Lesson 4: Introduction of Mass

The Concept of Mass

The Concept of MassWeight

• 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.

• The weight of an object is measured using spring balance or a compression balance.

• The SI unit for weight is Newton (N).

Weight devices

A Spring Balance A Compression Balance

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.

Balances to measure mass

A beam balance A lever balance

Differences between weight and mass

Weight Mass

•The pull of the Earth’s gravitational force on an object

•The amount of matter contained in an object

•Changes according to places

•Remains the same at all places.

•SI unit is Newton (N) •SI unit is kilogram (kg)

•Measured with a spring balance or compression balance

•Measured with a lever balance, a beam balance or an electronic balance

Measuring Tools

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 mm1 m = 100 cm1 km = 1000 m

• 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.

The correct eye position when taking a reading

Ruler

• 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.

Measuring the length of a curve line.

An Opisometer

Measuring the diameter of objectsa. 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.

Measuring the external diameter of a breaker

External calipers

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

Measuring the internal diameter of a breaker

Internal Calipers

Measuring Area

• 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 mm²1 m² = 10 000 cm²1 km² = 1 000 000

m²

• 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.

Measuring Volume• 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 ml1 l = 1000 ml = 1000 cm³1 m³ = 1 000 000 ml = 1 000 000 cm³

Measuring the Volume of Liquids

• 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.

The correct eye position when reading

Meniscus

• 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.

The correct eye position when taking a reading

Other Liquids Mercury

• 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.

Measurement of the volume of a liquid using a pipette

• 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.

Measurement of the volume of a liquid using a burette

Measurement the volume of solids

• 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.

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

Measurement of the volume of a stone

Final volumeInitial volume

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

Measuring the volume of a cork

Initial volume Final volume

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

Measuring the volume of a stone using a Eureka tin

Measurement skill• 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.

b. Using the right techniquesFor 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.

ReadingQuantity

1st 2nd 3rd

Length of pencil (cm) 7.1 7.2 7.0

Average of readings = 7.1 + 7.2 + 7.0 cm 3

Therefore, the length of the pencil is 7.1 cm

= 7.1 cm

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.

• 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.

See you next time!!