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# Physical Quantities, Units and Measurements

This is Unit 1 for Physics 5054. It comprises physical quantities and SI units; measurements.
by

## Khairunnisaa Hj Ariffin

on 15 January 2013

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#### Transcript of Physical Quantities, Units and Measurements

Basic physical quantities = base quantities Derived quantities The International System of Units (SI units) were developed to standardise the usage of units worldwide. Prefixes of units - to simplify the expression of quantities that are VERY large or VERY small. Length of a book = 0.16 m 1.1 Physical quantities and SI units 1.2 Measurement of length Learning objectives: Topic 1 Physical Quantities, Units and Measurements All other physical quantities are derived from the base quantities. Standard form can also be used to express quantities that are VERY large or VERY small. The SI unit for length is the metre (m). physical quantity magnitude unit Common instruments to measure length metre rule calipers vernier calipers micrometer screw gauge The smallest value that an instrument can measure = PRECISION Parallax errors are caused by the inaccurate positioning of the observer's eye while taking readings. How to avoid?
The eye should be positioned directly above the markings. Zero error is the reading shown when it should be zero. Accurate readings can still be taken using these instruments. To measure the internal and external diameters and the depth of objects. Has a precision of 0.01 cm Example

The diagram below shows the main scale and vernier scale of a pair of vernier calipers used to measure the diameter of a ball bearing. What is the diameter of the ball bearing if there is a) no zero error? b) a zero error of +0.03 cm c) a zero error of -0.03 cm? To measure lengths to a precision of 0.01 mm To measure objects of less than 1 cm. 1.3 Measurement of Time The SI unit for time is the second (s). Time is measured by observing events that repeat at regular intervals or periods. A simple pendulum consists of a heavy object, called a bob, attached to a string. The bob swings back and forth, or oscillates. The time taken for the pendulum to complete a full oscillation is called its period. A stopwatch can be used to measure the period T of a pendulum. The time taken for 20 oscillations is measured and then divided by 20 to calculate the average value. The period T of a simple pendulum is independent of
a)the amplitude of oscillation (angle of swing);
b)the mass of the bob. The period T of a simple pendulum is dependent on
a)the length l of the pendulum;
b)the acceleration due to gravity g. Example

The bob of the pendulum shown was pulled to position P and then released. The time taken for the pendulum to swing 20 oscillations is 10 s.

a)What is the period of the pendulum?
b)What is the time taken for the pendulum to swing from P to Q? 1.4 Measurement of Mass lever balance electronic balance spring balance
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