First Ten Minutes (Everyday) – Revision,
questions
 Then Learning/Pracs
 Homework…
 40+ Club

Textbook
 Student Book
 Scientific Calculator

Unit 3: Motion
 Electronics and Photonics

Unit 4: Electric Power
 Interaction of light and matter

Detailed Study (Homework)
 …Plus an extra project…?

Print off News article.
 Youtube video

Motion in One and Two Dimensions
Review of Motion
 Projectile Motion
 Momentum
 Energy
 Circular Motion
 Gravity and Satellites


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

CUPS 2
Scalar: Physical quantity represented by
only a number
 Eg Mass, temperature
 Vector: Physical quantity requiring a
direction AND number (magnitude)
 Eg Force, velocity

• 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: Scalar quantity. Magnitude only
Velocity: Vector quantity. Magnitude
AND Direction

To find velocity, when travelling at a
constant velocity (no acceleration) OR
to find the average velocity:
Δ𝑥
𝑣=
Δ𝑡
Tim travels 80m North up North Rd. He
then turns and travels 60m East along
east road. This travel takes 24s.
1) Draw a diagram of the situation
2) Calculate the total distance
3) Calculate the total displacement
4) Calculate the average speed
5) Calculate the average velocity

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

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


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 is a measure of how much
velocity changes over time
 Change in velocity:
Δ𝑣 = 𝑣 − 𝑢
𝑣 − 𝑢 Δ𝑣
 Acceleration: 𝑎 =
=

𝑡
𝑡
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 …

Zero Slope/
Gradient
Positive
Slope/
Gradient
Negative
Slope/
Gradient
Area under
graph

Prac: Graphing Motion
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
Gradient of the displacement-time graph is
the velocity
Questions:
1) Calculate the velocity over the first 10s
2) Calculate the velocity over the next 10s
3) Calculate the velocity over the last 10s
4) Calculate the average velocity over the
full 30s
Gradient of a velocity-time graph is
acceleration
 Area under the velocity graph is the
displacement
 Eg…

What is the initial velocity?
2) What is the change in velocity over the
30s of motion
3) What is the acceleration?
4) Is this a constant acceleration?
5) What is the total displacement between
0 and 30s?
1)

The area under an acceleration-time
graph is the change in velocity.
What is the acceleration at 0s?
2) What is the acceleration at 10s?
3) What could this graph be describing?
4) Find the change in velocity between 0s
and 10s
5) If the initial velocity is 2ms-1, what is the
velocity after 10s?
1)
Draw a displacement-time, velocitytime, 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

Any object moving with a constant
acceleration, use the equations of
motion
 𝑣 = 𝑢 + 𝑎𝑡

1
𝑎𝑡 2
2

𝑥 = 𝑢𝑡 +

𝑣 2 = 𝑢2 + 2𝑎𝑥

𝑥=
𝑢+𝑣
𝑡
2
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

1)
2)
3)
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 [Pg 12] 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 is accelerated motion
where the acceleration equals gravity
(10ms-2)
How high does the tennis ball go?
1) Time how long it takes to get to the top of
its flight
2) What was the initial velocity?
3) How high did the ball go?
After the ball left the hand, draw the forces
acting on it
5) When the ball was at the top of its flight,
draw the forces acting on it
4)
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?

When we add all the forces acting on a
body, we add the forces head to tail
 eg


The total force is found by drawing a
new arrow from the tail of the first to the
head of the last

A metal ring is to be held stationary by
three forces. Which configuration would
make the ring stationary, and why?
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:

Third Law: For every action force on
object A, there is an equal and opposite
reaction force on object B
 New Name:


Why is there misunderstanding in Newton’s
Laws?

Rules for next 6 examples.
Quietly think of which answer you like.
Text in answer
Then we will work together to decide how
most people think…
And what Newton’s Laws predict the
answer should be





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…?
A downwards force of gravity, along
with a steady decreasing upwards force
b) 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
c) An almost constant downwards force of
gravity
a)
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?

An elevator is being lifted by a steel cable
at a constant speed. The forces on the
elevator are…
a)
b)
c)
d)
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
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?

