Physics
Gravitational Field Strength
Quick fact
The gravitational field strength on Earth's surface averages 9.8 N/kg, but it is only about 1.6 N/kg on the Moon, which is why astronauts appear to bounce effortlessly.
Why this is interesting
You feel heavier on some parts of Earth and lighter on others – why does your weight change even when your mass stays the same?
Read the full explanation
Understanding Gravitational Field Strength
Imagine a gravitational field as an invisible 'force region' around a planet. Every object with mass creates such a field. The strength of this field at a point tells you how much gravitational force a 1 kg object would feel there. If you place a 1 kg mass on Earth, it feels a pull of about 9.8 newtons – that's Earth's gravitational field strength. If you place that same mass on the Moon, the pull is only 1.6 newtons. This force per unit mass is called gravitational field strength (g). Your weight is simply your mass multiplied by this field strength: W = mg. So your mass never changes, but g does, which is why your weight varies depending on where you are.
A deeper explanation
Newton's law of universal gravitation explains why gravitational field strength changes. Every mass M (like a planet) attracts any other mass m with a force F = G M m / r², where G is the gravitational constant and r is the distance between their centers. Gravitational field strength is defined as the force per unit mass experienced by a small test mass, so g = F/m = G M / r². This shows that g depends only on the attracting mass (M) and the distance from its center (r). That is why Earth's g is about 9.8 N/kg at the surface (r = Earth's radius), but decreases with altitude (larger r). It also varies slightly due to Earth's non-spherical shape and density differences. Understanding g is crucial because it controls free-fall acceleration (all objects fall at the same rate in a given field), satellite orbits, and even the force you feel when stepping on a scale. Without this concept, the difference between mass and weight remains mysterious, and the mechanics of planetary motion seem disconnected from everyday experience.