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## National 4/5 Physics

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Title: National 4/5 Physics

1
National 4/5 Physics
• In addition to set homework you will be expected
to
• finish off class notes and regularly review work
against
• the learning outcomes.
• You will be expected to take responsibility for
• learning and for seeking help when you need it.
At the
• end of each section, you must ensure all notes
are
• completed and examples attempted.

2
In unit 1 we will learn about the physics of
motion. We will focus on the language, principles
and laws which describe and explain the motion of
an object. Kinematics, also known as Mechanics is
the science of describing the motion of objects
using words, diagrams, numbers, graphs and
equations.
The goal is to develop mental models which
describe and explain the motion of real-world
objects.
3
• Key words vectors, scalars, distance,
• displacement, speed, velocity.
• By the end of this section you will be able to
• Describe what is meant by vector and scalar
quantities
• State the difference between distance and
• displacement
• State the difference between speed and velocity
• State that force is a vector quantity
• Use a scale diagram to find the magnitude and
direction
• of the resultant of two forces acting at right
angles to
• each other.

4
• Key words average speed
• By the end of this section you will be able to
• Describe how to measure an average speed
• Carry out calculations involving distance, time
• and average speed.

5
Which of these are units of speed?
metres
gallons
• miles per hour

seconds
minutes
amperes
miles
kilometres per second
miles per minute
watts
metres per second
Newtons
6
Speeds in.
• In Physics we normally use units
• m/s for velocity.

7
Average speed (m/s)
Snail
High speed train
Sound
270 m/s
13.4 m/s
0.006 m/s
UK town
Fast jet
747 jumbo jet
29790 m/s
10.3 m/s
97 m/s
Air molecule
Earth in orbit
Falcon
60 m/s
648 m/s
7500 m/s
Olympic sprinter
Earth satellite
UK motorway
500 m/s
300000000 m/s
340 m/s
Walking speed
Light speed
Concorde
1.7 m/s
31 m/s
833 m/s
8
Average speed ( m/s )
Light speed 300000000 m/s
Earth in orbit 29790 m/s
Earth satellite 7500 m/s
Concorde 648 m/s
Air molecule 500 m/s
Fast jet 833 m/s
747 jumbo jet 270 m/s
Sound 340 m/s
Falcon 97 m/s
High speed train 60 m/s
UK motorway 31 m/s
UK town 13.4 m/s
Walking speed 1.7 m/s
Olympic sprinter 10.3 m/s
Snail 0.006 m/s
9
What is speed?
• When we talk about speed we mean
• the distance covered by an object in a
• given time.

10
What is speed?
• If Hamish (the dog) runs 10 metres in 2
• seconds, what is his speed?

11
What is speed?
• His speed is 5 metres per second.
• So speed is

12
What is speed?
• If you forget the formula think of cars
travelling at 30 kilometres per hour

13
• Key words average speed
• By the end of this section you will be able to
• Describe how to measure an average speed
• Carry out calculations involving distance, time
• and average speed.

14
distance
time
speed
15
Speed Calculations
• A cyclist travels 100 m in
• 12 s. What is her speed?

16
• Step 1 write down what you know.

d 100 m t 12 s speed ?
17
• Step 2 write down your formula. You can use
no marks for this!

d 100 m t 12 s speed ?
d speed x t
18
• Step 3 substitute in your values.

d speed x t
d 100 m t 12 s speed ?
19
• Step 4 rearrange

d speed x t 100 speed x 12
d 100 m t 12 s speed ?
20
• Step 5 calculate

d speed x t 100 speed x 12 speed
8.33
d 100 m t 12 s speed ?
100
12
21
• Step 6 units!!!!

d speed x t 100 speed x 12 Speed
8.33 m/s
d 100 m t 12 s speed ?
100
12
22
• Key words average speed, instantaneous
• speed
• By the end of this section you will be able to
• Describe how to measure instantaneous speed.
• Identify situations where average speed and
• instantaneous speed are different.

23
Instantaneous and average speed
• Are instantaneous and average speed the same?

24
Instantaneous or average?
• A cars speed between North Berwick and
• Edinburgh
• Average

25
Instantaneous or average?
• The speed read from a cars speedometer
• Instantaneous

26
Instantaneous or average?
• A tennis balls speed as it crosses the net
• Instantaneous

27
Instantaneous or average?
• A racing cars speed over a lap of the track
• Average

28
Instantaneous or average?
• A parachutists speed as he/she lands
• Instantaneous

29
Scalars and Vectors
• Imagine a boat
• making a distress
• call to the
• coastguard.
• The boat tells the
• coastguard he is 60 km
• from Aberdeen.

30
Scalars and Vectors
• Is this enough
• information for the
• coastguard to find
• the boat?

31
Scalars and Vectors
32
Scalars and Vectors
• The coastguard needs both
• distance (size)
• and
• direction
• to find the boat.