A big truck and a small car collide head
on.
a) The truck exerts a bigger force on the
car than the car onto the truck
b) The car exerts a bigger force on the
truck than the truck on the car
c) The truck exerts a force on the car, but
the car doesn’t exert one on the truck
d) They exert an equal force on each
other
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?

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 …?
a) Speed up as there is no friction
b) Travel at a constant speed, and would only
be stopped by the edge of the ice rink
c) Slow down as the force of gravity works
against it
d) Slows down as it runs out of force from the
hockey stick hit
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?


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?

What is the new direction of the puck?
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?


Demo: A pulley is set up with a string
connecting two weights of equal
masses. But the masses are at different
heights. What happens…?
a)
b)
c)
d)
e)
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.
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?





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

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

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)


Forces, Pulleys and String
CUPS 3, 4, 5. [Monash University]
 Reminder, print A3 sheets


Experiment
A normal force (FN or N) always acts at
RIGHT ANGLES to a surface.
 Draw in the normal forces acting on the
circles below:

Fg
Draw the force of gravity on the ball
 Is the normal force bigger/same/smaller
than gravity?

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


Show how the two forces acting on the
ball add to give the total force
Draw in the right angle
 Draw in the angle of the ramp
 What is the size of the total force?
 𝐹 = 𝑚𝑔𝑠𝑖𝑛𝜃


𝑎=
𝐹
𝑚
= 𝑔𝑠𝑖𝑛𝜃
Sample Q 3b
 Sample Q 6a & 6c
 2012 Q 4a, b
 2012 Q 5a – d
 2011 Q 7, 8


… And the effects of air resistance

Projectile motion is made simple
because we can deal separately with
an object’s horizontal and vertical
components of its velocity.
𝑣 = 20𝑚𝑠 −1
45o
𝑣 = 20𝑚𝑠 −1
45o
𝑣 = 20𝑚𝑠 −1
𝑣𝑦
45o
𝑣𝑥
𝑂
𝐻
𝑣𝑦

𝑠𝑖𝑛45 =

𝑠𝑖𝑛45 =

𝑣𝑦 = 𝑣𝑠𝑖𝑛45 = 20 × sin 45

𝑣𝑦 = 14.14𝑚𝑠 −1
𝑣𝑦
𝑣
𝐴
𝐻
𝑣𝑥
𝑣

𝑐𝑜𝑠45 =

𝑐𝑜𝑠45 =

𝑣𝑥 = 𝑣𝑐𝑜𝑠45 = 20 × cos 45
𝑣𝑥 = 14.14𝑚𝑠 −1

𝑣 = 20𝑚𝑠 −1
45o
𝑣𝑥
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
d) Do your answers make sense?

a)
b) 𝑣𝑦 = 6.8𝑚𝑠 −1
c) 𝑣𝑥 = 18.8𝑚𝑠 −1
d) Of course!
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
 𝑣𝑥 = 140𝑐𝑜𝑠75 = 36𝑚𝑠 −1
 𝑣𝑦 = 140𝑠𝑖𝑛75 = 135.2𝑚𝑠 −1

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

Horizontal Component of Velocity: Use
𝑥
𝑣=
𝑡

Vertical Component of Velocity: Use the
equations of motion
0s
1s
2s
3s
4s
0s
1s
2s
3s
4s
0s
1s
2s
3s
4s

Horizontal Component of Velocity: Use
𝑥
𝑣=
𝑡

Vertical Component of Velocity: Use the
equations of 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)
𝑣 = 20𝑚𝑠 −1
20o


𝑣𝑥 = 18.8𝑚𝑠 −1 ; 𝑣𝑦 = 6.8𝑚𝑠 −1 ;

𝑥 = 𝑣𝑥 × 𝑡
𝑡 =? ? ? ? ?
Use y-component of velocity. Need to use an
equation of motion
𝑢 = 6.8𝑚𝑠 −1 , 𝑎 = −9.8𝑚𝑠 −2 ; 𝑥 = 0𝑚 𝑡 =?
Work out v first
𝑣 2 = 𝑢2 + 2𝑎𝑥
𝑣 2 = 𝑢2 . 𝑣 = ±6.8𝑚𝑠 −1
Why two answers?







