IGCSE Physics / Motion in the universe
Gravitational field strength on other planets
Edexcel International GCSE Physics (4PH1), chapter 26: Motion in the universe, specification point 8.3. Free lesson.
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Gravitational field strength, g, tells you how strongly gravity pulls on each kilogram of mass. It differs between worlds, so weight changes, but mass stays the same.
What g means on different worlds
Gravitational field strength, g, is the force of gravity on each kilogram of mass. It is measured in newtons per kilogram (N/kg). At the surface, g is about 10 N/kg on Earth, 3.7 N/kg on Mars and 1.6 N/kg on the Moon. This means the Moon pulls on each kilogram about six times less strongly than Earth does. Earth: g ≈ 10 N/kg. Mars: g = 3.7 N/kg. Moon: g = 1.6 N/kg.
What affects gravitational field strength
The strength of a planet's or moon's field depends on its own properties and how far you are from it. More mass creates a stronger pull. A larger radius puts the surface further from the centre, making g weaker at the surface. Moving further away from a planet also makes its field weaker. Distance from the Sun does not decide g at a planet's surface. More mass means a stronger field and a bigger g. A larger radius means the surface is further from the centre, so g is smaller there. The further you are from a planet, the weaker its field becomes.
How to calculate weight
Weight is the force of gravity on an object, measured in newtons (N). Use the equation W = m × g. Mass, m, is the amount of matter in an object, measured in kilograms (kg). The equation is on the equation list, so choose it and substitute the values. Weight changes when g changes, but mass stays the same everywhere. W = weight in N. m = mass in kg. g = gravitational field strength in N/kg.
Why mass stays the same
Feeling lighter on the Moon does not mean an astronaut has less mass. Mass measures the amount of matter in the astronaut, and that does not change when she travels. Her weight is smaller because the Moon's g is smaller. On the Moon, the astronaut's mass stays 70 kg. Her weight is less because gravity pulls less strongly there.
Why Mars has a smaller g than Earth
The field at a planet's surface comes from the planet's own mass. Mars has less mass than Earth, so its surface gravitational field strength is smaller. Mars being further from the Sun is not the reason. Use the planet's mass to explain differences in g at the surface. Do not use distance from the Sun as the cause.