• The gravitational field strength,?g?at a point describes how?strong or weak a gravitational field is?at that point
• g?in a?radial?field (such as a planet) is calculated using the equation:

• Where:
  • g?= gravitational field strength (N kg-1)
  • G?=?Newton’s Gravitational Constant
  • M?= mass of the body producing the gravitational field (kg)
  • r?= distance from the mass where you are calculating the field strength (m)

   

• Gravitational field strength,?g, is a vector quantity
• The direction of?g?is always towards the centre of the body creating the gravitational field
  • This is the same direction as the gravitational field lines

   

• On the Earth’s surface,?g?has a constant value of 9.81 N kg-1
• However, outside the Earth’s surface,?g?is not constant
  • g?decreases as?r?increases by a factor of 1/r2
  • This is an?inverse square law relationship?with distance

   

• When?g?is plotted against the distance from the centre of a planet, r has two parts:
  • When?r?<?R, the radius of the planet,?g?is directly proportional to?r
  • When?r?>?R,?g?is inversely proportional to?r2?(this is an ‘L’ shaped curve and shows that?g?decreases rapidly with increasing distance?r)

   

Graph showing how gravitational field strength varies at greater distance from the Earth’s surface

• Sometimes,?g?is referred to as the ‘a(chǎn)cceleration due to gravity’ with units of m s-2
  • Any object that falls freely in a uniform gravitational field on Earth has an acceleration of 9.81 m s-2