Now t.
 𝑣 = 𝑢 + 𝑎𝑡
 𝑣 − 𝑢 = 𝑎𝑡



𝑡=
𝑣−𝑢
𝑎
𝑡=
−6.8𝑚𝑠 −1 −6.8𝑚𝑠 −1
−9.8𝑚𝑠 −2
=
−13.6𝑚𝑠 −1
−9.8𝑚𝑠 −2
= 1.4𝑠
Now use t with the horizontal component
of velocity to work out the horizontal
distance
 𝑥 = 𝑣𝑥 × 𝑡 = 18.8𝑚𝑠 −1 × 1.4𝑠 = 26.1𝑚
 Don’t give up your day job if that is as far
as you can hit!

Summary:
 Split initial velocity into horizontal and
vertical components
 Horizontal component uses v=x/t
 Vertical component uses equations of
motion


Golf ball hit had an initial velocity of 50ms-1,
and angled at 35o to the horizontal. How far
does it go before hitting the ground?
(Assume no air resistance)
𝑣 = 50𝑚𝑠 −1
35o

Sample – Q5 a & b
 2011 – Q 12 and 13


Domenic fires a toy cannon, and the
projectile leaves the barrel with a velocity
of 24 ms-1 at an angle of 37o to the
horizontal as shown. Ignore air resistance
How long does it take for the projectile
to move from point A to point B?
 What is the maximum height of the
trajectory?

When you account for air resistance in
projectile motion, how does it change
the trajectory?
 Demo
 Draw trajectory


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
What is momentum?
 Mass x velocity
 𝑝 = 𝑚𝑣.
Units: kgms-1

What is it good for?
 Analysing collisions or when velocity has
changed

The reason it is so good for analysing
collisions is that…
 In a closed system, TOTAL MOMENTUM is
CONSERVED
 Conserved = Stays the same.


The man jumps from his boat to the
shore.
Write down what YOU think happens to
the boat.
 Why..?


Demo of what just happened

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?

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?

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.
What is the total speed of the two car
wreck after the collision.
2) First: Plan how you will do this
3) Then, do it!
1)
Sample Q1a
 2012 Q2

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!

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
???
Impulse!
Force
Change in momentum (lamborgini) =
final momentum – initial momentum.
 Change in momentum = Impulse = F x t

Δ𝑝 = 𝐼 = 𝐹𝑡

Sample Q6 b



Types of energy? Two types:
Kinetic
Potential (stored)











Kinetic
Electricity
Sound
Elastic
Gravitational
Heat
Chemical
Elastic
Nuclear
Light energy
Wave

Place them into kinetic energy or
potential energy
1
𝑚𝑣 2
2

Kinetic Energy =

Gravitational potential energy = mgh

Energy can never be created or
destroyed, only moved from one form to
another.
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 is defined as: how much an object
energy has changed by.
 𝑊 = 𝐹𝑑


Eg. A 1kg brick is lifted 1m vertically and
placed on a table. How much work has
been done?
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.

2012 1a & b
 2011 Q14 & 15


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


Sample Q 1b
Δx
Δx
Compressed
Normal
Extended
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?

𝐹 = −𝑘Δ𝑥
 k = spring constant (depends on the
spring).
 Eg, would this spring have a big spring
constant or a little spring constant?


Springs can also be used on the
horizontal …


Springs store energy with the equation
𝐸=
1
𝑘𝑥 2
2
Demo: Sonic Ranger
 Draw graphs of …
 Displacement, velocity, acceleration,
kinetic energy, elastic potential energy,
gravitational potential energy, total
energy

Sample Question 3a -b
 2011 Question 16-20

Things that travel in a circular motion…
 Bucket on the end of a string.
 Moon about the earth
 Earth about the sun
 Hammer throw
 Cyclist in a velodrome
 Car going around a corner

Speed = ??
 Speed = distance/time
 Speed = (2πr)/time

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!

