module 4 chpt 3 Effects of Forces

Module 4:
Gadgets Work Wonders (II)
Chapter 3 Effects Of Forces
1© Copyright Star Publishing Pte Ltd

3.1 What is a force?
3.2 What are the effects of forces?
3.3 How can length and time be measured
accurately?
3.4 How can speed and acceleration be
used to describe the motion of an
object?
3.5 How are distance-time graphs plotted
and interpreted?
3.6 What is moment of a force and its
application in daily life?

Objectives
 Recall that a force is a push or a pull

What is a force?
 A force is a push or a pull.
 The SI unit of force is the newton (N).
© Copyright Star Publishing Pte Ltd 4
pushing a car

What is a pull?
 A pull is a force which you exert to
move an object towards you.
 When you pull a handle of a drawer, the
drawer will move towards you.
pulling the handle of a drawer

Pushing and Pulling forces
(a) Pushing forces include: squeezing,
pressing and kicking
(b) Pulling forces: lifting, tugging,
stretching and gravitational
attractions
(c) Combination of pushing and pulling
forces: twisting and shaking
Activity Book Link
Activity 1.1
Measurement of Forces

accurately?
object?
and interpreted?

Objectives
 Understand that a force can change
the state of rest and motion of a body

How do Forces Affect Us?
 A force can cause the following
changes to the motion of an object:
(a) Make a stationary object move
(b) Make a moving object stop
(c) Make a moving object move
slower or faster
(d) Change the direction of a moving
object

Effects of Forces
a footballer kicks a stationary
ball to move it
a goalkeeper stops a
moving balla footballer heads a ball to
change its
direction of motion
Activity Book Link
Activity 3.2
Balanced and
Unbalanced
Forces

Uses of forces in our daily
lives
(a) We apply forces to change the
direction of a pen in order to write.
(b) We apply forces to stop and move
a trolley when shopping in a
supermarket.
(c) We apply a braking force to slow
down a bicycle or a car to avoid
hitting someone.

Calculating the net force
 When two or more forces act on an object, we
can calculate the net force.
 When the net force is zero, we say that the
forces are balanced. There will be no change to
the motion of the object.

3.3 How can length and time be
measured accurately?
object?
and interpreted?

Objectives
 Use instruments for measuring length:
rulers and measuring tape
 Use digital stopwatches for measuring
time intervals

How can length be measured accurately?
 In the past:
(a) the width
of a man’s
thumb was taken
as an inch
(b) the length of a
foot was taken
as a foot

How can length be measured accurately?
 Using body parts to
measure length causes
problems as different
people have different body
lengths.
 To solve this problem of
inconsistency, a set of
standard units is now
being used to make sure
that the unit of length (and
other quantities) is the
same everywhere.
 This set of units is called
the International System
of Units or SI units.
using length of the arm to
measure length

SI units
 The SI unit of length is the metre (m).
 There are other common units based on the
metre:
(a) millimetre (mm)
(b) centimetre (cm)
(c) kilometre (km)
 The millimetre and centimetre are used to
measure lengths shorter than a metre, while
the kilometre is used to measure longer
lengths.

Instruments for measuring lengths
 Two common instruments for
measuring lengths are ruler and
measuring tape. Both instruments
measure lengths in centimetres or
millimetres.
a length of 1.3 cm is
measured with a ruler

Avoiding error in reading
 When taking measurements with a
ruler, the eye should be placed
vertically above the point being read
to avoid error.

Conversion of Units
 We can perform conversions between the
different units of length.
 Examples:
2 km = (2 × 1000) m = 2000 m
3 m = (3 × 100) cm = 300 cm
50 cm = (50 ÷ 100) m = 0.5 m
Activity Book Link
Activity 3.3
Measuring Length

Measurement of Time
 In the past, people looked at the Sun and the
moon to tell time.
 For example, sunrise to sunset was taken to
be a day, and a month was taken as the time
between a new moon and the next.

How do we measure time accurately?
 As technology advanced, even more accurate
instruments such as clocks and watches were
invented which allow us to measure and keep
track of time easily and accurately.
 For example, digital stopwatches are used to
measure the time taken by athletes to finish a
race.
a digital watch
a digital stopwatch

Conversion of Units
 We can perform conversions between the
different units of time.
 Examples:
30 min = (30 ÷ 60) h
= 0.5 h
0.1 s = (0.1 × 1000) ms
= 100 ms
Activity Book Link
Activity 3.4
Making a Water Clock

accurately?
object?
and interpreted?

Objectives
 Explain what is meant by speed and
acceleration
 Calculate the average speed and
acceleration
 Predict changes in speed and
direction when a force acts on an
object

Speed
 Speed is defined as the distance moved per
unit time.
 A car that moves 1 metre every second is
said to have a speed of 1 metre per second
(1 m/s).
a car travelling at a constant speed of 1 m/s will move 1 m every second

SI unit for speed
 The SI unit for speed is metre per second
(m/s).
 Another common unit is kilometre per hour
(km/h).
 We can calculate the speed of an object by
dividing the distance travelled by the time taken.
a speedometer tells the
instantaneous speed of a car

Calculating the speed of an object
 Example:
 A car travels a distance of 200 m in 5
seconds at constant speed. Calculate
the speed of the car.
speed =
time
distance
=
5s
200m
= 40 m/s

Average Speed
 In real life, many objects do not move at
constant speed throughout a journey.
 For objects that do not move at a constant
speed, we can take the journey as a whole and
calculate the average speed by dividing the
total distance by the total time taken.
 Average speed is also measured in metre per
second (m/s) or kilometre per hour (km/h).

