Gravity and Weight

Gravity is a fundamental force that attracts two bodies with mass toward each other. On Earth, gravity gives us weight and keeps objects grounded. The force of gravity depends on the masses involved and the distance between them.

Weight

Weight is the force due to gravity acting on a mass: \[ W = mg \] where:

\( W \): weight (N)
\( m \): mass (kg)
\( g \): gravitational field strength (\( \approx 9.8 \, \text{m/s}^2 \) on Earth)

Mass vs Weight

Mass is the amount of matter in an object; it is scalar and constant everywhere.
Weight is a force that depends on local gravity; it is a vector and varies from place to place (e.g., Earth vs Moon).

An astronaut has a mass of 70 kg. What is their weight on:

Earth (\( g = 9.8 \, \text{m/s}^2 \))
Moon (\( g = 1.6 \, \text{m/s}^2 \))

Solution:

On Earth: \( W = mg = 70 \times 9.8 = 686 \, \text{N} \)
On Moon: \( W = mg = 70 \times 1.6 = 112 \, \text{N} \)

The astronaut's mass is the same, but their weight is significantly lower on the Moon due to the weaker gravity.

Written by Thenura Dilruk