Home » Handbook of O Level Physics » UNIT 4: MASS, WEIGHT AND DENSITY


  • Mass: It is the amount of matter (substance) in a body.

  • Weight: The downward force equal to product of mass and the gravitational field strength.

W = m g

  • Differences:



It is a force.

It is the amount of matter in a body.

Measured in N

Measured in kg


Scalar (,i.e., no direction)

Changes from place to place

Constant regardless of location

  • Inertia: It is the reluctance of mass to a change in its state of motion.

    • A truck has more mass and hence more inertia than a car.

  • Gravitational field strength of Earth is 10 N/kg. This means that the strength of Earth’s gravity is so strong that an object will have a weight of 10 N for every kg of mass that it has.

    • Unit: N/kg

  • Measuring instruments:

    • Spring balance: Measures weight.

      Using the formula W=mg, it can have a scale for mass too. The value of ‘g’ is assumed in such a case.

    • Beam balance: Measures mass by comparing the weight of a mass with the weight of known masses (e.g., discs).

  • Density: It is the mass per unit volume.

ρ = m / V

    • SI Unit: kg/m3

  • Measurement of volume: Volume can be found by:

    • Displacing a liquid

    • For regular shaped objects such as a block or cylinder, a formula may be used.

  • Less dense liquids and gases rise above more dense liquids and gases respectively.

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