This topic accounts for approximately 5% of your exam marks.
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Rare
Stable5%
Orbital motion, gravitational fields and circular motion in the solar system tested as shorter questions.
What gravity does
Every object with mass attracts every other object with mass. This pull is the gravitational force
The bigger the masses, the bigger the pull. The Sun pulls on planets; planets pull on moons; the Earth pulls on everything on its surface, including you
On a planet's surface, the force pulling everything towards the centre is what we call weight:
= the force on an object due to gravitational attraction
Definition of gravitational field strength
The , given the symbol g, is defined as:
g = the gravitational force per unit acting on an object placed in the field
Units: N/kg (newtons per kilogram)
The relationship between weight and mass is:
weight W = mass m × gravitational field strength g
W (N) = m (kg) × g (N/kg)
On Earth's surface, g ≈ 10 N/kg. So a 50 kg pupil has a weight of 50 × 10 = 500 N
Mass vs weight: keep them separate
Mass
Weight
What it measures
How much matter an object contains
The force that gravity exerts on it
Unit
kilogram (kg)
newton (N)
Type of quantity
Scalar
Vector, points downwards
Changes with location?
A 70 kg astronaut is always 70 kg, whether on Earth, on the Moon, or in deep space. But the astronaut weighs 700 N on Earth, only about 112 N on the Moon (70 × 1.6), and zero N in deep space far from any planet
How g varies around the Solar System
The larger the planet's mass and the smaller its radius, the stronger the gravitational field at its surface
Body
g at surface (N/kg)
Compared to Earth
Moon
≈ 1.6
About 1/6 of Earth's
Mercury
≈ 3.7
About 1/3
Mars
≈ 3.7
About 1/3
Earth
≈ 10
On the Moon, gravity is weak. The Apollo astronauts could bounce in long, slow strides. Lifting a 100 kg lump of rock there feels like lifting 16 kg on Earth
On Jupiter, gravity is so strong that a person standing on the cloud-tops (if they could) would weigh more than 2.5 times what they do on Earth. Human leg muscles cannot push against that much force, so a person on Jupiter would collapse to the floor and could not get back up
Why g varies with height
The strength of a gravitational field falls with distance from the centre of the planet
On the surface of Earth, g is about 10 N/kg
At the height of the International Space Station (about 400 km up), g has only dropped to about 8.9 N/kg, only 11% less than at the surface. Astronauts on the ISS are not weightless because gravity has vanished; they appear weightless because they are in free-fall, falling around the Earth at the same rate as the station
For ordinary problems at this level, treat g as constant at the surface of any given planet. It does not change appreciably between the top and bottom of a mountain
Exam tip
Why gravitational field strength is smaller on the Moon than on Earth
What comes up: a multiple-choice question asks which statement correctly explains why g on the Moon is less than g on Earth.
Write: the Moon has less mass than the Earth. A smaller mass produces a weaker gravitational field at its surface, so g is lower and an object weighs less there.
Watch out: the mark scheme rejects the ideas that atmosphere, distance from the Sun, or greater density cause the difference. Only mass (and radius) determine surface g — the Moon's lower mass is the key reason.