33
Scalars and Vectors - Definition
• A scalar is a quantity which has only
• magnitude (size). It is defined by a
• number and a unit.
• A vector is a quantity which has
• magnitude (size) and direction. It is
• defined by a number, a unit and a
• direction.

34
Distance and Displacement
A pupil walks from her house to her school. Her
brother makes the same journey, but via a shop.
How far has the girl walked? How far has her
brother walked?
500 m
300 m
400 m
35
Distance and Displacement
The girl has walked 500 m. Her brother has walked
700 m.
Distance is a scalar quantity it can be defined
simply by a number and unit.
500 m
300 m
400 m
36
Distance and Displacement
Distance is simply a measure of how much ground
an object has covered.
500 m
300 m
400 m
37
Distance and Displacement
But how far out of place is the girl? And her
brother? Displacement is a vector which requires
number, unit and direction.
38
Distance and Displacement
The girl has a displacement of 500 m at a bearing
of 117 East of North.
500 m
300 m
400 m
39
Distance and Displacement
What is her brothers displacement?
500 m
300 m
400 m
40
Distance and Displacement
Her brother has a displacement of 500 m at a
bearing of 117 (117 East of North).
500 m
300 m
400 m
41
Distance and Displacement
Their displacement (how far out of place they
each are) is the same.
500 m
300 m
400 m
42
Speed and Velocity
Speed is a scalar quantity requiring only
magnitude (number and unit). Velocity is a
vector, requiring magnitude and direction.
43
Speed and Velocity
Speed tells us how fast an object is
moving. Velocity tells us the rate at which an
object changes position.
44
Speed and Velocity
• Imagine a person stepping one step
• forward, then one step back at a speed of
• 0.5 ms-1.
• What is the persons velocity? Remember
• velocity keeps track of direction. The
• direction of the velocity is the same as
• the direction of displacement.

45
Speed and Velocity
46
• Key words vectors, scalars, distance,
• displacement, speed, velocity.
• By the end of this section you will be able to
• Describe what is meant by vector and scalar
quantities
• State the difference between distance and
• displacement
• State the difference between speed and velocity
• State that force is a vector quantity
• Use a scale diagram to find the magnitude and
direction
• of the resultant of two forces acting at right
angles to
• each other.

47
Distance and Displacement
48
Speed and Velocity
49
A physics teacher walks 4 meters East, 2 meters
South, 4 meters West, and finally 2 meters North.
The entire motion lasted for 24 seconds.
Determine the average speed and the average
velocity.

50

The physics teacher walked a distance of
12 meters in 24 seconds thus, her average speed
was 0.50 m/s. However, since her displacement
is 0 meters, her average velocity is 0 m/s.
Remember that the displacement refers to the
change in position and the velocity is based upon
this position change. In this case of the
teacher's motion, there is a position change of 0
meters and thus an average velocity of 0 m/s.
51
Scalar or Vector?
52
• Key words vectors, scalars, resultant, scale
diagram
• By the end of this lesson you will be able to
• Describe what is meant by vector and scalar
quantities
• State the difference between distance and
• displacement
• State the difference between speed and velocity
• State that force is a vector quantity
• Use a scale diagram to find the magnitude and
direction
• of the resultant of two forces acting at right
angles to
• each other.

53
Vectors
• Vectors can be represented by a line
• drawn in a particular direction.
• The length of the line represents the
• magnitude of the vector.
• The direction of the line represents the
• direction of the vector.

54
• When two or more scalars are added
• together, the result is simply a numerical
• sum.
• For example a mass of 3kg and a mass of
• 5 kg, when added, make a mass of 8kg.

55
• When two or more vectors are added
• together, providing they act in the same
• direction, the addition is straightforward.

5 N
3 N
8 N
56
• If they are acting in opposite directions

5 N
3 N
2 N
57
• Key words vectors, resultant
• By the end of this section you will be able
• to
• Use Pythagoras and Trigonometry to find
• the magnitude and direction of the
• resultant of two forces acting at right
• angles to each other.

58
• The resultant of two or more vectors
• which act at angle to each other can be
• found either using a scale diagram, or by
• Pythagoras and trigonometry.

59
To find the resultant of a set of vectors using a
scale diagram
• 1. Decide on a suitable scale and write this
• down at the start
• 2 Take the direction to the top of the page as
• North. Draw a small compass to show this.
• 3 Draw the first vector ensuring it is the
• correct length to represent the magnitude
• of the vector, and it is the correct
• direction.

