Question: Sorry if I haven't worded the title very well, feel free to improve.

Let me explain before I ask my question...

We have purchased a [car carrier](https://www.petbarn.com.au/playmate-airline-spec-open-top-pet-carrier-white-grey-medium) (small version) for our dog and also purchased [some straps](https://rads.stackoverflow.com/amzn/click/com/B005SH05E6) to secure it to the seat; however we have noticed that even though it is secured to the seatbelt it is still a little loose (you can move the carrier forward a bit before it attempts to prevent further movement).

I have heard someone else that has used these same straps state they have instead put the straps around the whole back of the seat rather than use the seatbelt and they have stated it is now much more secure/tighter against the seat and hence prevents much movement.

So my question is - which method is better/safer?

* Securing it around the seat, to me, *seems* like it may be a better option as it helps prevent movement, but I'm wondering, because of this lack of movement, would our dog now have to sustain more **g-force**?
* Securing it to the seatbelt, to me, would seem to propel the carrier forward on an impact until it is stopped and then our dog would be thrown forward into the front of the carrier in a more "jerky" fashion then when secure but *may* not have to sustain as much g-force!?

As a side note, if it matters, we have been told by [CPS](https://www.centerforpetsafety.org/) that the straps we purchased are **not** *strength rated*; which I assume means they have the potential to break at high points of impact, but we are open to purchasing better ones if need be, but I am also cautious about using straps not specifically made for carriers as I have heard using seatbelts around carriers [can potentially crush them](https://www.youtube.com/watch?v=YnES6NIMHsM).

Answer: 
A body in motion has an inertia of motion by virtue of which it tries to maintain it's previous state of motion(both speed and direction). For example it is noticed that on suddenly applying brakes to stop we are jerked in forward direction which is due to the inertia of motion.

Now when we are in enclosed space we must take care of the fact that if we don't stop, relative to the nearby objects, then it is quite probable that we may get hurt. So what to do then? Answer is to somehow attach ourselves to the surrounding so that when they stop our body stops with the them. But there is another problem which is of the case that on making our body to stop suddenly by applying constraints causes a lot of force on our body which can cause minor/sever damage and can be lethal. 

So what should we do then? 

The right thing is the case which is somewhere inbetween i.e., to make our bodies stop in such amount of time that a comparatively lesser force acts on us but we stop in such distance that our head or other part doesn't collide with the nearby object. So my suggestion is to do the thing in which both are satisfied. 

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$$\pmb {\underline {\text {Further Reading }}}$$

* [The Statistics: Car Safety and Pets in America](https://www.petprosupplyco.com/a/s/pages/car-accident-pet-statistics-america)
* [How to Keep Your Pets Safe When Traveling](http://www.petprosupplyco.com/pages/how-to-keep-your-pets-safe-when-traveling)
* [Pet Travel Safety - How To Travel Long Distance | American Red Cross](https://www.redcross.org/get-help/how-to-prepare-for-emergencies/pet-travel-safety.html)
* [Travel Safety Tips | ASPCA](https://www.aspca.org/pet-care/general-pet-care/travel-safety-tips)
I refer you Bob D's answer above for the physics.

At the moment of impact, the carrier will move forward until it reaches the limit of the restraining straps. The straps then undergo a deformation (elastic and inelastic), over the scale of a few centimetres, that decelerates the load. This process is the same whether the straps are loose or tight because the load is moving at the same speed at the moment when they reach their limit and begin to bite.

So it doesn't really matter if the straps are very tight or not. As long as they not so slack that the carrier can move forward enough to impact something in front.