Units and Measurements — Previous Year Questions (Class 11 Physics)
Measurement and units are fundamental to physics. Learn about SI units, dimensions, significant figures, and errors in measurements.
TL;DR: Measurement and units are fundamental to physics. Learn about SI units, dimensions, significant figures, and errors in measurements.
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Measurement and units are fundamental to physics. Learn about SI units, dimensions, significant figures, and errors in measurements.
Units and Measurements — Previous Year Questions with Solutions
Q (2023, 1 mark): Express 1 nanometer (nm) in terms of meter and scientific notation.
Answer: 1 nanometer = 10^-9 meter = 0.000000001 m
In scientific notation: 1 nm = 1 × 10^-9 m
Q (2023, 3 marks): Check the dimensional correctness of the equation: s = ut + (1/2)at², where s is displacement, u is initial velocity, a is acceleration, and t is time.
Answer: Dimensions:
- s (displacement): [L]
- u (velocity): [LT^-1]
- a (acceleration): [LT^-2]
- t (time): [T]
Left side: [s] = [L]
Right side:
First term: [ut] = [LT^-1][T] = [L] ✓
Second term: [(1/2)at²] = [LT^-2][T²] = [L] ✓
Both terms have dimension [L], so equation is dimensionally correct
Q (2022, 3 marks): A length is measured as 15.3 cm and mass as 4.20 kg. How many significant figures does each have? Calculate their product.
Answer: Significant figures in 15.3 cm: 3 (all digits are significant)
Significant figures in 4.20 kg: 3 (trailing zero after decimal is significant)
Product = 15.3 × 4.20 = 64.26 cm·kg
But result should have minimum significant figures = 3
Therefore: Product = 64.3 cm·kg (rounded to 3 significant figures)
Q (2021, 3 marks): Define absolute error, relative error, and percentage error. A length is measured as 10.2 cm when true value is 10.0 cm.
Answer: Absolute error (ΔL) = |Measured value - True value| = |10.2 - 10.0| = 0.2 cm
Relative error = Absolute error / True value = 0.2 / 10.0 = 0.02
Percentage error = Relative error × 100% = 0.02 × 100 = 2%
Note: Percentage error gives better idea of accuracy regardless of magnitude
Q (2023, 2 marks): The SI unit of force is newton. Express 1 newton in terms of fundamental SI units.
Answer: Force = mass × acceleration
F = m × a
1 N = 1 kg × 1 m/s²
1 N = 1 kg·m·s^-2
Or in base units: 1 N = 1 kg·m·s^-2
Q (2022, 3 marks): Using dimensional analysis, verify that the formula v = sqrt(2gh) is dimensionally correct, where v is velocity, g is acceleration due to gravity, and h is height.
Answer: Left side: [v] = [LT^-1]
Right side: [sqrt(2gh)]
= [sqrt(gh)] (2 is dimensionless)
= [sqrt([LT^-2][L])]
= [sqrt(L²T^-2)]
= [LT^-1]
Left side = Right side ✓
Therefore, the equation is dimensionally correct
Frequently Asked Questions
What is the difference between accuracy and precision?
Accuracy: How close a measurement is to the true/accepted value. Precision: How close repeated measurements are to each other. A measurement can be precise but not accurate, or accurate but not precise. Both are important in science.
Why are significant figures important in measurements?
Significant figures indicate the precision of a measurement and reflect the uncertainty in the measurement process. They prevent over-reporting precision beyond what was actually measured. Rules ensure consistency in reporting results.
More Class 11 Physics PYQs
- Current Electricity
- Ray Optics
- Electrostatics
- Moving Charges and Magnetism
- Electromagnetic Induction
- Alternating Current
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