60
To find the resultant of a set of vectors using a
scale diagram
• Draw an arrow to represent the second
• vector starting at the head of the first.
• 5 The resultant vector can now be determined
• by drawing it on the diagram from the tail
• of the first to the head of the last vector.
• The magnitude and direction of this vector
• 6 The final answer must have magnitude and
direction either a bearing from North or an
angle marked clearly on the diagram

61
Scale Diagrams
• Scale remember if the question is in ms-1 then
your scale should be a conversion from cm to
ms-1.
• Direction draw compass on page
• 1st vector length and direction
• 2nd vector tail of 2nd starts at tip of first
• Resultant vector tail of 1st to tip of last
• Answer must include magnitude (including units)
and direction

62
Scale Diagrams
• Direction should be given as a three
• figure bearing from North
• e.g. 045 or 175 or 035
• If you give any other angle, you must
• clearly mark it on the scale diagram.

63
• A car travels 100 km South, then 140 km
• East. The time taken for the whole
• journey is 3 hours.
• Using a scale diagram (and the six step
• process) find
• the cars total distance travelled
• its average speed
• its overall displacement
• its average velocity

64
Scale Diagrams
• Scale diagrams are used to find the
• magnitude and direction of the resultant
• of a number of a set of vectors.

65
The tropical island of Sohcahtoa
66
The tropical island of Sohcahtoa
67
The tropical island of Sohcahtoa
68
The tropical island of Sohcahtoa
69
hyp
opp
?
70
• The Old Arab Carried A
• Heavy Sack Of Hay
• SinOpp/Hyp

71
hyp
opp
?
72
N
3 km North
E
4 km East
Remember The vectors above are not tip to tail.
You must join them tip to tail
73
N
R ?
3 km North
?
E
4 km East
Bearing of 053.10
74
6N
6N North, 8N East - what is the resultant force
R ?
8N
R
?
75
• Key words acceleration, velocity
• By the end of this section you will be able to
• Explain the term acceleration
• State that acceleration is the change in
• velocity per unit time
• Carry out calculations involving the relationship
• between initial velocity, final velocity, time
and
• uniform acceleration.

76
Measuring Acceleration Activity
What do you expect to happen to the value of
acceleration as the light gate is moved further
up the slope?
Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope Position of light gate from bottom of slope
Acceleration (m/s2) 1st attempt 2nd attempt 3rd attempt Position 1 m Position 2 m Position 3 m Position 4 m
Acceleration (m/s2) 1st attempt 2nd attempt 3rd attempt
Average acceleration (m/s2)
77
What is acceleration?
• Acceleration is the change in velocity of an
object per second (in one second).
• Is acceleration a vector or scalar quantity?

78
Acceleration
• What is the definition of acceleration?
• Is it a vector or a scalar?

Acceleration is the rate of change of velocity
per unit time OR change in velocity per unit time.
Vector since velocity is a vector.
79
What is acceleration?
• The rocket starts off at 0 m/s and 1
• second later is travelling at 10 m/s.
• What is its acceleration?
• 10 metres per second per second
• 10 m/s2

change in speed
in one second
80
Calculating acceleration
• We need to know
• the change in velocity so
• initial velocity (u)
• final velocity (v)
• and
• time (t)

81
(No Transcript)
82
change in velocity
in one second
83
Acceleration
• a acceleration measured in m/s2
• u initial velocity measured in m/s
• v final velocity measured in m/s
• t time measured in s

84
Units of acceleration
final velocity initial velocity
a
time
acceleration is measured in m/s2
If the speed is measured in kilometres per hour,
acceleration can be measured in kilometres per
hour per second.
85
Acceleration
• An object accelerates at a rate of 4 m/s2.
• What does this mean?
• The object goes 4 m/s faster each
• second.

86
Acceleration
• The object goes 4 m/s faster each
• second.
• If the object is initially at rest, what
• is its velocity after
• 1s? 4 m/s
• 2s? 8 m/s
• 3s? 12 m/s
• 4s? 16 m/s

87
Acceleration
• What does it mean if an object has a negative
• value of acceleration?
• It means that it is slowing down.
• For example an object which has an
• acceleration of -2 m/s2 is becoming 2 m/s
• slower each second.

88
Acceleration Calculations
• A car, starting from rest, reaches a
• velocity of 18 m/s in 4 seconds. Find the
• acceleration of the car.
• What do I know?
• Initial velocity u 0 m/s
• Final velocity v 18 m/s
• time t 4 s

89
Acceleration Calculations
• What do I know?
• Initial velocity u 0 m/s
• Final velocity v 18 m/s
• time t 4 s
• Formula?

90
Acceleration Calculations
• A cheetah starting from rest accelerates
• uniformly and can reach a velocity of 24
• m/s in 3 seconds. What is the
• acceleration?
• Use technique and show all working!
• Units!!

91
Acceleration Calculations
• A student on a scooter is travelling at
• 6 m/s. 4 seconds later, she is travelling at
• 2 m/s. Calculate her acceleration.
• Use technique and show all working!
• Units!!
• What do you notice about her change in
• velocity?

