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Universal Gravitational Fields

When you jump, you always come back down, but did you know your body is also pulling the Earth slightly upwards towards you?

Matter (meaning any object with mass) creates its own gravitational field in the space around it.
A gravitational field is a region where any other mass will experience a non-contact, attractive force.
Gravity is exclusively attractive — it pulls objects together but never repels them.
This pulling force always acts directly towards the centre of mass of the object generating the field.

The Effect of Mass on Field Strength

If everything with mass has a gravitational field, why do we not feel pulled towards buildings or cars?

The gravitational field strength ( $g$ ) is directly proportional to the mass of the object creating it.
Objects with relatively small masses, like humans or cars, produce fields so extremely weak that their gravitational pull is completely negligible.
Massive objects, such as planets, natural satellites (moons), and stars, have enormous masses and therefore generate drastically stronger gravitational fields.
These strong fields stretch out into space, allowing these bodies to clear their orbital paths of debris and providing the centripetal force required to keep moons and planets locked in stable orbits.
Field strength is not constant everywhere; it decreases as you move further away from the object's centre.

Celestial Body	Type of Object	Approximate $g$ (N/kg)
The Sun	Star	$\sim 274$ to $293$
Jupiter	Gas Giant	$\sim 24.7$ to $26$
Earth	Rocky Planet	$10$ (OCR standard)
Mars	Rocky Planet	$\sim 3.7$
The Moon	Natural Satellite	$\sim 1.6$

Feature	Mass	Weight
Definition	The amount of matter contained within an object.	The force acting on an object due to gravitational attraction.
Type of Quantity	Scalar (has magnitude but no direction).	Vector (acts downwards towards the centre of mass).
Unit of Measurement	Kilograms (kg)	Newtons (N)
Variability	Constant regardless of location in the universe.	Changes depending on the local gravitational field strength.

Students frequently confuse mass and weight — remember that an object's mass in kg NEVER changes, but its weight in N will decrease if it moves to a planet with a weaker gravitational pull.

Always use $g = 10$ N/kg for Earth in your OCR Gateway Physics A calculations unless a specific question explicitly gives you a different value (like $9.8$ N/kg).

In explanation questions comparing planetary gravity, explicitly state that a lower gravitational field strength is caused by the celestial body having a smaller mass.

If a multiple-choice question asks for the two defining properties of a gravitational field, always look for the options stating that it is 'attractive' and 'greater for massive objects'.

A region of space where an object with mass experiences a non-contact attractive force due to the presence of another mass.

The force per unit mass exerted by a gravitational field on an object, measured in Newtons per kilogram (N/kg).

A measure of the amount of matter in an object, which remains constant everywhere in the universe.

The force acting on an object due to gravitational attraction, calculated by multiplying mass by gravitational field strength.

The single point through which the entire weight of an object can be considered to act.

A field pattern where lines point directly towards or away from a central point, such as the gravitational field around a planet.

A field where the strength and direction are exactly the same everywhere, represented by equally spaced parallel lines.

Any physical substance that has mass and occupies space.

A non-contact force that pulls objects together; gravitational fields are always attractive and never repulsive.

Large bodies, such as stars and planets, that possess enough mass to generate significant gravitational field strengths.

The resultant force directed towards the centre of a circle that keeps an object moving in a circular orbit.

A physical quantity that is described by magnitude (size) only, without a specific direction.

A physical quantity that is described by both magnitude (size) and a specific direction.

A region of space where an object with mass experiences a non-contact attractive force due to the presence of another mass.

The force per unit mass exerted by a gravitational field on an object, measured in Newtons per kilogram (N/kg).

A measure of the amount of matter in an object, which remains constant everywhere in the universe.

The force acting on an object due to gravitational attraction, calculated by multiplying mass by gravitational field strength.

The single point through which the entire weight of an object can be considered to act.

A field pattern where lines point directly towards or away from a central point, such as the gravitational field around a planet.

A field where the strength and direction are exactly the same everywhere, represented by equally spaced parallel lines.

Any physical substance that has mass and occupies space.

A non-contact force that pulls objects together; gravitational fields are always attractive and never repulsive.

Large bodies, such as stars and planets, that possess enough mass to generate significant gravitational field strengths.

The resultant force directed towards the centre of a circle that keeps an object moving in a circular orbit.

A physical quantity that is described by magnitude (size) only, without a specific direction.

A physical quantity that is described by both magnitude (size) and a specific direction.

Gravitational field

Universal Gravitational Fields

The Effect of Mass on Field Strength

Visualising Field Patterns

Comparing Gravitational Field Strengths

Comparing Mass and Weight

Calculating Weight

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Key Terms(13)

Key Terms(13)