Acceleration in a circle is called
“centripetal acceleration”
 NOT “centrifugal acceleration”

𝑣2
𝑟

𝑎=

Substitute in the formula for the speed of
an object in circular motion

𝑎=
4𝜋2 𝑟
𝑇2
Velocity

What is the direction of the
acceleration?
Velocity

Centripetal Acceleration is always
towards the centre

Flies off at tangent

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?



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!)


Sample Q 2
Mass = 150g
1.
2.
3.
4.
5.
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?

Do on board
𝑟
1.5𝑚

sin 60 =

1.5 × sin 60 = 𝑟 = 1.3𝑚
Mass = 150g

Gravity and Tension
60o
FT
How do we add forces?
 Head to tail!

Fg
FG
1.47N
FT
60o
2.94N

𝐹𝐺 = 9.8 × 0.15𝑘𝑔 = 1.47𝑁
𝐹𝑇 = ? ?

cos 60 =

1.47𝑁
; 𝐹𝑇
𝐹𝑇
=
1.47𝑁
𝑐𝑜𝑠60
= 2.94𝑁
FG
1.47N



60o
FT
2.94N
Σ𝐹 = 1.47 × tan 60 = 2.55𝑁
𝑎=
𝐹
𝑚
𝑣2
=
2.55𝑁
0.15𝑘𝑔
= Blah
𝑎 = ; 𝑣 2 = 𝑎 × 𝑟;
𝑟
4.7𝑚𝑠 −1
𝑣=
𝑎 × 𝑟 = 𝐵𝐿𝐴𝐻 × 1.3𝑚 =
Which direction is the acceleration?
 Which direction is the net force?
 What forces add together to give the
net force?

Draw the direction of the total force (the
centripetal force)
 Draw the forces that make up this total
force

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

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!


Imagine you are in the roller coaster car
below, and it travels with a constant
speed of 8ms-1 along the track

Describe what you feel as you get to
point A
A

Describe what you feel as you get to
point B
B
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

Do on board (together with slides…)

At point A

𝑎=
𝑣2
𝑟

𝐹 = 𝑚𝑎 = 80𝑘𝑔 × 6.4𝑚𝑠 −2
𝐹 = 512𝑁 – 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.




=
82
10
= 6.4𝑚𝑠 −2

Together these two forces add up to
512N
512N
What is the normal force equal to?
 1296N


At point B

𝑣2
𝑟
𝑎=
=
82
10
= 6.4𝑚𝑠 −2
𝐹 = 𝑚𝑎 = 80𝑘𝑔 × 6.4𝑚𝑠 −2
 𝐹 = 512𝑁 – This is the total force on the
man.
 Direction = Downwards.
 So centripetal force is the same, but in
different direction


Together gravity and the normal force
add up to 512N
512N
What is the normal force equal to?
 272N

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
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
Demo: water in the bucket…
 How to keep the water in the bucket?...
 What is the minimum speed…?


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 𝑎 =

𝑣= …
𝑣2
;𝑎
𝑟
= 9.8𝑚𝑠 −2
Why?
 If they have a centripetal acceleration
greater than gravity, they move around
the roller coaster faster than they “fall”
 Eraser example

Sample Q 7
 2011 Q 4, 5, 6
 2011 Q 9, 10, 11


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?


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)
𝐺𝑀𝑚
𝑟2

𝐹=

G = 6.67x10-11
If I have a mass of 80kg, what is the force
of gravity on me?
 𝐹 = 𝑚𝑔 = 80 × 9.8 = 784𝑁
 Using the earth’s mass of 6x1024kg and
radius of 6.4x106m
𝐹 = …
 𝐹 = 784𝑁


If 𝐹 =
𝐺𝑀𝑚
𝑟2
And 𝐹 = 𝑚𝑔
 What does 𝑔 =?


𝑔=
𝐺𝑀
𝑟2
= 9.8𝑚𝑠 −2 at the earth’s surface
Using Newton’s law, what happens to
earth’s gravitational field as you move
further away from it?
 It gets smaller


A field is a series of arrows, which show
the direction of a certain force
The arrows are the same distance apart
and direction because…
 Distance apart = strength of field
 At the earths surface gravity is the same.