92
Rearranging the acceleration equation
v-u a
t
93
Rearranging the acceleration equation
94
• Key words acceleration, velocity
• By the end of this section you will be able to
• Explain the term acceleration
• State that acceleration is the change in
• velocity per unit time
• Carry out calculations involving the relationship
• between initial velocity, final velocity, time
and
• uniform acceleration.
• Graph results

95
Acceleration using two light gates
• The length of the mask is 5 cm. Calculate
• the acceleration.
• Remember calculate u (initial velocity) and
• v (final velocity) and use

96
• The length of each section mask is 4 cm. The gap
is also 4 cm. Calculate the acceleration.
• Remember calculate u (initial velocity) and
• v (final velocity) and use

97
• Key words acceleration, velocity, displacement
• By the end of this seection you will be able to
• Draw velocity-time graphs of more than one
• constant motion.
• Describe the motions represented by a
• velocity-time graph.
• Calculate displacement and acceleration, from
• velocity-time graphs, for more than one constant
• acceleration.

98
Graphing Motion
• Information about the motion of an
• object can be obtained from velocity-time
• graphs.
• Similarly, we can graph motion based on
• descriptions of the motion of an object.

99
Velocity-time graph
• The motion of a moving object can be
• represented on a velocity time graph.

100
Vectors and Direction
• When dealing with vector quantities we
• must have both magnitude and
• direction.
• When dealing with one-dimensional
• kinematics (motion in straight lines) we
• use and to indicate travel in opposite
• directions. We use to indicate acceleration
• and to indicate deceleration.

101
Velocity-Time Graphs
Describe the motion of this object.
Constant velocity does not change with time
0
0
102
Velocity-Time Graphs
Describe the motion of this object.
Increasing with time constant acceleration
0
0
103
Velocity-Time Graphs
Describe the motion of this object.
Decreases with time constant deceleration
0
0
104
Velocity-Time Graphs
Describe the motion of this object.
0
0
105
Speed-Time Graphs
Calculate the distance covered by the object in
the first 10 s of its journey.
The area under the graph tells us the
distance travelled.
0
0
106
Speed-Time Graphs
Calculate the distance covered by the object in
the first 10 s of its journey.
The area under the graph tells us the
distance travelled.
0
0
107
• Key words forces, newton balance, weight, mass,
gravitational field strength.
• By the end of this section you will be able to
• Describe the effects of forces in terms of their
ability to
• change the shape, speed and direction of travel
of an object.
• Describe the use of a newton balance to measure
force.
• State that weight is a force and is the Earths
pull on an
• object.
• Distinguish between mass and weight.
• State that weight per unit mass is called the
gravitational
• field strength.
• Carry out calculations involving the relationship
between weight, mass and
• gravitational field strength including situations
where g is not equal to 10

108
What effect can a force have?
• Force is simply a push or a pull.
• Some forces (e.g. magnetic repulsion, or
• attraction of electrically charged
• objects) act at a distance.

109
What is force?
• A force can
• change the shape of an object
• change the velocity of an object
• change the direction of travel of an object

110
Units of Force?
• Force (F) is
• measured in
• newtons (N).

111
Measuring Forces
• A Newton (or
• spring) balance can
• be used to measure
• forces.

112
Mass and Weight
• We often use the words mass and weight
• as though they mean the same
• but do they?

113
Mass and Weight
• An objects mass is
• a measure of how much stuff makes up
• that object how much matter, or how
• many particles are in it.
• Mass is measured in
• grams or kilograms.

114
Mass and Weight
• An objects weight is
• the force exerted by gravity on a mass.
• Since it is a force, weight must be
• measured in
• newtons.

115
Investigating the relationship between mass and
weight
• How can we find the relationship between
• mass and weight?
• A newton balance can be used to find the
• weight of known masses.

116
Results
Mass Weight in N
100g
200g
300g
400g
500g
1kg
2kg
5kg
117
Relationship between mass and weight
• From this we can see a relationship
• between mass and weight
• 100g 0.1 kg -gt 1 N
• 1kg -gt 10 N
• To convert kg -gt N multiply by 10
• To convert N -gt kg divide by 10

118
Gravitational Field Strength (g)
• Gravitational field strength on Earth is
• 10 N / kg

119
What is gravitational field strength?
• This is the pull of gravity on each
• kilogram of mass.
• So on Earth, the pull of gravity on a 1kg
• mass is

10 N
120
What is gravitational field strength?
• and the pull of gravity on a 2 kg mass is

20 N
121
Definition
• A planets gravitational
• field strength is the
• pull of gravity on
• a 1 kg mass.

122
Gravity in the universe
• Is gravitational field strength always the
• same?
• No! It varies on different planets.
• http//www.exploratorium.edu/ronh/weight/index.htm
l

123
• Use the website to find your weight on
• different planets for a mass of 60 kg (a
• weight of 600 N on Earth).
• From this calculate the gravitational field
• strength for each planet.

