Measuring length and time
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Measuring length and time

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Measuring length and time Measuring length and time Presentation Transcript

  • 1.5 How do we measure length? • The metre rule and half-metre rule are used to measure lengths to a precision of 0.1 cm. • A vernier caliper is used to measure lengths to a precision of 0.01 cm. • A micrometer is used to measure length to a precision of 0.01 mm.
  • 1.5 How do we measure length? • Correct way to read the scale on a ruler • Position eye perpendicularly at the mark on the scale to avoids parallax errors • Another reason for error: object not align or arranged parallel to the scale
  • 1.5 How do we measure length? Zero Error • Many instruments do not read exactly zero when nothing is being measured • Happen because they are out of adjustment or some minor fault in the instrument • Add or subtract the zero error from the reading shown on the scale to obtain accurate readings
  • 1.5 How do we measure length? • The table below shows the range and precision of some measuring instruments Instrument Range of measurement Accuracy Measuring tape 0−5 m 0.1 cm Metre rule 0−1m 0.1 cm Vernier caliper 0 − 15 cm 0.01 cm Micrometer screw gauge 0 − 2.5 cm 0.01 mm
  • 1.5 How do we measure length? Vernier Caliper • allows measurements up to 0.01 cm • Consists of a 9 mm long scale divided into 10 divisions
  • 1.5 How do we measure length? Vernier Caliper • The object being measured is between 2.4 cm and 2.5 cm long. • The second decimal number is the marking on the vernier scale which coincides with a marking on the main scale
  • 1.5 How do we measure length? Vernier Caliper •Here the eighth marking on the vernier scale coincides with the marking at C on the main scale •Therefore the distance AB is 0.08 cm, i.e. the length of the object is 2.48 cm
  • 1.5 How do we measure length? Using the Vernier Caliper
  • 1.5 How do we measure length? Micrometer Screw Gauge • Used to measure diameter of fine wires, thickness of paper and small lengths • The micrometer has two scales: – Main scale on the sleeve – Circular scale on the thimble • There are 50 divisions on the thimble. • One complete turn of the thimble moves the spindle by 0.50 mm.
  • 1.5 How do we measure length? Micrometer Screw Gauge • Two scales: main scale and circular scale • One complete turn moves the spindle by 0.50 mm. • Each division on the circular scale = 0.01 mm
  • 1.5 How do we measure length? Using the micrometer
  • 1.5 How do we measure length? Precautions when Using a Micrometer 1. Never tighten thimble too much – Modern micrometers have a ratchet to avoid this 2. Clean the ends of the anvil and spindle before making a measurement – Any dirt on either of surfaces could affect the reading 3. Check for zero error by closing the micrometer when there is nothing between the anvil and spindle – The reading should be zero, but it is common to find a small zero error – Correct zero error by adjusting the final measurement
  • 1.6 How do we measure time? Time • Measured in years, months, days, hours, minutes and seconds • The SI unit for time is the second (s).
  • 1.6 How do we measure time? Stopwatch • Measure short intervals of time • Two types: digital stopwatch, analogue stopwatch • Digital stopwatch more accurate as it can measure time in intervals of 0.01 seconds. • Analogue stopwatch measures time in intervals of 0.1 s.
  • 1.6 How do we measure time? Errors Occur in Measuring Time • If a digital stopwatch is used to time a race, should not record time to the nearest 0.01 s. • This is because the reaction time in starting and stopping the watch will be more than a few hundredths of a second. • Here an analogue stopwatch would be just as useful.
  • 1.6 How do we measure time? Ticker-tape Timer • An electrical device making use of the oscillations of a steel strip to mark short intervals of time • Steel strip vibrates 50 times a second and makes 50 dots a second on a paper tape being pulled past it
  • 1.6 How do we measure time? Ticker-tape Timer • Time interval between two consecutive dots is 0.02 s. • If there are 10 spaces on a pieces of tape, time taken is 10 × 0.02 s = 0.20 s • Counting of the dots starts from zero. • A 10–dot tape is shown below.
  • 1.6 How do we measure time? Period of a pendulum • The oscillation of a simple pendulum is an example of a regularly repeating motion. • The time for 1 complete oscillation is referred to as the period of the oscillation.