1.03 measuring length and time -1

7/25/2019 1.03 Measuring Length and Time -1

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1.03

UnderstandingMeasurement


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Physics is a science which involvesmeasurements of various physicalquantities.

Choosing an appropriate instrumentto measure a physical quantity is

important to ensure that themeasurement is accurate.


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Common Measuring Instrument

Measuring tape

Metre rule Vernier Callipers

Micrometer Screw Gauge


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Measuring Tape

Measuring length of several metres.Measuring the circumference of a

round object.

Accuracy up to 1 cm.


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Metre Rule

Measure lengths from a few cm to 1m.

Accuracy up to 0.1 cm. Precaution to be taken when using a ruler:

a) Ensure that the object is in contact

with the ruler.b) Avoid parallax errors.

c) Avoid zero errors.


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Vernier Callipers

Measure length of less than 10cm.Measure internal or external

diameter of round object and the

depth of a hole. Accuracy up to 0.01cm.


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MicrometerScrew Gauge

Measure small lengths ranging

between 0.10mm and 25.00mm.

Measure diameters of wires and

thickness of steel plates.

Accuracy up to 0.01mm.


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Parts and Functions

Inside jaws

To measure the internal diameter

of an object.

Outside jawsTo measure the external diameteror length of an object.


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Parts and Functions

Tail

To measure the depth of an object.

Screw clamp

Tighten up the vernier scale toensure that the reading does notchange while the scale is being read.


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Measuring the internaldiameter of a beaker


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Measuring the diameter of apendulum bob


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Measuring the depth of atest tube


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Steps

Read the mark on the main scale precedingthe 0 mark on the vernier scale.

Read the mark on the vernier scale that isexactly in line with any mark on the mainscale.


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Main scale = 4.50 cm

Vernier scale = 0.00 cm

Reading of vernier callipers= main scale + vernier scale

= 4.50 cm + 0.00 cm

= 4.50 cm


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= 2.40 cm + 0.09 cm

= 2.49 cm


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= 3.80 cm + 0.07 cm

= 3.87 cm


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= 12.50 cm + 0.00 cm

= 12.50 cm


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Zero Errors

No zero error

Positive zero error

Negative zero error

Correct reading= Callipers reading Zero error


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No zero error

0 cm 1

0 105


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Positive zero error

0 cm 1

0 5 10

Zero error= + 0.02cm


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Negative zero error

0 cm 1

0 105

Zero error= -[0.10-0.09]

= - 0.01cm


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Parts and Functions

Lock

To ensure that the thimble does not

rotate while you take the reading.

Ratchet knob

Prevents over-tightening and exertingtoo much pressure on the object.


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Main scale = 8.50 mm

Thimble scale = 0.48 mm

Reading of micrometer screw gauge= main scale + thimble scale

= 8.50 mm + 0.48 mm

= 8.98 mm


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Reading of micrometer screw gauge= main scale + thimble scale

= 0.50 mm + 0.40 mm

= 0.90 mm


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Reading of micrometer screw gauge

= main scale + thimble scale

= 2.00 mm + 0.19 mm

= 2.19 mm


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= 8.50 mm + 0.49 mm

= 8.99 mm


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= 3.00 mm + 0.46 mm

= 3.46 mm


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No zero error

00


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Positive zero error

0

5

0

Zero error= + 0.02 mm


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Negative zero error

0

45

Zero error

= - 0.03 mm


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Consistency

Consistency of an instrument is theability of the instrument to

measure a quantity with little or nodeviation among measurements.


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A deviation is the difference betweenthe measured value and its mean valueor the average value.


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Accuracy

Accuracy of a measurement is howclose the measurement made is to

the actual value.


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Consistent and accurate

Consistent but notaccurate


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Not consistent and notaccurate

Not consistent butaccurate


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Sensitivity

Sensitivity of an instrument is itsability to detect a small change in

the quantity to be measured.

The smaller the change which can

be measured by an instrument, themore sensitive is the instrument.

Th iti it f i t t


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The sensitivity of some common instrumentsfor measuring length.

Instrument Smallest reading

Measuring tape 1cm

Metre rule 0.1cm

Vernier Callipers 0.01cm

Micrometer screwgauge

0.001cm


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Errors in Measurement

There are two main types of errors inmeasurements.

Systematic errors

Random errors


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Systematic Errors

Systematic errors are due to incorrectcalibration of instruments.

Another cause of systematic error isthe zero error.

Zero error is due to non-zero readingwhen the actual reading should be zero.


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Random Errors

Random errors arise from unknown andunpredictable variations in condition.

Random error occur due to

parallax error

natural error (e.g. change in wind) wrong technique


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Random Errors

Random errors can be minimised byrepeating the measurement several

times and taking the average ormean value of the readings.


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Parallax Errors

Parallax error is error in the readingdue to the position of the observerseye.


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1.03 measuring length and time -1

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