# If direction and speed are always relative, what's with G Forces in the remoteness of space?

Let’s say I am traveling along in the remoteness of space at a constant speed and direction relative to, say, the sun.

I am so far away from any large body that I am practically unaffected by any gravitational pull.

If I suddenly stop and “change direction”, I would feel G forces, right?

But why would I feel G forces? Although I may have been moving along at constant speed compared to the sun, haven’t I also been perfectly stationery relative to Asteroid Flobbermawobby?

If you are suddenly stopping and changing direction you are experiencing acceleration and so experience a g-force.

But relative to the asteroid, I didn’t.

Being “perfectly stationery” is also “moving along at constant speed” … it’s just that the speed is 0.

If you measure your motion relative to the the sun, you’re slowing and stopping.

But if you measure your motion relative to Asteroid F, you’re speeding up and going.

Either way, you’re accelerating, and you experience G forces.

Sure you did. Previously you were stationary relative to the asteroid. Now you’re moving.

Isn’t it covered by Newton’s First and Second laws of motion? First law is that an object in motion will tend to stay in motion, while second law explains how a force will affect an object. Since the space traveller is in motion, changing the direction will require the application of force, which the space traveller will feel as a form of g force.

There doesn’t need to be another object in the frame to make relativity work. It can be relative to what you would have been doing if the applied forces had been different.

Furthermore, all of you is not experiencing the g-force at the same time: it has to be transmitted to different parts of your body and/or spacecraft. So, the particles are briefly moving relative to other particles in the spacecraft even without a frame of reference outside of the spacecraft.

If you were previously stationary relative to the asteroid, and you then “stop (relative to another body) and change direction” then you are no longer stationary relative to the asteroid.

If you accelerate in any direction, then your velocity relative to any body will have changed. You might be going slower relative to asteroid X but faster relative to asteroid Y; that doesn’t matter - all that matters is that you have accelerated.

Picture this scenario: you’re heading away from the sun at 1,000km/h. Directly ahead of you out of the windscreen of your spaceship is asteroid A stationary relative to the sun (and so, relatively speaking, moving towards you at 1,000km/h). Directly behind you is asteroid B, heading away from the sun at 1,000km/h (and so stationary relative to you).

You now decelerate to zero, relative to the sun. You feel a “g-force” pushing you towards the windscreen, due to the deceleration from 1,000km/h away from the sun to 0km/h.

From an observer on asteroid A, you have gone from moving at 1,000km/h towards the asteroid, to stationary. Again, the g-force is the same.

From an observer on asteroid B, you have gone from stationary, to moving at 1,000km/h “backwards” towards the asteroid (because the asteroid is approaching the rear of your craft at that velocity). Again the g-force is the same (imagine reversing fast in your car from a standstill - the “g-force” you feel is towards the windscreen, the same as it is when braking hard while travelling forwards).

Are you suggesting that at the exact same time that you stopped and changed direction, so did Asteroid Flobbermawobby?

You did stop and change direction relative to yourself, and that’s why you feel the g-forces. Even if Asteroid Flobbermawobby magically stopped and changed direction at the same time, it doesn’t affect you at all.

D’Alembert’s principle

D’Alembert’s principle describes how accelerating frames can be modelled as non-acclerating frames with the addition of inertial forces (e.g. “g-forces”)

The equivalnce principle is a central feature of general relatvity and describes how inertial forces and graviational forces are locally equivalent in that theory (mathematically they are both due to non-vanishing Christoffel symbols in a frame at the particualr event in spacetime).

mach’s principle is the conjecture that inertial forces arise out of acceleration relative to a universal centre of mass frame. It’s an idea that physcists like a lot, but no-ones managed to incorporate it fully in to a satisfactory theory of gravitation as of yet.

The problem you are having is that you interpret the phrase “G Force” to mean a force due to the gravity of some other body. This is wrong. Here, on the surface of the Earth, we experience a “force” pulling us down (and an equal, and opposite, force “pushing” up against our feet) due to the acceleration of Gravity. We have arbitrarily ascribed the value of “1 g” to this force. When the Space Shuttle lifts off, at maximum acceleration the occupants experience a force approximately three times that, and are pressed forcibly back into their seats. We say they are experiencing a force of “3 g.” In the early days of the space program, when they wanted to determine a human’s ability to withstand the stresses of acceleration, they put volunteers in “Rocket Sleds,” which achieved forces in excess of 46 g. At no time were these forces ascribed to the action of another celestial body—they were entirely the result of the object in question being accelerated.

In the case of your example, you posit that you are initially moving at a “constant speed and direction.” Then, you “suddenly stop and ‘change direction.’” This can only be accomplished by firing a retro-rocket, or in some other fashion accelerating (to slow down is negative acceleration, but acceleration it is, still.) It is this acceleration, and this acceleration alone, which causes you to experience G Forces, whether there are any other objects nearby, and regardless of how you may be moving with respect to them.

Well, if you are travelling along in your spaceship at a constant speed, and suddenly push a button that fires the rockets along a vector, the transition is not immediate. The rocket will “push” (I’m using the term metaphorically here, please don’t call me out on this) on the ship, which will put some stress on the frame. If you accelerate too fast you could damage or destroy the ship (just like you could, in theory, damage a car by accelerating or breaking too fast). YOU, personally, will continue to travel through space in the same direction until the ship pushes you along the new trajectory (e.g. via seat belt, or by you smacking your head against a bulkhead) In either case, you will feel some force as your body’s direction adjusts.

(in retrospect, others above have said the same thing).

Remember that real space travel is not like video games - applying a thrust vector is not instantaneous, but is continuous.