When we zoom back…

As we move out, the arrows are further
apart = less gravity

Using 𝐹 =
𝐺𝑀𝑚
;
2
𝑟
and 𝑔 =
𝐺𝑀
𝑟2
[Mass earth: 6x1024kg ; Radius:6400km]
 At…
 400km (ISS), g = … 8.7ms-2
 36000km (comm sat), g = … 0.22ms-2


Something to think about …
There is still gravity there…
 Newton’s thought experiment


[Draw on board]
So, satellites go around the earth, in
circular motion
 What force keeps them in motion?
 Gravity
 What equations can we remember from
circular motion?



𝑎=
𝑣2
𝑟
Therefore, 𝐹 =
𝑚𝑣 2
𝑟

What do we know about the force of
gravity?
𝐺𝑀𝑚
𝑟2

𝐹=

Equate the two
𝐺𝑀𝑚

𝑟2

=
𝑚𝑣 2
𝑟
What cancels?
𝐺𝑀
= 𝑣2
𝑟
Whoop de doo basil – what does it all
mean?
𝐺𝑀
= 𝑣2
𝑟
 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 …

𝑣 = 𝑑/𝑡
 In a circular motion, 2𝜋𝑟

2𝜋𝑟
𝑇

𝑣=

So equation from last page becomes
𝐺𝑀

𝑟
= 𝑣 2 … Working on the board
𝐺𝑚
 3
𝑟
4𝜋2
= 2
𝑇
𝐺𝑀
𝑟3
= 2
2
4𝜋
𝑇


Or
𝐺𝑀
𝑟
This is similar to
= 𝑣 2 , but we have the
time period instead of the velocity

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.

A geostationary satellite is one that has
an orbiting period exactly equal to one
day (24 hours in earth’s case)
Calculate the orbiting radius of a
geostationary orbit
2) Calculate the orbiting time of the ISS
[400km height]
1)

Note… Tricky little question…
And iridium flares
 Who has seen a satellite going across?
 http://www.heavens-above.com
 http://spotthestation.nasa.gov/


Graphs
ISS orbits at 400km (g=8.7ms-2)
 But …

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”

Getting to the top of a lift
 Starting the lift (going down)
 Driving over the top of a small hill
 Falling

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)


Video
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


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!
Sample Q 4 a-c
 2012 Q8, b


Make a concept map with Forces in the
middle.

And do some questions for revision…
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

Review of electronics
 Voltage dividers and thermistors
 Diodes
 Amplification
 Photonics systems and modulation

Symbols
 Voltage
 Current
 Resistance
 Ohms Law
 Power
 Series
 Parallel

Device
Symbol
Device
Symbol
Wires crossed
(not joined)
Cell
Wires joined
Battery of cells
Resistor or other
load
AC supply
CC
VV
Resistor
Ammeter
A
Filament Lamp
Voltmeter
Diode
DC Supply
Earth or ground
Switch
V
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”

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)


Revision of Electronics 1


All electrical components have a
resistance
𝑅=
𝑉
𝐼
for an Ohmic resistor
An Ohmic resistor has constant
resistance over it when different voltages
are applied over it
 Has a straight line graph for V-I

P=VI
 Can be calculated for each electrical
component (power used)
 Or calculated for battery (power
supplied)
 Measured in Watts (W)


This is an example of light bulbs in series…
In a series part of the circuit…
 Current doesn’t change
 Voltage is used up


𝑅𝑇 = 𝑅1 + 𝑅2 + …

eg

Eg, Find the total resistance of these
bulbs
100Ω 100Ω
100Ω

Find the total resistance of these sets of
bulbs
100Ω
50Ω
30Ω
20Ω
50Ω
30Ω

This is an example of bulbs in 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

1
1
1
=
+
+ …
𝑅𝑇 𝑅1 𝑅2

eg

Find the total resistance
6Ω
3Ω

Find the total resistance
6Ω
12Ω
1Ω
12Ω

Eg
4Ω
4Ω
4Ω
4Ω
4Ω
4Ω
Revision Prac.
 Similar to the question in sample.
 Set up, which bulb is the brightest for
maybe three different circuits

