Bright Tutorials Bright Tutorials
ICSE Class 7 Physics Question 9 of 18

Physical Quantities and Measurement — Question 12

Back to all questions
12
Question

Question 12

The mass of a lead piece is 115 g. When it is immersed into a measuring cylinder, the water level rises from 20 mL mark to 30 mL mark. Find:

(i) the volume of the lead piece,

(ii) the density of the lead in kg m-3.

Answer

(i) Mass = 115 g;
Initial volume V1 = 20 mL;
Final volume V2 = 30 mL;
Volume of lead piece = V2 - V1 = 30 - 20 = 10 mL

1 mL = 1 cm3
10 mL = 10 cm3

So, volume of the lead piece = 10 cm3.

(ii) Density = ? Density=MassVolumeDensity=11510=11.5g cm3\text{Density} = \dfrac{\text{Mass}}{\text{Volume}} \\[1em] \text{Density} = \dfrac{\text{115}}{\text{10}} = 11.5 \text{g cm}^{3}

1 g cm-3 = 1000 kg m-3
11.5 g cm-3 = 1000 x 11.5 = 11500 kg m-3

So, density of lead = 11500 kg m-3.

ICSE Class 7 Physics — Physical Quantities and Measurement: Complete Study Guide

Physical Quantities and Measurement is the foundational chapter in ICSE Class 7 Physics, carrying approximately 12 marks in the annual examination. This chapter introduces students to the scientific method of measurement, which forms the basis for all subsequent physics topics. Understanding units, instruments, and conversions at this stage is critical for success in higher classes.

A physical quantity is any property that can be measured and expressed as a number with a unit. Students learn to classify quantities as fundamental (length, mass, time, temperature) or derived (area, speed, density). The SI system — the internationally accepted standard — uses metre, kilogram, second, and kelvin as base units. Mastering unit conversions (km to m, kg to g, km/h to m/s) is a skill that will be used throughout the physics curriculum.

The chapter also covers measurement instruments such as the metre rule, measuring tape, vernier callipers, and physical balance. Students learn about least count (the smallest measurement an instrument can make), parallax error (reading error from wrong eye position), and proper measurement techniques. These hands-on skills connect classroom learning to real-world applications in science, engineering, and everyday life.

Formula / RelationDetails
1 km = 1000 mLength conversion
1 m = 100 cm = 1000 mmLength conversion
1 kg = 1000 gMass conversion
1 hour = 3600 sTime conversion
1 km/h = 5/18 m/sSpeed unit conversion
1 g/cm³ = 1000 kg/m³Density unit conversion

Must-Know Concepts

  • Fundamental quantities are independent: length (m), mass (kg), time (s), temperature (K)
  • Derived quantities are combinations: area (m²), speed (m/s), density (kg/m³)
  • SI system is the universal standard — always use correct symbols (m, kg, s, not M, KG, sec)
  • Least count of a ruler = 1 mm = 0.1 cm
  • Parallax error is avoided by reading perpendicular to the scale

Common Mistakes

  • Writing wrong unit symbols: "KG" instead of "kg", "M" instead of "m"
  • Multiplying when you should divide in conversions (or vice versa)
  • Forgetting to include units in final answers
  • Confusing mass (kg) with weight (N)
  • Measuring from the edge of a ruler instead of the zero mark

Exam Tips

  • Always show conversion steps in numericals — do not skip to the answer
  • Use tables for "differentiate" questions (SI vs CGS, fundamental vs derived)
  • Underline your final answer and include the correct unit
  • For diagram questions, label instruments clearly with least count noted
What is a physical quantity?

A physical quantity is any property of a body or phenomenon that can be measured and expressed as a numerical value along with a unit. Examples include length (5 m), mass (10 kg), time (30 s), and temperature (25°C).

What is the difference between fundamental and derived quantities?

Fundamental quantities are independent quantities that cannot be expressed in terms of other quantities (e.g., length, mass, time). Derived quantities are obtained by combining fundamental quantities (e.g., speed = distance/time, density = mass/volume).

How do you convert km/h to m/s?

Multiply the speed in km/h by 5/18. For example, 36 km/h = 36 × 5/18 = 10 m/s. To convert m/s back to km/h, multiply by 18/5.

What is parallax error and how can it be avoided?

Parallax error occurs when the observer's eye is not directly in line with the measurement mark, causing an incorrect reading. It is avoided by positioning the eye directly above (perpendicular to) the scale at the point being measured.

Why is the SI system preferred worldwide?

The SI system is preferred because it provides a universal standard that all scientists and engineers worldwide can use. This ensures consistency in measurements, accurate communication of scientific data, and fair international trade.