Weight•The Earth pulls all objects to its centre. This pull is calledthe force of gravity or gravitational force.•The weight of an object is the pull of the Earth on theobject.•The weight of an object can change. It depends on thegravitational force that acts an object at the place.Because of this, the weight of an object differs fromplace to place.•The weight of an object becomes less when the objectsis further away from the centre of the Earth. Thus, it isless on top of a high mountain than at sea level.
•The weight of an object is measured using springbalance or a compression balance.•The SI unit for weight is Newton (N).Weight devicesA Spring Balance A Compression Balance
Mass•The mass of an object is the quantity of mattercontained in the object.•Unlike weight, the mass of an object is constanteverywhere. This is because the quantity of matter inan object is the same wherever the object is.•The mass of an object is measured using a leverbalance, 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 massA beam balance A lever balance
Weight Mass•The pull of the Earth’sgravitational force onan object•The amount of mattercontained in an object•Changes according toplaces•Remains the same atall places.•SI unit is Newton (N) •SI unit is kilogram(kg)•Measured with aspring balance orcompression balance•Measured with a leverbalance, a beambalance or anelectronic balance
Measuring length•Length is the distance between two points.•The SI unit for length is metre (m).•Short lengths are measured in centimetres (cm) ormillimetres (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 orobjects.a. A ruler such as the metre rule can be used tomeasure the length of short straight lines orobjects. The metre rule gives an accuracy of0.1 cm.b. The correct reading is obtained only whenthe eyes are vertically above the mark on theruler.c. Parallax error occurs if the position of the eyeis wrong when taking a reading.d. A measuring tape can be used to measurethe length of long straight lines.
•Measuring the length of the curved lines.a. The instrument that can be used to measure thelength of a curve are a piece of thread and a metrerule.b. First, the thread is placed along the curved line. Theend of the curve is marked on the thread.c. Then, the length of the thread is measured using aruler.d. The length of a curved line can also be measuredusing 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 usingcalipers and a ruler.b. There are two types of calipers, namely externalcalipers and the internal calipers.c. The external calipers is used to measure the externaldiameter of an object.
d. The internal calipers is used to measure theinternal diameter of an object.
•The SI unit for area is square metre (m²)•Square kilometre (km²) can be used to measure largeareas. Other units for smaller areas are squarecentimetre (cm²) and square millimetre (mm²)•The relation between the units of the area:1 cm² = 100 mm²1 m² = 10 000 cm²1 km² = 1 000 000m²
•The area of objects with regular shapessuch as a rectangle, a triangle or a circlecan be calculated using mathematicalformulae.•The area of an irregular shape can beestimated using a graph paper.a. First, the shape of the object is tracedon 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 incm².
•The SI unit for volume is cubic metre (m³)•Other units of volume are cubic centimetre (cm³) andcubic millimetre (mm³)•The volume of solids is usually measured in cm³ and m³units.•We usually measure the volume of liquids in metricunits 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 000000 cm³
•The volume of a liquid can bemeasured using a measuringcylinder.•A more accurate volume of liquidcan be measured using either apipette or a burette.•The level of liquid in anymeasuring tool is curved. Thiscurve is known as the meniscus.
•When taking a reading, ensure that the position ofthe eye is at the same level as the bottom of themeniscus of the liquid to prevent errors. This mustbe done for all liquids except mercury.•The meniscus of water is concave while themeniscus of mercury is convex.
•When a measuring cylinder isused, make sure that it is placedon a flat surface when taking areading.•When a pipette is used, theliquid is sucked into the pipetteuntil the bottom of the meniscusreaches the mark on the pipette.This can be done using a pipettepump.•Then, the accurately measuredliquid is released from thepipette into an empty container.
•To use a burette, you must first clamp it vertically toa retort stand. Then, the liquid is poured into itthrough a filter funnel. The clip is turned slowly torelease the liquid into an empty container until thelevel of the liquid inside the burette reaches the zeromark.
•The volume of regularly and irregularly shapedsolids can be measured by using the waterdisplacement 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 measuringcylinder. The final volume is recorded.•The difference between the two readings is thevolume of the solid object.
•The figure below shows the volume of a stone ismeasured using the water displacement method.
For solids less dense than water (like a cork), a weightis tied to it before being immersed in water.
A Eureka tin can also be used to measure the volumeof regular and irregular shaped solids.
•Measuring is an important skill in scientificinvestigations.•We say that a measurement is accurate if it isvery close to the actual value.•Inaccurate measurements may lead a scientistto make a wrong conclusion to an experiment.•All measurement cannot be 100% accurate.However, we can increase the accuracy ofmeasurements by:a. Using suitable measuring tools.For example, to measure 1 ml of water, weshould use a burette instead of measuringcylinder. The division on the scale of aburette are smaller.
b. Using the right techniquesFor example, employing the correct eye position whentaking a reading.c. Taking several readings. Then, the average of thereadings is determined and taken as themeasurement.ReadingQuantity1st 2nd 3rdLength of pencil (cm) 7.1 7.2 7.0Average of readings = 7.1 + 7.2 + 7.0 cm3Therefore, the length of the pencil is 7.1 cm= 7.1 cm
The Importance of Standard Units•The earlier system of measurement were based onour body parts. These include the palm or thebreadth of the hand and the foot. This system gaverise to many problems because the size of the footor hand is different for different people.•More sophisticated systems of measurement werethen introduced. However, different countries useddifferent system of measurement. For example, inEngland, they used units such as inch, foot, yard,chain and mile in measuring length. Units such aspound and ounce were used in measuring mass.
•With the increase in global trade and travelling, itwas necessary to adopt a standard system ofmeasurement.•In 1960, the SI units or the International System ofUnits were taken as the standard units ofmeasurement for the world over.•The use of standard units has made internationaltrading, travelling and communication amongscientists easier and smoother.