124
• Mass on Earth 60 kg
• Weight on Earth 600 N
• Gravitational field strength
• Weight on Mercury 226.8 N g
• Weight on Venus 544.2 N g
• Weight on the Moon 99.6 N g
• Weight on Mars 226.2 N g
• Weight on Jupiter 1418.4 N g
• Weight on Saturn 549.6 N g

125
Units for g
• We found g by dividing weight in newtons
• by mass in kilograms.
• What are the units for g?

10 N / kg
126
• Which of the planets has the greatest
• gravitational field strength?
• Why do you think this is the case?

127
Weight, mass and gravity
• We have seen that there is a link between
• weight, mass and gravity.
• On Earth
• 1 kg acted on by 10 N / kg weighs 10 N

m x g W
mass
Gravitational field strength g
weight
128
Weight, mass and gravity
Why is weight measured in newtons?
• W mg

Gravitational field strength measured in N / kg
Mass measured in kg
Weight measured in newtons
129
• Key words friction, force
• By the end of this section you will be able to
• State that the force of friction can oppose
• the motion of an object.
• Describe and explain situations in which
• attempts are made to increase or decrease
• the force of friction.

130
Frictional Forces
• Moving vehicles such as cars can slow
• down due to forces acting on them.
• These forces can be due to
• road surface and the tyres
• the brakes
• air resistance.

131
Frictional Forces
• The force which tries to oppose motion is
• called the force of friction.
• A frictional force always acts to slow an
• object down.

132
Increasing Friction
• In some cases, we want to increase
• friction. Some examples of this are
• Car brakes we need friction between
• the brake shoes and the drum to slow
• the car down
• Bicycle tyres we need friction to give
• grip on the surface

133
Increasing Friction
• On the approach to traffic lights and
• used to increase friction compared with

134
Decreasing Friction
• In some cases, we want to decrease
• friction. Some examples of this are
• Ice skating
• Skiing
• Aircraft design

135
Reducing Friction
• Friction can be reduced by
• Lubricating the surfaces this generally
• means using oil between two metal
• surfaces. This is done in car engines to
• reduce wear on the engine metal parts
• arent in contact because of a thin layer
• of oil between them.

136
Reducing Friction
• Friction can be reduced by
• Separating surfaces with air (e.g. a
• hovercraft).
• Making surfaces roll (e.g. by using ball
• bearings).

137
Reducing Friction
• Friction can be reduced by
• Streamlining. Modern cars are designed
• to offer as little resistance (or drag) to
• the air as possible, reducing friction on
• the car.

138
Streamlining
• Cars, aeroplanes and rockets are streamlined
(that is, have their
• drag coefficient reduced) by
• Reducing the front area
• Having a smooth body shape

139
• Key words force, vector, balanced
• forces
• By the end of this section you will be able
• to
• State that force is a vector quantity.
• State that forces which are equal in size but
• act in opposite directions on an object are
• called balanced forces and are equivalent to
• no force at all.
• Explain the movement of objects in terms of
• Newtons first law.

140
Force
• Force is a vector quantity. What do we
• mean by this?
• To describe it fully we must have size
• and direction.

141
Balanced Forces
F
F
• Balanced forces are EQUAL FORCES which act in
OPPOSITE DIRECTIONS. They CANCEL EACH OTHER OUT.

142
• If balanced forces act on a STATIONARY OBJECT,
it REMAINS STATIONARY.

If balanced forces act on a MOVING OBJECT, it
continues moving in the same direction with
CONSTANT VELOCITY.
143
• This is summarised by NEWTONS FIRST LAW which
states

An object remains at rest, or moves in a
straight line with constant velocity unless an
UNBALANCED FORCE acts on it.
144
• To understand NEWTONS FIRST LAW remember

An object tends to want to keep doing what it is
doing (so if it is sitting still it wants to stay
that way, and if it is moving with constant
velocity it wants to keep going).
145
• This reluctance to change motion is known as
inertia.
• The greater the mass, the greater the
reluctance.
• Think! Is it easier to stop a tennis ball
travelling towards you at 10 m/s or to stop a car
travelling towards you at 10 m/s?

