Gravitational Field Strength (AQA A Level Physics): Revision Note

Gravitational force

• There is a universal force of attraction between all matter with mass

  • This force is known as the ‘force due to gravity’ or the weight

• The Earth’s gravitational field is responsible for the weight of all objects on Earth

• A gravitational field is defined as:

  A region of space where a mass experiences a force due to the gravitational attraction of another mass
  

• The direction of the gravitational field is always towards the centre of the mass

  • Gravitational forces cannot be repulsive

• Gravity has an infinite range, meaning it affects all objects in the universe

  • There is a greater gravitational force around objects with a large mass (such as planets)

  • There is a smaller gravitational force around objects with a small mass (almost negligible for atoms)

Gravitational field strength

• The strength of this gravitational field g at a point is the force F per unit mass m of an object at that point:

$g=\frac{F}{m}$

• Where:

  • $g$ = gravitational field strength (N kg^-1)

  • $F$ = force due to gravity, or weight (N)

  • $m$ = mass (kg)

• This equation shows that:

  • the larger the mass of an object, the greater its pull on another object

  • on planets with a large value of $g$, the gravitational force per unit mass is greater than on planets with a smaller value of $g$

• An object's mass remains the same at all points in space

  • However, on planets such as Jupiter, the weight of an object will be a lot greater than on a less massive planet, such as Earth

  • This means the gravitational force would be so high that humans, for example, would be unable to fully stand up

• Factors that affect the gravitational field strength at the surface of a planet are:

  • the radius (or diameter) of the planet

  • the mass (or density) of the planet