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Title: Discover PHYSICS for GCE


1
Discover PHYSICS for GCE O Level Science
  • Unit 1 Measurement

2
1.1 What is Physics?
  • Physics is the study of Matter and Energy.
  • This includes sub-topics like
  • General Physics
  • Thermal Physics
  • Light
  • Waves
  • Sound
  • Electricity
  • Magnetism

3
  • Figure 1.1 What is Physics - a pictorial overview

4
1.2 Physical Quantities and SI units
  • In this section, youll be able to
  • understand that all physical quantities consist
    of a numerical magnitude and a unit
  • recall the seven base quantities and their units
  • use prefixes and symbols to indicate very big or
    very small SI quantities

5
1.2 Physical Quantities and SI units
  • What is a Physical Quantity?
  • A physical quantity is a quantity that can be
    measured. It
  • consists of a numerical magnitude and a unit.

6
1.2 Physical Quantities and SI units
  • The 7 base quantities and 7 base SI units are
    shown in
  • the table below.

Table 1.1 The seven base quantities and their SI
units
7
1.2 Physical Quantities and SI units
  • All other physical quantities can be derived from
    these
  • seven base quantities. These are called derived
    quantities.

Table 1.2 Some common derived quantities and units
8
1.2 Physical Quantities and SI units
  • Some common SI prefixes are listed in the table
    below.

Table 1.3 Common SI prefixes
9
1.2 Physical Quantities and SI units
  • Worked Example 1.1
  • Donovan Bailey broke the 100 m sprint world
    record
  • at the 1996 Atlanta Olympics, with a time of 9.84
    s.
  • In contrast, a dog runs at a speed of 30 km h1.
    If
  • the dog chases Donovan Bailey, will the dog catch
  • up with him?

10
1.2 Physical Quantities and SI units
  • Solution
  • First, we calculate the average speed of Donovan
    Bailey.

11
1.2 Physical Quantities and SI units
  • Solution (Continued)
  • In order to make meaningful comparisons of speed,
  • the units must be the same. So Baileys speed
  • should be converted to km h1.

Since Baileys speed of 36.7 km h 1 gt 30 km h
1, Bailey will outrun the dog over a distance of
100 m.
12
1.2 Physical Quantities and SI units
  • Key Ideas
  • A physical quantity has a numerical magnitude and
    a unit.
  • The are seven base quantities length, mass,
    time, electric current, temperature, luminous
    intensity and amount of substance.
  • The units of these seven base quantities are
    known as the SI base units
  • m, kg, s, A, K, cd, mole

13
1.2 Physical Quantities and SI units
  • Test Yourself 1.2
  • 1. Express the weight of a Quarter Pounder in
    grams, given that 2.205 pounds (lb) is equal to 1
    kilogram (kg).
  • 2. The worlds smallest playable guitar is 13 ?m
    long. Express the length in standard form.

Figure 1.5 Quarter Pounder
Figure 1.6 Nanoguitar
14
1.2 Physical Quantities and SI units
  • Solutions
  • 1.



2. 13 ?m 13 ? 10-6 m
1.3 ? 10-7 m (in standard form)
15
1.3 Measurement of Length
  • In this section, you will be able to
  • Have a good sense of the orders of magnitude
  • Describe how to measure a variety of lengths
    using the appropriate instruments (e.g. metre
    rule, vernier calipers, micrometer)
  • Use a vernier scale

16
1.3 Measurement of Length
  • The SI unit for length is the metre (m).

Figure 1.7 There is a wide range of lengths in
the natural world.
17
1.3 Measurement of Length
  • Some of the common instruments that we use to
  • measure lengths are the
  • Metre rule
  • Tape measure
  • Calipers
  • Vernier Calipers
  • Micrometer screw gauge

18
1.3 Measurement of Length
  • Metre rules can measure lengths up to 1 m.
  • Tape measures can measure lengths up to a few
    metres.

Figure 1.11 Using a metre rule to measure the
depth of a pond
Figure 1.10 Using a tape measure to measure the
width of a pond
Figure 1.9 Tape measure
19
1.3 Measurement of Length
  • Precision of an Instrument
  • The precision of an instrument is the smallest
    unit
  • that the instrument can measure.
  • What is the precision of the metre rule? The
    smallest
  • unit the metre rule can measure is 0.1 cm or 1
    mm.
  • Hence, we say that the metre rule has a precision
    of
  • 0.1 cm.

