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Questions and Answers
Define a physical quantity.
A physical quantity is a quantity that can be measured and consists of a numerical magnitude and a unit.
Which of the following are base quantities in the System International of Units (SI)?
The SI unit for mass is ________________.
kilogram
The prototype kilogram based on a metal cylinder is still used today.
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Match the following prefixes with their corresponding symbols and factors:
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How can accuracy be enhanced when doing estimations?
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How should we read the volume of a liquid with a meniscus?
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Which of the following instruments is more accurate for time measurements?
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Period of oscillation in a pendulum is affected by the mass of the bob.
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What is the result of 5.24 + 4.1?
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What is the result of 26.28 - 19.2?
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Calculate the sum of 17.2, 17.5, and 17.4, then divide the result by 3.
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What is the result of 24 × 14.8?
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What is the result of 82 ÷ 12.3?
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Calculate 34.6 divided by 20.
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What is the speed of an object that traveled 10 m in 2.0 s?
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Which term best describes an apparatus that has difficulty reproducing the same reading?
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Study Notes
Physical Quantities and Units
- A physical quantity is a quantity that can be measured and consists of a numerical magnitude and a unit.
- Example: 4.5 m is a physical quantity, where 4.5 is the numerical value and m is the unit.
Base Quantities
- The System International of Units (SI) defines 7 base quantities and units:
- Length: meter (m)
- Mass: kilogram (kg)
- Time: second (s)
- Electric current: ampere (A)
- Temperature: kelvin (K)
- Amount of substance: mole (mol)
- Luminous intensity: candela (cd)
- The definitions of base quantities are based on specific physical measurements that can be reproduced accurately in laboratories around the world.
Derived Quantities
- Derived quantities are physical quantities that are derived from a combination of base units using a defining equation.
- Examples:
- Area: length × width (m²)
- Volume: length × width × height (m³)
- Speed: distance / time (m/s)
Prefixes
- Prefixes are used to indicate a decimal multiple or sub-multiple of a unit.
- Examples:
- Pico (p): 10^(-12)
- Nano (n): 10^(-9)
- Micro (μ): 10^(-6)
- Milli (m): 10^(-3)
- Centi (c): 10^(-2)
- Deci (d): 10^(-1)
- Kilo (k): 10^3
- Mega (M): 10^6
- Giga (G): 10^9
- Tera (T): 10^12
Unit Conversion
- Unit conversion is the process of converting a quantity from one unit to another similar unit.
- Example: converting 0.800 g/cm³ to kg/m³
Measuring Instruments
- Various measuring instruments are used to measure different physical quantities, such as:
- Measuring cylinder: measures volume of liquids
- Metre rule: measures length
- Digital calipers: measures length and diameter
- Digital micrometer screw gauge: measures small lengths
- Electronic balance: measures mass
- Stopwatch: measures time
Area Measurements
- Area measurements can be done using:
- Formulae for regular shapes (e.g., square, rectangle, triangle, circle)
- Estimation by dividing the figure into small squares
- Tracing the outline of the figure and counting the number of squares
Volume Measurements
- Volume measurements can be done using:
- Measuring cylinder: measures volume of liquids
- Pipette: measures specific volumes of liquids
- Burette: measures volume of liquids with high accuracy
Time Measurements
- Time measurements can be done using:
- Stopwatch: measures short intervals of time
- Pendulum: measures the period of oscillation
- Atomic clock: measures time with high accuracy### Significant Figures and Rules for Rounding Off
- The number of significant figures is directly related to the precision of the measuring instrument.
- A calculated value can be no more precise than the values used to obtain it.
- When giving answers, quote to the same number of significant figures as the least precise measurement.
Rules for Rounding Off
- Raw data are measurements taken directly from a measuring instrument, expressed to a fixed number of decimal places dictated by the units used and the precision of the instrument.
- Processed data are readings obtained from the calculation of one or more sets of raw data.
- Rules for rounding off in calculation of processed data:
- Addition and subtraction: refer to decimal place, answer has same number of decimal places as the least number of decimal places or place value in any of the numbers.
- Multiplication and division: refer to significant figure, answer has same number of significant digits as the least number of significant digits in any one of the numbers.
Practice Exercises
- Answers to practice exercises:
- a. 5.24 + 4.1 = 9.3
- b. 26.28 – 19.2 = 7.1
- c. 17.2 + 17.5 + 17.4 = 17.4
- d. 24 × 14.8 = 360
- e. 82 ÷ 12.3 = 6.7
- f. 34.6 ÷ 20 = 1.73
The Use of Algebra (Quantity and Number Algebra)
- Scientific equations are statements written in the form of complete sentences, using universal shorthand (Quantity Algebra).
- Good practice: initial substitution of original units facilitates comprehension and checking.
- Bad practice: omission of original units when substituting leads to incorrect statements.
Advanced Topic: The Ticker Tape Timer
- The ticker-tape timer is an electrical device used in certain Physics experiments to measure short intervals of time.
- It marks short intervals of time by making 50 dots per second on a paper tape being pulled past it.
- Between two consecutive dots, there is a time interval of 1/50 s or 0.02 s.
Advanced Topic: Accuracy and Precision
- Accuracy refers to the degree of closeness to the true value.
- Precision refers to the degree to which an instrument or process will repeat the same value.
- An inaccurate apparatus has a systematic error and is unable to provide a true value of the reading.
- An imprecise apparatus has difficulty reproducing the same reading; hence it will make many random errors.
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Description
This quiz covers physical quantities and units, including base quantities and their units. It is based on Chapter 1 of the Physics Matters textbook.
