What are Gs? And why do they increase with speed?

2 Answers
Feb 9, 2018

We describe large accelerations as multiples of the acceleration due to gravity.


In my experience, G is reserved to be the symbol for the constant of Universal Gravitation. It is used in Newton's formula for gravitational force between 2 bodies. I suspect you are asking about g's. It refers to acceleration. On Earth, the acceleration of a body in free-fall accelerates at 1 g, which has a value of #9.8 m/s^2#. This acceleration is also called "acceleration due to gravity".

Note: after writing the above paragraph, I did a search and found several sites using G where I expected g. They were not sites that were strict on Physics conventions, but ... now I feel like a science snob. Sorry.

Imagine you had a car that could achieve twice the acceleration of free-fall -- that would be #19.6 m/s^2# (and that would be amazing). That would be called acceleration of 2 g's. It is convenience when acceleration is significantly larger than the acceleration due to gravity to describe it as a multiple of 1 g.

Regarding your 2nd question: why do they increase with speed?
In general, they do not increase with speed. The acceleration might increase with speed in some specific situations. Where did you hear or read this? What was the situation?

I hope this helps,

Feb 10, 2018

Firstly, the previous answer (Steve’s) is correct, I just wanted to add something on the end in relation to speed.


For this to make sense you (probably) need to alter your understanding of how forces work a little.

When an object turns a corner, or moves in circular motion there only needs to be one force acting (there may be more, but only 1 is required) and that force is centripetal i.e. acting towards the centre of the circle. A good example is the orbit of the moon (for this argument, a circle) around the earth. The only force acting is gravity and that acts towards the earth (centre of mass, close to the earth, but again we’ll ignore that for now) causing it to accelerate.

Now, to develop this idea we need to clarify what we mean by an acceleration, and Newton’s 1st law. An object can have a speed in a straight line, but this tells you nothing about the direction of travel. For that, you need a concept called velocity which describes both the speed and direction (in Physics we call these vectors.)

Acceleration is defined as a change in velocity per unit time, and that allows either the speed or the direction to alter as time goes by. In other words, turning a corner at constant speed means you are accelerating (because the direction changes) and thus requires a net force. Newton’s 1st law states that an object will continue in uniform motion (a straight line at the same speed) unless acted upon by an external force. So if there is no net force, an object keeps going in the same direction at the same speed.

Secondly, why does this not tally with your experience of being in a car on a roundabout? You seem to feel a force outwards, that people call centrifugal, but is in fact just the effect of mass not wanting to change where it is (or where it is going) called inertia. Masses, in effect, resist acceleration, including turning their direction, and the greater the mass, the more resistance there is.

Now we can finally relate the question (say an F1 car taking a high speed turn) to the so called “g force” experienced by the driver. As the mass requires a force to turn it, given by #F = (mv^2)/r#. This shows that as speed increases the force required to turn the mass increases exponentially (as a squared term) and as the radius reduces (the turn on the track becomes ‘tighter’) so the force also rises. This required force (on a level track) comes from the friction between tyre and track and points inwards (centripetally) towards the inside of the turn.

The inertia of the mass (driver) creates an apparent force pointing outwards, but we now know this is just mass being “stubborn” and not willing to accelerate. This “force” is described in units of “g” or “G” to compare it to the force we experience as gravity, in other words in units of 9.81 Newtons for every kilogram of mass.

Sorry for the long answer, but it isn’t simple! Any clearer?