The Earth pulls all objects to its centre. This pull is called the force of gravity or gravitational force. The weight of an object is the pull of the Earth on the object.

2. Weight
•The Earth pulls all objects to its centre. This pull is called
the force of gravity or gravitational force.
•The weight of an object is the pull of the Earth on the
object.
•The weight of an object can change. It depends on the
gravitational force that acts an object at the place.
Because of this, the weight of an object differs from
place to place.
•The weight of an object becomes less when the objects
is further away from the centre of the Earth. Thus, it is
less on top of a high mountain than at sea level.

3. •The weight of an object is measured using spring
balance or a compression balance.
•The SI unit for weight is Newton (N).
A Spring Balance A Compression Balance
Weight devices

4. Mass
•The mass of an object is the quantity of matter
contained in the object.
•Unlike weight, the mass of an object is constant
everywhere. This is because the quantity of matter in
an object is the same wherever the object is.
•The mass of an object is measured using a lever
balance, a beam balance or an electronic balance.
•The SI unit for mass is the kilogram (kg).
•The weight of an object having a mass of 1 kg is 10 N.
•Weight and mass are two different quantities.

5. A beam balance A lever balance
Balances to measure mass

6. Weight Mass
•The pull of the Earth’s •The amount of matter
gravitational force on contained in an object
an object
•Changes according to •Remains the same at
places all places.
•SI unit is Newton (N) •SI unit is kilogram
(kg)
•Measured with a •Measured with a lever
spring balance or balance, a beam
compression balance balance or an
electronic balance

7. Measuring length
•Length is the distance between two points.
•The SI unit for length is metre (m).
•Short lengths are measured in centimetres (cm) or
millimetres (mm).
•Long distances are measured in kilometres (km).
•The relation between the units of length:
1 cm = 10 mm
1 m = 100 cm
1 km = 1000 m

8. •Measuring the length of straight lines or
objects.
a. A ruler such as the metre rule can be used to
measure the length of short straight lines or
objects. The metre rule gives an accuracy of
0.1 cm.
b. The correct reading is obtained only when
the eyes are vertically above the mark on the
ruler.
c. Parallax error occurs if the position of the eye
is wrong when taking a reading.
d. A measuring tape can be used to measure
the length of long straight lines.

10. •Measuring the length of the curved lines.
a. The instrument that can be used to measure the
length of a curve are a piece of thread and a metre
rule.
b. First, the thread is placed along the curved line. The
end of the curve is marked on the thread.
c. Then, the length of the thread is measured using a
ruler.
d. The length of a curved line can also be measured
using an opisometer and a ruler.

11. Measuring the length of a curve line.
An Opisometer

12. Measuring the diameter of objects
a. The diameter of objects can be measured using
calipers and a ruler.
b. There are two types of calipers, namely external
calipers and the internal calipers.
c. The external calipers is used to measure the external
diameter of an object.

13. d. The internal calipers is used to measure the
internal diameter of an object.

14. •The SI unit for area is square metre (m²)
•Square kilometre (km²) can be used to measure large
areas. Other units for smaller areas are square
centimetre (cm²) and square millimetre (mm²)
•The relation between the units of the area:
1 cm² = 100 mm²
1 m² = 10 000 cm²
1 km² = 1 000 000
m²

15. •The area of objects with regular shapes
such as a rectangle, a triangle or a circle
can be calculated using mathematical
formulae.
•The area of an irregular shape can be
estimated using a graph paper.
a. First, the shape of the object is traced
on the graph paper.
b. Then, every square that is fully
covered, half – covered and more than
half – covered is ticked.
c. The total number of ticks is counted.
This gives you the estimated area in
cm².

16. •The SI unit for volume is cubic metre (m³)
•Other units of volume are cubic centimetre (cm³) and
cubic millimetre (mm³)
•The volume of solids is usually measured in cm³ and m³
units.
•We usually measure the volume of liquids in metric
units such as millimetre (ml) and litre (l).
•The relation between the units of volume:
1 cm³ = 1 ml
1l = 1000 ml = 1000 cm³
1 m³ = 1 000 000 ml = 1 000
000 cm³

17. •The volume of a liquid can be
measured using a measuring
cylinder.
•A more accurate volume of liquid
can be measured using either a
pipette or a burette.
•The level of liquid in any
measuring tool is curved. This
curve is known as the meniscus.

18. •When taking a reading, ensure that the position of
the eye is at the same level as the bottom of the
meniscus of the liquid to prevent errors. This must
be done for all liquids except mercury.
•The meniscus of water is concave while the
meniscus of mercury is convex.

19. •When a measuring cylinder is
used, make sure that it is placed
on a flat surface when taking a
reading.
•When a pipette is used, the
liquid is sucked into the pipette
until the bottom of the meniscus
reaches the mark on the pipette.
This can be done using a pipette
pump.
•Then, the accurately measured
liquid is released from the
pipette into an empty container.

20. •To use a burette, you must first clamp it vertically to
a retort stand. Then, the liquid is poured into it
through a filter funnel. The clip is turned slowly to
release the liquid into an empty container until the
level of the liquid inside the burette reaches the zero
mark.

21. •The volume of regularly and irregularly shaped
solids can be measured by using the water
displacement method.
•First, a measuring cylinder is half – filled with water.
The initial volume of the water is recorded.
•A solid object is slowly lowered into the measuring
cylinder. The final volume is recorded.
•The difference between the two readings is the
volume of the solid object.

22. •The figure below shows the volume of a stone is
measured using the water displacement method.

23. For solids less dense than water (like a cork), a weight
is tied to it before being immersed in water.

24. A Eureka tin can also be used to measure the volume
of regular and irregular shaped solids.

25. •Measuring is an important skill in scientific
investigations.
•We say that a measurement is accurate if it is
very close to the actual value.
•Inaccurate measurements may lead a scientist
to make a wrong conclusion to an experiment.
•All measurement cannot be 100% accurate.
However, we can increase the accuracy of
measurements by:
a. Using suitable measuring tools.
For example, to measure 1 ml of water, we
should use a burette instead of measuring
cylinder. The division on the scale of a
burette are smaller.

26. b. Using the right techniques
For example, employing the correct eye position when
taking a reading.
c. Taking several readings. Then, the average of the
readings is determined and taken as the
measurement.
Reading 1st 2nd 3rd
Quantity
Length of pencil (cm) 7.1 7.2 7.0
Average of readings = 7.1 + 7.2 + 7.0 cm = 7.1 cm
3
Therefore, the length of the pencil is 7.1 cm

27. The Importance of Standard Units
•The earlier system of measurement were based on
our body parts. These include the palm or the
breadth of the hand and the foot. This system gave
rise to many problems because the size of the foot
or hand is different for different people.
•More sophisticated systems of measurement were
then introduced. However, different countries used
different system of measurement. For example, in
England, they used units such as
inch, foot, yard, chain and mile in measuring length.
Units such as pound and ounce were used in
measuring mass.

28. •With the increase in global trade and travelling, it
was necessary to adopt a standard system of
measurement.
•In 1960, the SI units or the International System of
Units were taken as the standard units of
measurement for the world over.
•The use of standard units has made international
trading, travelling and communication among
scientists easier and smoother.