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

# 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:

Weight

Mass

It is a force.

It is the amount of matter in a body.

Measured in N

Measured in kg

Vector

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.