Sample Q9a-c [Together]
 2012 A2 Q1
 2011 A2 Q1-4

In the following circuit, what would the
voltage be, measured over…
 Bulb A
 Bulb B?
12V

100Ω 100Ω
A
B

Yes, in a series circuit, the voltage is
divided between the components
What about in the following circuit. What
is the voltage over
 Bulb A
 Bulb B
12V

100Ω 50Ω
A
B
What about now?
 Bulb A
 Bulb B

12V
100Ω 300Ω
A
B
An now…
 Bulb A
 Bulb B

12V
100Ω 40Ω
A
B
Make a voltage divider
 Make one with a variable resistor


What is a general rule for how the
voltage is divided in a series circuit?
 𝑉𝑜𝑢𝑡
=
𝑅1
𝑉𝑖𝑛
𝑅1 +𝑅2
What are they good for?
 Demo: Variable resistor
 Room in the book to draw the circuit
diagram.


If we swap the variable resistor from
before with a thermistor or LDR, we can
get a cool “control circuit”
Thermistor: Is a resistor, whose resistance
changes depending on its temperature
 Symbol:
 Graph (Board)

LDR: Light Dependant Resistor.
 A resistor whose resistance depends on
the amount of light falling on it.
 Symbol:

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.

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!

Samp. Q 13
 2011 – A2 – Q 5 & 6


Quick Prac – Diodes. Increase voltage.
Reverse Bias. Make graph
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:


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…

Consider the following circuit. The diode
has a threshold voltage of 3V.
12V
A
90Ω
Will the bulb glow??
2. How could you make it glow?
3. Assume the diode is now the correct
way around.
4. What is the voltage used by the diode?
5. What is the voltage used by the bulb?
6. What is the current measured at point
A?
7. What is the power used by the bulb?
1.
Different types of diodes include Light
Emitting Diode (LED), and photo diode.
 Both still have the same characteristics
as a diode

Sample Q10 a-b
 2012 A2 Q2


A typical electrical signal, that is
transmitting sound, might look like this…

Draw what it might look like if it was
amplified 2x…

This is known as the “gain”
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

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?


What is the gain of this graph? Is it
inverting/non inverting?
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 achieved with a transistor

More readily done today with an IC
(integrated circuit) that has many
transistors/resistors and capacitors built
into it

Demo / 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 awkward
voltage because many of the newer opamps are designed for the 3v or 5v range
and won't go up to 9v; the older ones, as you know, are often designed for
at least +/-5v, that is, 10v single supply.
Don't fret too much about the rated supply voltage. You can actually get
decent audio performance out of even those ones rated for at least 10v, on a
9v battery. It's one of those things where the manufacturer won't promise
it 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 audio
work. It has the advantage of very low supply current. It is, however,
very noisy and has crappy frequency response - that's the tradeoff. For
better sound at the expense of more supply current, the TL072 is a good
opamp. The LM358 has also been widely used for 9v audio, although it does
have some shortcomings.
Note that both the TL062 and TL072 have improved versions, the TLE2062 and
TLE2072 respectively, with better specs. The TLE2072 uses 1.8mA per
channel, is rated for supply voltages as low as 4.5v (single supply!), and
has a 10MHz gain bandwidth. I've used it in a number of battery-powered
audio applications with good success.
As GregS points out, the OPA2134 is a truly excellent opamp, and is rated
for 5v single supply. It does consume 3 times the current of the TLE2072,
though.
Sample 11 a-b
 2012 A2 Q4
 2011 A2 Q11, 12

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.

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”


Diagram
Has encoder/modulator
 Emitter
 Transfer medium
 Receiver
 Demodulator

LED (Light emitting diodes)
 Laser Diodes – These are LED’s with a
laser cavity.

LDR – Light dependant resistors
 Photodiodes – Light dependant diodes


LDR prac
Why
 How
 Modulation and demodulation of the
carrier wave.


Draw

La Trobe. (Find out specifics)