146
Forces and Supported Bodies
• A stationary mass m
• hangs from a rope.
• What is the weight of
• the mass? In what
• direction does
• this act?
• W mg downwards

m
147
Forces and Supported Bodies
• The mass is stationary.
• Newtons law tells us
• that the forces must
• be
• balanced forces.
• The weight is
• counterbalanced by a
• force of the same size
• acting upwards due to
• the tension in the
• string.

m
148
Forces and Supported Bodies
• A book of mass m
• rests on a shelf.
• What is the weight of
• the book? In what
• direction does
• this act?
• W mg downwards

m
149
Forces and Supported Bodies
• The mass is stationary.
• Newtons law tells us
• that the forces must be
• balanced forces.
• The weight is
• counterbalanced by a
• force of the same size
• acting upwards due to
• the shelf.

m
150
What forces are acting on this stationary
hovering helicopter?
W mg
lift
W mg
151
Newtons First Law
• Newtons first law tells us that when the
• forces on an object are balanced, a
• stationary object will remain stationary.
• But it also says that if when forces are
• balanced, an object moving at constant
• velocity will continue in the same direction
• with the same velocity.

152
A moving car If a car moves with constant
velocity, then what forces are acting on it?
The ENGINE FORCE and the FRICTION FORCE
must be equal.
153
• Newtons Law Car Seat Belts

If a car stops suddenly, someone inside the car
appears to be thrown forwards. In fact, they
simply carry on moving with the cars previous
speed. A seat belt prevents this happening by
applying an unbalanced force to the person, in
the direction opposite to motion. This causes
rapid deceleration.
154
No seatbelt whats going to happen when the car
hits the wall? Explain this in terms of Newtons
1st law.
155
Whats going to happen when the motorbike hits
the wall? Explain this in terms of Newtons 1st
law.
156
Air bags Air bags produce a similar effect to
seatbelts. They apply a force which opposes the
motion, causing rapid deceleration. The large
surface area also spreads the force of impact,
reducing the pressure and reducing injury.
157

Forces in a Fluid
Terminal velocity Any free-falling object in a
fluid (liquid or gas) reaches a top speed, called
terminal velocity.
158
Terminal Velocity The air resistance acting
on a moving object increases as it gets
faster. Terminal velocity is reached when the
air-resistance (acting upwards) has increased to
the same size as the persons weight (acting
downwards)
159
time 0s, velocity 0 m/s, friction 0 N
Friction Ff(air resistance) 0 N
a -10 m/s2
W weight
160
Ff
a lt -10 m/s2
v
W weight
161
Equal opposite forces Acceleration
zero Terminal velocity
Ff
a 0 m/s2
v
W weight
162
Velocity Time Graph
velocity (m/s)
Terminal velocity
0
0
time (s)
163
air resistance
Terminal velocity is reached when the air
resistance balances the weight.
weight
164
Terminal Velocity
• What effect does opening a parachute
• have on the terminal velocity?
• When the parachute is opened, air resistance
• increases a lot. There is now an unbalanced force
• upwards, which causes deceleration. The velocity
• decreases, and the air resistance decreases until
• the forces are balanced again. The parachutist
• falls to the ground with a lower terminal
velocity.

165
• Key words Newtons second law,
• unbalanced forces, mass, force,
• acceleration
• By the end of this section you will be able to
• Describe the qualitative effects of the change of
• mass or of force on the acceleration of an object
• Define the newton
• Carry out calculations using the relationship
• between a, F and m and involving more than
• one force but in one dimension only

166
• The example of the parachutist accelerating
until the forces are balanced helps us to
understand NEWTONS SECOND LAW which states

When an object is acted on by a constant
UNBALANCED FORCE the body moves with constant
acceleration in the direction of the unbalanced
force.
167
Force, mass and acceleration
Acceleration (m/s2)
• F ma

Force (N)
mass (kg)
168
Force, mass and acceleration
• One newton (1N) is the force required to
• accelerate 1 kg at 1 m/s2

169
F ma
• Find the unbalanced force required to accelerate
a 4 kg mass at 5 m/s2
• What do I know?
• m 4kg
• a 5m/s2

F ma F 4 x 5 F 20 N
170
• Key words free body diagrams, resultant
• force
• By the end of this section you will be able
• to
• Use free body diagrams to analyse the forces
• on an object
• State what is meant by the resultant of a
• number of forces
• Use a scale diagram, or otherwise, to find the
• magnitude and direction of the resultant of
• two forces acting at right angles to each
• other.

171
Newtons First Law
• A body remains at rest, or continues at
• constant velocity, unless acted upon by an
• external unbalanced force.
• (that is objects have a tendency to keep
• doing what they are doing)

172
Newtons Second Law
• Newtons Second Law is about the
• behaviour of objects when forces are not
• balanced.
• The acceleration produced in a body is
• directly proportional to the unbalanced
• force applied and inversely proportional to
• the mass of the body.

173
Newtons Second Law
• In practice this means that
• the acceleration produced increases as
• the unbalanced force increases
• the acceleration decreases as the mass of
• the body increases

174
Which forces?
• An object may be acted upon by a number
• of forces but
• only an overall unbalanced force
• will lead to acceleration in the direction
• of that force.

