Measurements

By Mr Tan KK

Measuring Instruments

Rulers or measuring tapesAvoid 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.

Measuring Instruments

Measuring Instruments

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

Measuring Instruments

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

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.

Vernier Calipers

Vernier Calipers

Precautions

What are the 2 precautions that you should take when taking a reading from the vernier calipers?Video on using the vernier calipersJava AppletWebsite: More Info on the Vernier Calipers

Using the Vernier Calipers

Correcting Zero Errors

Correcting Zero Errors

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

Micrometer Screw Gauge

Precautions: MicrometerThe 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. VideoApplet on the usage of micrometerWebsite: More Info on the micrometer

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.?

Correcting Zero Errors

Zero error = _____________mm

Observed reading = __________ mm

Corrected reading = __________ mm

Correcting Zero Errors

Zero error = _____________mm

Observed reading = __________ mm

Corrected reading = __________ mm

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?

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.

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.

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?

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

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.

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?

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.

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?

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?

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.

Measuring Time

In this case, an analogue stopwatch would be just as useful.

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.

Ticker Tape Timer

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.

Ticker Tape Timer

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

Reading Measuring Cylinder

Meniscus Reading