Physics: Quantities and Units

1. What is a Quantity?

A quantity is something we can measure or count.

Example: length, weight, time, temperature.

2. What is a Unit?

A unit is the standard we use to measure a quantity.

It tells us how big or how much the measurement is.

Example:

– Unit for length → meter (m)

– Unit for time → second (s)

– Unit for mass → kilogram (kg)

3. Two Types of Quantities

A. Base Quantities

These are the main quantities. We do not make them from other quantities.

There are 7 base quantities in physics:

B. Derived Quantities

These are made by combining two or more base quantities.
We calculate them, not measure directly.

Examples:

– Area = Length × Width → Unit: square meter (m²)

– Volume = Length × Width × Height → Unit: cubic meter (m³)

– Speed = Length ÷ Time → Unit: meter per second (m/s)

4. Important Rules

1. Always write number + unit together.
→ Example: 5 kg, not just 5.

2. Do not mix units in one calculation.
→ If one is in meter and another in centimeter, change them to the same unit first.

📝 Practice Questions: Quantities and Units

Part A: Fill in the blanks

1. Something that we can measure or count is called a __________.

2. The standard used to measure a quantity is called a __________.

3. The SI unit for length is __________.

4. The symbol for the unit of mass is __________.

5. Time is measured in __________.

6. Quantities that are made by combining two or more base quantities are called __________ quantities.

7. Area is calculated by multiplying length and __________.

8. The unit for speed is meter per __________.

9. Electric current is measured in a unit called __________.

10. We must always write the number and the __________ together when we write a measurement.

Part B: Choose the correct answer

1. Which of the following is a Base Quantity?
a) Volume
b) Mass
c) Speed

2. What is the unit of Temperature that we use in daily life?
a) Degree Celsius (°C)
b) Meter (m)
c) Second (s)

3. Which one is a Derived Quantity?
a) Length
b) Time
c) Volume

4. What is the symbol for the unit Kilogram?
a) g
b) kg
c) km

5. Speed = Length ÷ __________.
a) Mass
b) Time
c) Area

6. The unit m² (square meter) is used for measuring __________.
a) Area
b) Volume
c) Time

7. Which statement is wrong?
a) The pencil is 15 cm long.
b) The book is 200 g heavy.
c) The rope is 5 long.

8. How many main (base) quantities are there in physics?
a) 3
b) 5
c) 7

9. Luminous intensity is a base quantity. What is its unit?
a) Candela
b) Mole
c) Ampere

10. The unit m³ (cubic meter) is the unit for __________.
a) Area
b) Volume
c) Speed