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Mass, Weight, Density & Pressure

Basic Science & Engineering · Measurement, Drawing & Mechanics

Useful for: Railway RRB

Overview

Mass is the amount of matter; weight is the force of gravity on it; density is mass per volume; pressure is force per area.

Key points

Weight W = mg (newton); mass in kg does not change with place.
Density = mass ÷ volume; water ≈ 1,000 kg/m³.
Pressure = force ÷ area; unit pascal (N/m²).
Pascal’s law: pressure in a confined fluid is transmitted equally (hydraulic press, brakes).
Archimedes’ principle: buoyant force equals the weight of fluid displaced.

Explanation

Atmospheric pressure at sea level ≈ 101.3 kPa.
A body floats if its density is less than the fluid’s.
Sharp tools concentrate force on a small area → high pressure.
Relative density = density of substance ÷ density of water.

Step by step

Identify the concept, tool or rule involved.
Recall the definition, value or procedure.
Check units, safety logic and the direction of the effect.

Examples

A 2 kg mass weighs 2 × 9.8 = 19.6 N.
500 kg in 0.5 m³ → density 1,000 kg/m³.
100 N on 0.5 m² → pressure 200 Pa.

Traps and advanced points

Trap: Using kg for weight.
Trap: Multiplying force and area for pressure.
Trap: Thinking mass changes on the Moon.

Quick revision

W = mg.
ρ = m/V.
P = F/A.

Practice MCQs

The SI unit of weight is the
1. newton
2. kilogram
3. pascal
4. joule
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Answer: (a) newton. Weight is a force, W = mg.
On the Moon, the weight of an object compared with its weight on Earth is about
1. half
2. the same
3. six times
4. one-sixth
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Answer: (d) one-sixth. Mass stays the same; the Moon's gravity is about 1/6 of Earth's.
The density of pure water at 4 °C is about
1. 1,000 kg/m³
2. 100 kg/m³
3. 10,000 kg/m³
4. 1 kg/m³
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Answer: (a) 1,000 kg/m³. That is 1 g/cm³.
Pressure is defined as
1. force × distance
2. mass per unit volume
3. force per unit area
4. work per unit time
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Answer: (c) force per unit area. P = F/A, measured in pascal.
Standard atmospheric pressure at sea level is about
1. 9.8 × 10⁵ Pa
2. 1.013 × 10³ Pa
3. 1.013 × 10⁵ Pa
4. 1 Pa
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Answer: (c) 1.013 × 10⁵ Pa. It equals 76 cm of mercury.
Sharp knives cut easily because a small area produces
1. low pressure
2. high pressure
3. less force
4. more mass
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Answer: (b) high pressure. For the same force, pressure rises as the area falls.

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Notes and questions are written by Kundali IQ for practice. They are not official papers; always check the latest official notification for the syllabus and pattern.