Measurements

1. Measurements By Mr Tan KK

2. Measuring Instruments • Rulers or measuring tapes • Avoid parallax errors – due to the wrong positioning of the eye or the object is not at the same level as the markings of the scale. • Correction: Eye must be positioned perpendicularly at the mark of the scale.

3. Measuring Instruments

4. Measuring Instruments • How about objects without any flat sides, e.g. sphere? • Can we use a ruler to make measurements?

5. Measuring Instruments • A pair of engineer’s calipers can be used which consist of a pair of steel jaws hinged at the base.

6. Measuring Instruments • Metre Rule or measuring tape – accurate to 0.1 cm. • For greater precision, we need special instruments: vernier calipers and micrometre screw gauge.

7. Vernier Calipers

8. Vernier Calipers

9. Precautions • What are the 2 precautions that you should take when taking a reading from the vernier calipers? • Video on using the vernier calipers • Java Applet • Website: More Info on the Vernier Calipers

10. Using the Vernier Calipers

11. Correcting Zero Errors

12. Correcting Zero Errors

13. Micrometer Screw Gauge • For measuring the diameter of fine wires, the thickness of paper and similar small distances, a micrometer screw gauge (commonly shortened to micrometer) is used.

14. Micrometer Screw Gauge

15. Precautions: Micrometer • The screw should never be tightened too much. • Some modern micrometers are fitted with a ratchet to avoid this, but in older models it is possible to damage the screw by overtightening. • Video • Applet on the usage of micrometer • Website: More Info on the micrometer

16. Precautions: Micrometer • It is a good experimental technique to clean the ends of the anvil and spindle before making a measurement. • Any dirt on either of these surfaces could affect the reading. • Lastly, check for systematic errors, e.g.?

17. Correcting Zero Errors Zero error = _____________mm Observed reading = __________ mm Corrected reading = __________ mm

18. Correcting Zero Errors Zero error = _____________mm Observed reading = __________ mm Corrected reading = __________ mm

19. Measuring Time • Time is measured in years, months, days, hours, minutes and seconds but the SI unit for time is the ________________. • All timing devices make use of some regular process. • Can you name some regular process?

20. Measuring Time • Most clocks use a process which is a regularly repeating motion, such as the swing of a pendulum. • Such repeating motions are termed oscillations. • The most accurate clock is the caesium clock (an atomic clock) which depends on the oscillation of a caesium-133 atom.

21. Measuring Time • This clock is accurate to a 1 second loss or gain in 6 000 years. • Since such oscillations are very regular, the time in which one oscillation occurs should be constant. • This time is referred to as the period of the oscillation.

22. Measuring time • The pendulum clock is used to measure long intervals of time. • It measures time in hours, minutes and seconds. • The gravitational potential energy from the descending mass is used to keep the pendulum swinging. • How about clocks and watches?

23. Measuring Time • In clocks that are wound up, this energy is stored in coiled springs as elastic potential energy. • Watches are also used to measure long intervals of time. • Most modern watches and clocks depend on the vibration of quartz crystals to keep time accurately

24. Measuring Time • The energy to keep these crystals vibrating comes from a small battery. • However, there are still many watches making use of coiled springs to supply the needed energy.

25. Measuring Time • Stopwatches are used to measure short intervals of time. • There are two type of stopwatches: the digital stopwatch and the analogue stopwatch. • Which do you think is more accurate?

26. Measuring Time • The digital stopwatch is more accurate than the analogue stopwatch as can measure time in intervals of 0.01 seconds while the latter can only measure time in intervals of 0.1 seconds.

27. Measuring Time • Before giving the result of the time measurement, it is important to consider other errors involved. • For instance, if you use a digital stopwatch to time a race, you should not give the time to the nearest 0.01 s. Why?

28. Measuring Time • Before giving the result of the time measurement, it is important to consider other errors involved. • For instance, if you use a digital stopwatch to time a race, you should not give the time to the nearest 0.01 s. Why?

29. Measuring Time • The watch may be accurate, but your reaction time in starting and stopping the watch will be more than a few hundredths of a second typically 0.3 s.

30. Measuring Time • In this case, an analogue stopwatch would be just as useful.

31. Ticker Tape Timer • One timer used only in certain Physics experiments is the ticker-tape timer. • This is an electrical device making use of the oscillations of a steel strip to mark short intervals of time. • It consists of a steel strip which vibrates 50 times a second and makes 50 dots a second on a paper tape being pulled past it.

32. Ticker Tape Timer

33. Ticker Tape Timer • Between two consecutive dots there is a time interval of 1 50s or 0.02 s. • If there are 10 spaces on a piece of tape, what is the time taken for the tape to pass through? • 10 x 0.02 s = 0.20 s.

34. Ticker Tape Timer • This section of the tape is also known as a 10-dot tape. Note that the counting starts from zero.

35. Reading Measuring Cylinder • Meniscus Reading