AQA A Level Physics

Revision Notes

7.1.2 Gravitational Field Strength

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 Attractive Force, downloadable AS & A Level Physics revision notes

The Earth’s gravitational field produces an attractive force

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:

Gravitational Field Strength Equation_2

  • 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 not be unable to fully stand up

gravitational field strength, downloadable AS & A Level Physics revision notes

A person’s weight on Jupiter would be so large that a human 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

Worked Example

Calculate the mass of an object with weight 10 N on Earth.

Exam Tip

There is a big difference between g and G (sometimes referred to as ‘little g’ and ‘big G’ respectively), g is the gravitational field strength and G is Newton’s gravitational constant. Make sure not to use these interchangeably!

Remember the equation density ρ = mass m ÷ volume V, which may come in handy with some calculations

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