20
1.3 Measurement of Length
  • Avoiding Reading Errors
  • When using the metre rule, position your eye
    directly
  • above the markings to avoid parallax errors. By
  • taking several readings and taking the average,
    you
  • will minimise reading errors.

Figure 1.12(a) No parallax errors
Figure 1.12(b) Inaccurate measurement due to
parallax errors
21
1.3 Measurement of Length
  • Calipers An instrument for measuring the
    diameters
  • of cylinders or circular objects.

Figure 1.13(a) Inverting the jaws of the calipers
to measure inner diameters
22
1.3 Measurement of Length
Figure 1.13(b) Calipers used to measure outer
diameters.
23
1.3 Measurement of Length
  • Vernier Calipers
  • A useful instrument to measure both internal and
  • external diameters of objects. It consists of a
    main
  • scale and a sliding vernier scale.
  • The vernier calipers has a precision of 0.01 cm.

24
1.3 Measurement of Length
Figure 1.14 Parts and uses of the vernier calipers
25
1.3 Measurement of Length
  • Using the Vernier Calipers
  • Before using the vernier calipers, it is
    important to
  • check the instrument for zero error.
  • This is to check that the zero mark on the main
    scale
  • coincides with the zero mark on the vernier scale
  • when not measuring anything between the jaws.
  • Table 1.4 of the textbook shows how to deal with
  • zero errors.

26
1.3 Measurement of Length
  • Guide to Using Vernier Calipers

Figure 1.15 Using the vernier calipers to
measure the diameter of a ball bearing.
27
1.3 Measurement of Length
Table 1.4 Checking and correcting zero errors
when using vernier calipers
28
1.3 Measurement of Length
  • Micrometer Screw Gauge
  • This instrument can measure to a precision of
    0.01 mm. It
  • is used to measure the diameters of wires or ball
    bearings.

29
1.3 Measurement of Length
Figure 1.16 Step by step guide to using the
micrometer screw gauge
30
1.3 Measurement of Length
  • Table 1.5 Checking and correcting zero errors
    when using the
  • micrometer screw gauge

31
1.3 Measurement of Length
  • Key Ideas
  • 1. Instruments with their range and precision.

32
1.3 Measurement of Length
  • 2. Errors to take note for each instrument

33
1.3 Measurement of Length
  • Test Yourself 1.3
  • 1. Figure 1.17 shows a voltmeter with a strip of
    mirror mounted under the needle and near the
    scale. Suggest how this may help to reduce errors
    when taking a reading.
  • Answer When taking a reading, ensure that your
  • vision is placed directly above the needle so
    that the
  • image of the needle coincides with the needle.
    This
  • helps to reduce parallax error.

Figure 1.17 Voltmeter scale with mirror mounted
under the needle
34
1.3 Measurement of Length
2. Vernier calipers are used to measure the
diameter of a ball bearing. What is the reading
of the vernier scale? Answer Step 1
Main scale reading 2.5 cm Step 2 Vernier
coincides with 3rd line. Vernier reading
is 0.03 cm. Step 3 Reading of diameter 2.5
0.03 cm 2.53 cm
35
1.3 Measurement of Length
  • 3. The diameter of a wire is measured using a
    micrometer
  • screw gauge. A student takes an initial zero
    reading
  • and then a reading of the diameter. What is
    the
  • corrected diameter of the wire in mm?
  • A 3.37 B 3.85 C 3.89 D 3.87
  • Answer
  • The zero reading Z 0.02 mm
  • The diameter reading D 3.87 mm
  • Hence the corrected diameter reading
  • Dcorrected D Z 3.87 (0.02)
  • 3.85 mm
  • Therefore the answer is B.

36
1.4 Measurement of Time
  • In this section, youll be able to
  • Describe how to measure periods of time using the
    pendulum, stopwatch and other appropriate
    instruments.

37
1.4 Measurement of Time
  • Using a Pendulum to Measure Time
  • A simple pendulum consists of a bob attached to a
  • string.
  • A complete to-and-fro motion from R to S and back
    to R is one complete oscillation.
  • The period T is the time taken for one complete
    revolution.