175
Forces are measured in?
• Newtons Second Law can be written as
• or more commonly

176
Forces are measured in?
• which gives us the definition of the Newton
• 1N is the resultant (or unbalanced)
• force which causes a mass of 1kg to
• accelerate at 1m/ s2

177
Quick Quiz
Unbalanced force (N) Mass (kg) Acceleration (m/ s2)
10 2
20 2
20 4
2 5
10 10
5
10
5
10
1
178
Direction of force
• Consider the oil drop trail left by the car
• in motion.
• In which direction is the acceleration?
• In which direction is the unbalanced
• force?

To the right
To the right
179
Direction of force
• Consider the oil drop trail left by the car
• in motion.
• In which direction is the unbalanced
• force?

To the left the car is moving to the right and
slowing down.
180
Newtons First and Second Laws
• Remember
• Forces do not cause motion
• Forces cause acceleration

181
Free-Body Diagrams
• Free body diagrams are special
• examples of a vector diagram.
• They show the relative magnitude
• and direction of all forces acting
• on an object.
• the magnitude and direction of an
• unbalanced Force acting on an
• object.

182
Using Newtons Second Law
• In the simplest case

m
Fun
183
Using Newtons Second Law
Direction of acceleration? Direction of
unbalanced force? Formula for calculating
acceleration?
184
Solving Problems
• Always draw a diagram showing all known
• quantities (forces magnitude and
• direction, resultant acceleration and
• direction, mass of object(s) )
• Remember that Funma can be applied to
• the whole system
• When working in the vertical direction
• always include the weight

185
• Key words acceleration, gravitational
• field strength, projectiles
• By the end of this section you will be able
• to
• Explain the equivalence of acceleration due to
• gravity and gravitational field strength
• Explain the curved path of a projectile in
• terms of the force of gravity
• Explain how projectile motion can be treated
• as two separate motions
• Solve numerical problems using the above method
• for an object projected horizontally.

186
Acceleration due to Gravity
Definition A planets gravitational field
strength equals the force of gravity PER UNIT
MASS. Units? N/kg
To calculate an objects weight, use this
equation -
187
Acceleration due to Gravity
• Near a planets surface all objects experience
the same gravitational acceleration.
• This acceleration is numerically equal to the
planets gravitational field strength.

188
Acceleration due to Gravity
For example, on Earth g 10
N/kg A free-falling object will experience
acceleration of a -10 m/ s2 What does the
ve sign tell you?
189
Gravitational field strength
• Is the gravitational field strength the same on
each
• planet?
• How does distance affect gravitational field
strength?
• It decreases the further away you are from the
planets
• surface.
• What will happen to the weight of an object as it
gets
• It will decrease.

190
• The force of gravity near
• the Earths surface gives
• all objects the same
• acceleration.
• So why doesnt the
• feather reach the ground
• at the same time as the
• elephant?

191
• Why are the gaps
• between the balls
• increasing?

192
• An object is released from rest close to the
Earths
• surface. Which formula can be used to find its
velocity
• at a given time?
• v u at
• where v ? u 0 a t
• What is its velocity
• At the time of release?
• After 1 second?
• After 2 seconds?
• After 3 seconds?
• After 4 seconds?

193
Projectiles
194
Forces acting on projectiles
• What would happen to a ball kicked off a
• cliff, in the absence of gravity?

195
Forces acting on projectiles
• There would be no vertical motion
• therefore the ball would continue at
• constant speed in a straight
• line (remember Newtons first law)

196
• What is the initial vertical speed of a
• projectile fired horizontally?
• How will the horizontal speed vary during
• the objects flight?

0 m/s
It will remain the same as the initial horizontal
speed.
197
• Describe the vertical motion of an object
• projected horizontally

It will accelerate downwards due to gravity.
198
• What formula can be used to find the
• horizontal displacement of an object
• fired horizontally if horizontal velocity
• and time of flight are known?

time of flight (s)
horizontal displacement (m)
sh vht
horizontal velocity (m/s)
199
Which ball will hit the ground first?
http//www.fearofphysics.com/XYIndep/xyindep_corre
ct.html
200
Summary
Horizontal motion Vertical motion
Forces Are there forces present? If so, in what direction are they acting? No Yes The force of gravity acts downward
Acceleration Is there acceleration? If so, in what direction? What is the value of the acceleration? No Yes Acceleration "g" downward at 10 m/s2
Velocity Constant or changing? Constant Changing by 10 m/s each second
201
Solving Numerical Problems
• Always write down what you know many questions
have a lot of text surrounding the Physics so
pick out the information from the question
• Write down other relevant information you have
e.g. acceleration due to gravity
• Select formula this isnt a test of memory so
while you should learn your formulae, dont be
afraid to check against the data book or text
book
• Substitute values and rearrange formula
• Write answer clearly remembering magnitude and
direction, and units.

