Complete Study Material for JEE, NEET, CBSE Board Class 11 Exams
What are Physical Quantities ?
A: Physical quantities are those quantities that can be measured and used to explain various physical phenomena in the form of laws. Examples include length, mass, time, and force. Non-measurable quantities such as happiness or sorrow are not physical quantities.
What is Measurement ?
A: Measurement is a technique in which the properties of an object are determined by comparing them to a standard. Measurement requires tools to provide scientists with a quantity.
Why is Measurement Important?
A: Measurement helps determine the magnitude of a physical quantity, compare similar physical quantities, and verify physical laws or equations.
Define a unit.
Any given physical quantity is measured in comparison to a basic unit that is arbitrarily chosen, internationally accepted. The result of a measurement is expressed by a number which is accompanied by a unit.
How is a Physical Quantity Represented?
A: A physical quantity is expressed using magnitude and unit.
- Example: 10 meters (where 10 is the numerical value and meter is the unit).
- Formula: Physical Quantity (Q) = Magnitude × Unit = n × u
- If the unit (u) changes, the numerical value (n) changes inversely, but their product remains constant.
Types of Physical Quantities
1. Ratio (Numerical Value Only)
These physical quantities are expressed as the ratio of two similar quantities and have no unit.
- Relative density = Density of object / Density of water at 4°C
- Refractive index = Velocity of light in air / Velocity of light in medium
- Strain = Change in dimension / Original dimension
**Note: Angle is an exception as it requires a unit (degrees or radians) despite being a ratio.
2. Scalar Quantities (Magnitude Only)
These quantities have only magnitude and no direction. When you measure them, you get a numerical value with a unit, like 5 kg or 25°C.
- Examples: Length, time, work, energy
- Scalars can be negative; the negative sign indicates a decrease but does not specify direction.
- Scalars follow the normal laws of addition and subtraction.
3. Vector Quantities (Magnitude and Direction)
These quantities have both magnitude and direction. When you measure vector quantities, you get a numerical value along with a unit and a specific direction, such as 20 m/s east or 10 N north.
- Examples: Displacement, velocity, acceleration, force
- Vectors follow special vector laws of addition and subtraction, different from scalar addition.
**Note: Some quantities, like moment of inertia, are neither scalar nor vector. These are called Tensors because their values change with direction but do not behave like vectors.
Multiple Choice Questions (MCQs) with Answers & Explanations
Q1: Which of the following is NOT a physical quantity?
A) Length
B) Force
C) Happiness
D) Time
Answer: C) Happiness
Explanation: Happiness cannot be measured or expressed in the form of laws, making it a non-physical quantity.
Q2: Which of the following is a vector quantity?
A) Work
B) Speed
C) Force
D) Time
Answer: C) Force
Explanation: Force has both magnitude and direction, making it a vector quantity.
Q3: Which statement is true about scalar quantities?
A) Scalars have direction.
B) Scalars follow vector addition laws.
C) Scalars have only magnitude.
D) Scalars are always positive.
Answer: C) Scalars have only magnitude.
Explanation: Scalars do not have direction and follow ordinary mathematical operations.
Test Your Knowledge
Worksheet on Physical Quantities
Q1: Define a physical quantity and give two examples.
Q2: Explain why measurement is necessary in physics.
Q3: What is the difference between scalar and vector quantities?
Q4: Why is angle considered an exceptional ratio quantity?
Q5: Classify the following as scalar or vector quantities:
- Mass
- Acceleration
- Work
- Momentum
Frequently Asked Questions (FAQs)
Q1: Why is measurement important in science?
A: Measurement allows us to define physical quantities accurately, compare values, and formulate scientific laws.
Q2: What is the difference between scalar and vector quantities?
A: Scalars have only magnitude, while vectors have both magnitude and direction.
Q3: Why does moment of inertia not behave as a vector?
A: It does not change when the sense of rotation is reversed and has different values along different axes, making it a tensor quantity.
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