Gravitational field strength — Physical education, 14–17 years
How a planet’s gravitational pull is measured, and why the same object can have a different weight on another world.
A pull measured locally
Gravitational field strength tells us how strongly gravity pulls on each kilogram at a particular place. Near Earth’s surface it is about 9.8 newtons per kilogram, so a 1 kg mass feels a 9.8 N pull. It can differ on the Moon or higher above Earth.
Why use this measure?
Saying that an object is “heavy” is not enough when comparing Earth, the Moon and other planets. Scientists needed a local number that describes gravity without depending on the object chosen. Dividing weight by mass gives that number: gravitational field strength.
Finding the field strength
An astronaut has a mass of 80 kg and weighs 128 N on a planet. Use g = weight ÷ mass. So g = 128 N ÷ 80 kg = 1.6 N/kg. The astronaut’s mass stays 80 kg, but the planet’s weaker field makes the weight smaller than on Earth.
Mass is not weight
A common mistake is saying that a person’s mass changes on the Moon because their bathroom scale would show a smaller number. This is reasonable because everyday scales usually sense force, not mass directly. Mass is the amount of matter; weight is the gravitational force on it.
Mapping other worlds
Spacecraft use changes in gravitational field strength to study planets, moons and asteroids. Tracking how a probe speeds up or changes direction reveals where more mass is concentrated underground. This helps scientists make maps and choose safe landing sites.
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