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Tag: SI Units

  • Physics test papers

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  • Density of some substances

    Density of some substances

    The table below shows density of some common substances.

    Please note that density of water being 1.0gcm-3 can be used as a relative density to compare densities of other substances. For example , density of gold is 19.3 times that of water.

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  • Questions on measurements

    Questions on measurements

    1. Distinguish between basic and derived quantity. Give two examples in each case. (4mk

    2. Complete the table below. (7mks)

    QuantitySI UnitsSymbols of SI Units
    Luminous Intensity
    Ampere
    K
    Kilograms
    s
    Mole
    Length
    table about basic physical quantities

    3. Draw a burette filled with water to a volume of 28cm3.     (2mks)

    4. 60 drops fell from a burette. The first and final readings were 28cm3 and 42cm3 respectively. What is the average volume of one drop.            (3mks)

    5. Determine the density in SI Units of a solid of mass 40g with dimensions 30cm by 4cm by 3cm. (4mks

    6. 1600cm3 of fresh water of density 1g/cm3 are mixed with1200cm3 of sea water of density 1.2g/cm3. Determine the density of the mixture.   (4mks)

    7. A sphere of diameter 6.0cm is molded into a thin uniform wire of diameter 0.2mm. Calculate the length of  the wire in metres. ( Take π=22/7)   (3mks)

    8. Find the area of the shaded region in the figure 1 below (use π = 3 .14)(3mks)

    9. A test-tube  has a diameter of 3cm. how many turns would a piece of thread of length 90.42cm make round the test tube.(Take π=22/7)  (3mks)

    10.  A cylindrical column of fat has diameter 17.5cm and height 10cm. Calculate the density in g/cm3 of fat if the column has a mass of 2kg. (3mks)

    11. The following figure represents a piece of land . The two ends are semicircles of radius 70m each.

    1. Calculate
    2. The perimeter of the land                                                                               (2mks)
    • The area of the land in hectares                                                                     (3mks)
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  • International System of Units (SI): Definition, Basic Quantities, and Derived Quantities

    International System of Units (SI): Definition, Basic Quantities, and Derived Quantities

    In order to measure we need to know or define the quantity to be measured and the units for measuring it. In 1971 in Paris France, a system known as the International System of Units (Systeme’ Internationale) with seven basic physical quantities and units were agreed upon as shown in table 1.1 below.

    Basic quantitySI unitsSymbols 
    LengthMeterm
    MassKilogramkg
    TimeSeconds
    Electric currentAmpereA
    Thermodynamic temperatureKelvinK
    Luminous intensityCandelaCd
    Amount of substanceMole mol
    table 1.1 the seven basic Physical quantities

    before the SI Units were developed, scientists were using different units of measurements depending on their immediate environment.

    some of the earlier units of measurements used includes:

    • inch which is equivalent to 0.0254 m
    • mile which is equal to 1061m
    • pint which is equal to 0.57 litres
    • gallon which is equal to 4.55 litres
    • pound which is equal to 0.45kg
    • tonne which is equal to 1000kg
    • acre which is equal to 0.41 Hectares
    • grams
    • centimeters
    • seconds

    Basic Physical quantities

    A physical quantity is a property of a material or system that can be quantified by measurement.

    Basic physical quantities are quantities whose property cannot be derived from any other quantity

    Other quantities can be obtained from these basic quantities and are referred to as derived quantities.

    Derived quantities

    Derived quantities are quantities obtained by multiplication or division of basic physical quantities. derived quantities includes:

    • Area
    • volume
    • density
    • speed
    • Force
    • acceleration
    • Magnetic flux density
    • velocity
    • weight

    As an example, Area can be obtained from multiplying two lengths together. The rectangle below has its shorter length labelled w and it’s longer length labelled l. it’s area is obtained by multiplying l with w. that is, area= l x w

    Area is derived quantity because two lengths are being multiplied together. As an example, consider Speed is defined as distance covered per unit time and is usually given as distance covered/time taken. distance is a length which is a basic quantity and so is the time hence speed is obtained by dividing two basic quantities together.

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