National Physical Laboratory

What are 'g-forces' and are they caused by gravity? (FAQ - Force)

The term g-force, or Gs is not well defined but is sometimes used to convey something like:

  • a value of force, expressed as a proportion of the nominal gravitational force experienced when standing on the earth's surface or
  • a value of acceleration, expressed as a proportion of the nominal gravitational acceleration experienced when in free-fall just above the earth's surface.

Astronauts, fighter pilots, and Formula One drivers might, for example, be said to experience a force or acceleration of so many 'g' when performing a particular manoeuvre. Unfortunately, use of the letter 'g' sometimes leads to the belief that the force involved is produced gravitationally, which is rarely the case. For example, when an astronaut alters trajectory, a pilot changes speed or direction, or a racing driver goes around a corner, it is a rocket, jet, or internal combustion engine that provides the force needed to accelerate them, not gravity. In these cases the magnitude of the forces involved can be several times higher than the value of the Earth's gravitational force and hence, for example, they are said to experience '2g', '3g', '6g' etc.

Note that to accelerate means to change velocity and in turn velocity is the rate of change of an object's position in a straight line - where an object is something that has mass. Moving in a circular path, whether it is vertically (with respect to the surface of the Earth), horizontally or anywhere in between requires a force to be applied continuously - a force that accelerates the object in question towards the centre of the turning circle and stops if careering off in a straight line. The force is needed to overcome the inertial resistance that mass has to acceleration.

Although the term 'g-force' tends to be used when the forces involved are substantially higher than the '1 g' experienced standing still on the Earth's surface there is no cut-off point for force/mass/acceleration phenomena. When you walk around a corner a force is needed to accelerate your body mass - it is just that you are so used to it and its magnitude is so low (mostly) that you don't notice it. Even in a number of hi-tech situations - for example controlling the stability of an orbiting observatory, the forces and accelerations involved are so small that the sub-multiple unit used to describe them is typically 'micro-g' (ie 1 micro-g = 0.000 001 x g).

The effect of high acceleration rates on humans can be very significant and most humans start to pass out when subjected to a sustained acceleration of a few g. But it is only when the acceleration is sustained that such an effect is noticeable, let alone significant; jumping from the lowest step of a stairway onto a hard floor, for example, can produce a deceleration of many g on landing but only for an instant (depending on what type of shoes you have on, how straight your knees are etc).

  • on 13 July 1977 British racing driver David Purley survived a deceleration from 173 km/h to zero in a distance of about 0.66 m, enduring 180 g (*)
  • the beak of the red-headed woodpecker hits the bark of a tree with an impact velocity of over 21 km/h, subjecting the bird's brain to a deceleration of approximately 10 g when its head snaps back (*)
  • when 'jack-knifing' into the air to escape predators, the click beetle averages 400 g (*)

     (*) Source: Guinness Book of Records

Last Updated: 25 Mar 2010
Created: 8 Oct 2007


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