A physical quantity is a property of matter or energy that can be measured. The table below shows common physical quantities and the instruments used to measure them:

What is Accuracy?

Accuracy refers to how close a measured value is to the true or accepted value. If you measure the length of a desk that is truly 100 cm and your instrument reads 101 cm, your measurement is close to the true value, so it is accurate.

What is Precision?

Precision refers to how consistent repeated measurements are when taken using the same instrument under the same conditions. If you measure the desk three times and get 101 cm, 101 cm, and 101 cm, your measurements are precise (they agree with each other), even if they are not exactly 100 cm.

The Relationship Between Precision and Accuracy

• A测量 can be precise but not accurate (consistently wrong in the same way)
• A测量 can be accurate but not precise (sometimes close to true value, sometimes far off)
• A good measurement system should be both precise and accurate

Example: A student measures the mass of a stone three times using the same scale:

These readings are precise because they are consistent. If the true mass is 48 g, they are also accurate. However, if the true mass is 50 g, the readings are precise but not accurate.

Different instruments have different levels of precision. For example:

• A metre ruler can measure length to the nearest millimetre (1 mm)
• A vernier caliper can measure length to the nearest 0.1 mm
• A stopwatch with centisecond display measures time to 0.01 s

Using a more precise instrument generally gives more consistent results, but accuracy still depends on:

• Correct use of the instrument
• Proper calibration
• Avoiding parallax errors
• Taking multiple readings and finding the average

Activity: Measuring Length with Different Instruments

Aim: To compare precision when measuring the same object with different instruments.

Materials: Metre ruler, vernier caliper, a wooden block.

Procedure:

1. Measure the length of the block using a metre ruler. Record your reading.
2. Measure the same length using vernier calipers. Record your reading.
3. Repeat each measurement three times.

Observation: The vernier caliper gives more precise readings because it can measure smaller divisions than the metre ruler.

Activity: Measuring Volume of a Liquid

Aim: To measure volume accurately using a measuring cylinder.

Procedure:

1. Pour some water into a measuring cylinder.
2. Read the level at the bottom of the meniscus (the curved surface of the liquid).
3. Keep your eye level with the meniscus to avoid parallax error.
4. Record the volume to the nearest millilitre.

Important: Always read the scale at eye level to ensure accuracy.

1. Parallax error: Reading the scale from an angle instead of directly in front.
2. Zero error: When an instrument does not read zero before taking measurements.
3. Instrument limitations: Using an instrument with insufficient precision for the required measurement.
4. Human error: Misreading scales or not following proper procedures.

In Tanzanian daily life, relating measuring instruments to physical quantities is essential in markets and shops. For example, when buying maize flour or rice at a local market in Dar es Salaam or Arusha, vendors use a scale (balance) to measure the mass of the product. A digital scale gives more precise readings than a traditional beam balance, ensuring customers get the exact amount they pay for. Similarly, when a nurse at a local clinic measures a patient's temperature using a thermometer, understanding precision helps read the scale correctly to determine if the patient has a fever.