Figure 1.22 A pendulum completes one full
oscillation when the bob moves from R to S and
back to R.
38
1.4 Measurement of Time
  • Instruments for Telling Time
  • All instruments use some kind of periodic motion
    to
  • tell time e.g. mechanical watches or clocks use
    the
  • oscillations of springs, quartz watches use the
  • natural vibrations of crystals.
  • Stopwatches can measure time to a precision of
    0.1 s.
  • Digital stopwatches can show readings to two
    decimal places of a second. However, human
    reaction time introduces an error of about
  • 0.30.5 s.

39
1.4 Measurement of Time
  • Experiment 1.1
  • Objective To calibrate a simple pendulum to
    measure
  • time in seconds.
  • Apparatus pendulum, stopwatch, metre rule,
    retort
  • stand and clamp

40
1.4 Measurement of Time
  • Procedure
  • 1. Fasten the metre rule vertically.
  • 2. Tie the pendulum to the clamp
  • and measure the length of the
  • string, l in metres.
  • 3. Measure the time taken t for the
  • pendulum to make 20 oscillations.
  • 4. Vary the length l between
  • 60 cm and 100 cm.

Figure 1.24
41
1.4 Measurement of Time
  • Complete the table below.
  • Plot a graph of period T/s against l/m and find
    the
  • length of pendulum with a period of one second.
  • Plot also a graph of T2/s2 against length l/m.

42
1.4 Measurement of Time
  • Results
  • The length of pendulum with a period of 1 second
    can be
  • read off the graph.

Figure 1.25(b) Graph of T2/s2 vs. l/m
Figure 1.25(a) Graph of T/s vs. l/m
43
1.4 Measurement of Time
  • Question 1 Why do we need to take the average
  • time of 20 oscillations?
  • Answer We take the average to account for human
  • reaction time. Human reaction time is about 0.3 s
    for
  • most people. It would not be accurate to stop a
  • stopwatch to measure the time taken for just one
  • oscillation.

44
1.4 Measurement of Time
  • Question 2 What can you observe about the
  • graph of T/s vs. l/m?
  • Answer The period of the pendulum, T, increases
  • with length l, but not linearly.

45
1.4 Measurement of Time
  • Question 3 What does the plot of T2/s2 vs. l/m
    tell us?
  • Answer It tells us that the square of the
    period, T2
  • is directly proportional to the length, l. This
    gives rise
  • to the straight line graph when we plot T2/s2
    against
  • l/m. By extending the straight line graph, we can
  • easily predict the period of the pendulum for
    lengths
  • that are not included in the graph we have
    plotted.

46
1.4 Measurement of Time
  • Key Ideas
  • Time intervals are measured by observing events
    that repeat themselves.
  • Clocks can be used to measure time intervals in
    minutes or hours.
  • Stopwatches can be used to measure time intervals
    to a precision of 0.1 s.
  • The period T is the time taken for the pendulum
    to swing from one end to the other and back again
    to its starting position.

47
1.4 Measurement of Time
  • Test Yourself 1.4
  • 1. How can you measure the average time taken by
    a bus to travel from home to school?
  • Answer At the beginning of the week e.g.
    Monday,record
  • the time on your watch when you board the bus.
    Record the
  • time when you alight the bus. The difference
    between the
  • two times is the time taken for the journey.
    Repeat steps
  • 2-3 over the course of the week until Friday.
    Take the
  • average of the time taken during the journey over
    the 5 days.

48
1.4 Measurement of Time
  • 2. How can you determine the period of the swing
    in the playground?
  • Answer Start the swing in its to-and-fro motion.
  • When the motion is steady, start the stopwatch
    when
  • the swing is at one end of its motion. Stop the
  • stopwatch after 20 oscillations. Record the time
    t1.
  • Repeat steps 2-3 for another set of reading t2.
  • Take average t
  • The period T is given by T

49
1.4 Measurement of Time
  • 3. Figure 1.26 shows an oscillating pendulum. If
    the
  • time taken for the pendulum to swing from A to
    C
  • to B is 3 s, what is the period of the
    pendulum?
  • Answer
  • Moving from A to C to B only
  • covers three-quarters of the
  • oscillation. Hence,

Figure 1.26
50
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