Mass and weight

Mass and weight are two distinct but related physical quantities.

Mass

• Is a measure of the quantity of matter in an object at rest relative to the observer
• Is measured in kilograms
• Remains constant regardless of location

Weight

• Is the gravitational force on an object that has mass
• Is measured in newtons
• Changes whenever the gravitational field strength changes
• Because weight is a vector, it always acts downwards towards the centre of the massive body producing the field

Key Definition Gravitational field strength, g, is defined as force per unit mass.

Gravitational field strength is calculated using

g = W / m

This value is numerically equal to the acceleration of free fall, approximately 9.8 N kg⁻¹ (or 9.8 m s⁻²) on Earth's surface. The equation can be rearranged to give

W = mg

This shows that weight is directly proportional to both mass and gravitational field strength — doubling either quantity doubles the weight.

Beam balance

• Compares masses and gives consistent readings regardless of location, because both sides experience the same g

Spring balance

• Measures weight directly and therefore gives different readings on bodies with different gravitational field strengths

Extended Extended students should understand weight as the effect of a gravitational field on a mass, connecting the force directly to the field model rather than treating weight as a standalone property.

At a Glance