module 4 chpt 3 Effects of Forces

Module 4:Gadgets Work Wonders (II)Chapter 3 Effects Of Forces

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Chapter 3 Effects Of Forces

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?


ObjectivesRecall 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).


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 LinkActivity 1.1Measurement of Forces








ObjectivesUnderstand 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 stationaryball to move it

a goalkeeper stops a moving balla footballer heads a

ball to change itsdirection of motion

Activity Book LinkActivity 3.2Balanced 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 forceWhen 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.© 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





ObjectivesUse 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 unitsThe 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 UnitsWe can perform conversions between the

different units of length.

Examples:2 km = (2 × 1000) m = 2000 m3 m = (3 × 100) cm = 300 cm50 cm = (50 ÷ 100) m = 0.5 m


Activity Book LinkActivity 3.3Measuring 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. © Copyright Star Publishing

Pte Ltd

a digital watcha digital stopwatch

Conversion of UnitsWe can perform conversions between the

different units of time.

Examples:30 min = (30 ÷ 60) h= 0.5 h0.1 s = (0.1 × 1000) ms= 100 ms


Activity Book LinkActivity 3.4Making a Water Clock




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?


ObjectivesExplain what is meant by speed

and accelerationCalculate the average speed and

accelerationPredict 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 speedThe 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 theinstantaneous speed of a

car

Calculating the speed of an objectExample: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 LinkActivity 3.5Finding the Speed of a Moving Marble

AccelerationWhen 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.


Acceleration

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


Calculating accelerationTo calculate the acceleration, we divide the

change in speed by the time interval.


Examples on Calculating Speed and AccelerationBen’s father took 45 min to drive 37.5 m from

hisoffice to East Coast Canoe Centre to fetch Benfrom his CCA. He waited 15 min for Ben at thecentre. 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 hishome?


Examples on Calculating Speed and Acceleration(a) from his office to the Canoe Centre


speed = taken time Totaltravelled distance Total

= 0.75h37.5km

= 50 km/h

time = 45 min = 45 ÷60 h = 0.75 h

Examples on Calculating Speed and Acceleration

(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

Examples on Calculating Speed and Acceleration(c) for the whole journey from his office to hishome?


speed = taken time Totaltravelled distance Total

= 1.5h67.5km

= 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

Examples on Calculating Speed and Acceleration

A rocket accelerates from 100 km/h to 500 km/h in 10 min. Calculate the acceleration of the rocket.


Acceleration = taken Timespeed Initial - speed Final

=

h61

km/h 100 – km/h 500

= 2400 km/h2

Final speed = 500 km/hInitial speed = 100 km/hTime taken = 10 min

= 10 ÷ 60 h = h

61

h61km/h 400 =

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 fasterwhen a force is applied in the same

direction


A moving trolley will slow down when a force is applied in the opposite direction.


a moving trolley will slow down whena force is applied in the opposite

direction


The moving trolley will change direction when a force is applied at an angle.





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



ObjectivesPlot and interpret distance-time

graphs of real situations


Distance-Time GraphsA 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, thespeed is constant.

To calculate the speed, we use the formula:


Example on Distance-Time GraphThe diagram shows the distance-time graph

of anobject.


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.









ObjectivesUnderstand 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 lifeWe 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 tosweep 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 LinkActivity 3.6Moments in Daily LifeActivity 3.7Factors Affecting Moments


Education

module 4 chpt 3 Effects of Forces