12V

A

90Ω

Example
• Will the bulb glow??
• How could you make it glow?
• Assume the diode is now the correct way around.
• What is the voltage used by the diode?
• What is the voltage used by the bulb?
• What is the current measured at point A?
• What is the power used by the bulb?
Diodes
• Different types of diodes include Light Emitting Diode (LED), and photo diode.
• Both still have the same characteristics as a diode
Question
• Sample Q10 a-b
• 2012 A2 Q2
Amplification
• A typical electrical signal, that is transmitting sound, might look like this…
Amplification
• Draw what it might look like if it was amplified 2x…
• This is known as the “gain”
Amplification
• Amplifiers can de-amplify.
• Amplifiers can be inverting as well.
• Draw the signal if it was amplified with an inverting amplifier with gain of -5
Amplification
• A typical voltage in/ voltage out graph looks like …
• The gain is the slope of the graph.
• What is the gain of this graph? Is it inverting/non inverting?
Amplification
• What is the gain of this graph? Is it inverting/non inverting?
Amplification
• Clipping can occur, if you attempt to amplify a signal larger than the amplifier can supply
• This is called saturation of the amplifier
• The saturation voltage is the largest that the amplifier can output
• If the signal has structure, this can result in a distortion of the signal
• (Draw on board)
Amplification
• Amplification achieved with a transistor
Amplification
• More readily done today with an IC (integrated circuit) that has many transistors/resistors and capacitors built into it
Prac:
• I'm looking for an opamp similar to a 5532 that I can operate using a> single 9 volt battery for the power supply. I use 5532's for general> audio circuits but my datasheet recommends a minimum supply voltage of> 10 volts for this device.>> Anyone know of a good low voltage opamp for audio aplications? BTW, a> deviced that is second sourced would be nice.It depends a bit on what you're trying to do. 9v is a bit of an awkwardvoltage because many of the newer opamps are designed for the 3v or 5v rangeand won't go up to 9v; the older ones, as you know, are often designed forat least +/-5v, that is, 10v single supply.Don't fret too much about the rated supply voltage. You can actually getdecent audio performance out of even those ones rated for at least 10v, on a9v battery. It's one of those things where the manufacturer won't promiseit but hundreds of thousands of audio devices have proven it does work.The TL062 is probably the most common opamp that I encounter for 9v audiowork. It has the advantage of very low supply current. It is, however,very noisy and has crappy frequency response - that's the tradeoff. Forbetter sound at the expense of more supply current, the TL072 is a goodopamp. The LM358 has also been widely used for 9v audio, although it doeshave some shortcomings.Note that both the TL062 and TL072 have improved versions, the TLE2062 andTLE2072 respectively, with better specs. The TLE2072 uses 1.8mA perchannel, is rated for supply voltages as low as 4.5v (single supply!), andhas a 10MHz gain bandwidth. I've used it in a number of battery-poweredaudio applications with good success.As GregS points out, the OPA2134 is a truly excellent opamp, and is ratedfor 5v single supply. It does consume 3 times the current of the TLE2072,though.
Questions
• Sample 11 a-b
• 2012 A2 Q4
• 2011 A2 Q11, 12
Photonics
• Photonics is the transfer of information or signals using light.
• We have it because electrical wire could only transfer one phone call per wire
• Fibre optics can transfer up to 1000 phone calls per fibre cable!
• Plus, its cheaper! Electrical cables are made from copper. Fibre is made from glass (silica), which is made from sand.
How might you use light to transfer information
• Morse code?
• What would we need?
• Something to produce the light
• Something to direct the lights travel
• Something to receive the light and to “translate it”
A photonics system
• Diagram
• Has encoder/modulator
• Emitter
• Transfer medium
• Demodulator
Emitters
• LED (Light emitting diodes)
• Laser Diodes – These are LED’s with a laser cavity.