202
Example
• A flare is fired horizontally out to sea from a
• cliff top, at a horizontal speed of 40 m/s. The
• flare takes 4 s to reach the sea.
• What is the horizontal speed of the flare after 4
s?
• There are no forces acting in the horizontal. The
• horizontal speed remains the same 40 m/s.

203
Example
• (b) Calculate the vertical speed of the flare
after 4s
• final speed v ?
• initial vertical speed u 0 m/s Initial
vertical speed is always 0 m/s!
• acceleration a 10 m/s2
• time t 4 s
• v u at
• v 0 10 x 4
• v 40 m/s

204
Example
• (c) Draw a graph to show how vertical speed
varies with time.
• Initial vertical speed 0 m/s
• Final vertical speed 40 m/s

205
Example
• (d) Use this graph to calculate the height of the
cliff.

Displacement area under velocity-time graph ½
bh ½ x 4 x 40 80 m Height of cliff 80 m
206
• Key words Newtons third law, newton
• pairs
• By the end of this section you will be able to
• State Newtons third law
• Identify Newton pairs in situations involving
• several forces
• State that momentum is the product of mass
• and velocity.
• State that momentum is a vector quantity.

207
Forces acting between objects
• Newton realised that

When a body is acted upon by a force there must
be another body which also has a force acting on
it. The forces are equal in size but act in
opposite directions.
208
Newtons Third Law
• If object A exerts a force on object B, then B
exerts an equal and opposite force on A

Forces always occur in equal and opposite pairs
For every action there is an equal and opposite
reaction
209
Firing a gun
• Force of GUN on BULLET

Force of BULLET on GUN
210
Starting a sprint
• Force of RUNNER on BLOCKS

Force of BLOCKS on RUNNER
211
A falling apple
• Force of EARTH on APPLE

Force of APPLE on EARTH
212
A Rocket
Force of GAS on ROCKET
• Force of ROCKET on GAS

213
• Key words work done, energy, force,
• distance, power, time
• By the end of this section you will be able
• to
• State that work done is a measure of the
• energy transferred.
• Carry out calculations involving the
• relationship between work done, force and
• distance.
• Carry out calculations involving the
• relationship between work done, power and
• time.

214
Work done?
• What is meant by work done in Physics?
• When a force acts upon an object to
• cause a displacement of the object, it is
• said that work was done upon the object.

215
Work done?
• There are three key ingredients to work
• force, displacement, and cause.
• In order for a force to qualify as having done
• work on an object, there must be a
• displacement and the force must cause the
• displacement.

Note at this level, we can use distance instead
of displacement.
216
Work done?
• Formula linking work done, force and
displacement?
• Examples of work done?
• a horse pulling a plough through the field
• a shopper pushing a grocery cart down the aisle
of a supermarket
• a pupil lifting a backpack full of books upon her
shoulder
• a weightlifter lifting a barbell above his head
• an Olympian launching the shot-put, etc.
• In each case described here there is a force
exerted upon an
• object to cause that object to be displaced.

217
Work done
• A dog pulls a 4 kg sledge for a distance on
• 15 m using a force of 30 N. How much
• work does he do?
• What do I know?
• F 30N
• d 15m

218
Work done
• What do I know?
• F 30N
• d 15m
• Formula?

219
Power
• Power is the rate of doing work i.e. if
• work is done then the work done per
• second is the power.

Energy in joules
time in seconds
Power in watts (joules per seconds)
220
Power
• A dog pulls a 4 kg sledge for a distance on
• 15 m using a force of 30 N in 20 s.
• Calculate the power of the dog.
• What do I know?
• F 30N
• d 15m
• t 20s

221
Power
• What do I know?
• F 30N
• d 15m
• t 20s
• Formula?

222
Power
• What do I know?
• F 30N
• d 15m
• t 20s
• Ew 450J
• Formula?

223
• Key words gravitational potential energy,
• mass, gravitational field strength, kinetic
• energy
• By the end of this section you will be able
• to
• Carry out calculations involving the relationship
• between change in gravitational potential energy,
• mass, gravitational field strength and change in
• height.
• Carry out calculations involving the relationship
• between kinetic energy, mass and velocity.

224
Gravitational Potential Energy
• is the potential energy
• gained by an object when
• we do work to lift it
• vertically in a gravitational
• field.

225
Gravitational Potential Energy
• The work done in lifting an
• object vertically

What force is required?
226
Gravitational Potential Energy
To lift the object we must overcome the weight
Wmg
227
Gravitational Potential Energy
Vertical distance we call this height h
228
Gravitational Potential Energy
229
Kinetic Energy
• is the energy associated with a moving object.

230
Kinetic Energy
• depends on
• The mass of the object

231
Kinetic Energy
• depends on
• The velocity of the object

232
Kinetic Energy
233
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