Calculating average speed
Activity Book Link
Activity 3.5
Finding the Speed
of a Moving Marble

Acceleration
 When a force makes an object move
faster, we say that the object
accelerates.
 When a force makes an object move
slower, we say that the object
decelerates.
 Acceleration is the rate of change of
speed.
 The SI unit for acceleration is m/s2.© Copyright Star Publishing Pte Ltd 31

Acceleration
 A car with a constant acceleration of
2 m/s2 will increase its speed by 2
m/s every second.

Calculating acceleration
 To calculate the acceleration, we divide the
change in speed by the time interval.

Examples on Calculating Speed and Acceleration
 Ben’s father took 45 min to drive 37.5 m from
his
office to East Coast Canoe Centre to fetch
Ben
from his CCA. He waited 15 min for Ben at
the
centre. It took them another 30 min to drive
30 km home. What was his average speed
(a) from his office to the Canoe Centre?
(b) from the Canoe Centre to his home?
(c) for the whole journey from his office to his
home? © Copyright Star Publishing Pte Ltd 34

(a) from his office to the Canoe Centre
speed = takentimeTotal
travelleddistanceTotal
= 0.75h
37.5km
= 50 km/h
time = 45 min
= 45 ÷60 h
= 0.75 h

(b) from the Canoe Centre to his
home?
speed =
time
distance
=
0.5h
30km
= 60 km/h
time = 30 min
= 30 ÷60 h
= 0.5 h

(c) for the whole journey from his office to his
home?
speed = takentimeTotal
travelleddistanceTotal
=
1.5h
67.5km
= 60 km/h
Total distance of journey = 37.5 km + 30 km
= 67.5 km
Total time for journey = 45 min + 15 min
+ 30 min
= 90 min
= 1.5 h

 A rocket accelerates from 100 km/h to 500 km/h in
10 min. Calculate the acceleration of the rocket.
Acceleration = takenTime
speedInitial-speedFinal
= h
6
1
km/h100–km/h500
= 2400 km/h2
Final speed = 500 km/h
Initial speed = 100 km/h
Time taken = 10 min
= 10 ÷ 60 h
= h
6
1
h
6
1
km/h400
=

Predicting Changes in the Motion of
an Object
 An object at rest will remain stationary unless
a force acts on it.
 When a force is applied in the same direction
as the motion of a moving object, the object will
move faster.
a moving trolley will move faster
when a force is applied in the same direction

Predicting Changes in the Motion of
an Object
 A moving trolley will slow down when a
force is applied in the opposite direction.
a moving trolley will slow down when
a force is applied in the opposite direction

Predicting Changes in the Motion of an
Object
 The moving trolley will change direction
when a force is applied at an angle.

accurately?
object?
3.5 How are distance-time graphs
plotted and interpreted?

Objectives
 Plot and interpret distance-time
graphs of real situations

Distance-Time Graphs
 A distance-time graph shows how the
distance travelled by an object changes with
time.
Distance-time graph of a car

Distance-Time Graphs
 From the graph, we can obtain:
(a) the distance travelled at a particular time.
(b) information on the speed. In this case, the
speed is constant.
 To calculate the speed, we use the formula:

Example on Distance-Time Graph
 The diagram shows the distance-time graph of an
object.

Example on Distance-Time Graph
(a) Calculate the speed between 0 s and 2 s.
(b) Describe the motion of the object between 2 s and 5
s.
(c) Calculate the average speed of the object during the
5 s.

accurately?
object?
and interpreted?

Objectives
 Understand what is meant by moment
of a force and apply this to everyday
examples

What is the moment of a
force?
 When we apply a force to push a door, it
will turn about its hinges. This turning
effect is called the moment of the
force.

Moment of a force
 For a force to produce a moment,
there must be a pivot ( the hinges ).
 The moment of a force depends on
two factors:
(a) size of the force
The larger the force, the larger the
moment.
(b) perpendicular distance
between force and pivot
The larger the perpendicular
distance, the larger the moment.
large perpendicular
distance
small perpendicular
distance

Perpendicular distance and the moment
of a force
 When we push a door at its handle, the
perpendicular distance of the force to the
hinges is large.
 Therefore, only a small force is needed to
produce the moment needed to open the door.
 When we push the door near the hinges, the
perpendicular distance of the force to the
hinges is small.
 Therefore a large force is needed to produce the
moment needed to open the door.

Moments in daily life
 We make use of
moments of forces in
many areas of our daily
lives.
(a) When we use a
broom to sweep
the floor, one of
our hands acts as
the pivot while the
other hand applies
a force to produce
a moment.
using moment to
sweep the floor

Moments in daily life
 (b) When we use a fishing
rod, the end of the rod
that pushes against our
body acts as the pivot
while our hand applies a
force to produce a
moment to lift the catch.
 (c) When we use a
wheelbarrow, the axis
of the wheel acts as a
pivot while our hands
apply a force to produce
a moment to lift the
load.
Activity Book
Link
Activity 3.6
Moments in
Daily Life
Activity 3.7
Factors
Affecting
Moments

module 4 chpt 3 Effects of Forces

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