> While I see the appeal of presenting the problem this way - in terms of force - it's not really a great way to perform this kind of analysis, precisely because of the issue you cite of not knowing the interval over which the deceleration occurred. Speaking as a physicist, I think a better approach is energy; the total kinetic energy of the ship that would have to be absorbed by the impact over whatever time period the ship is stopped. This would be the key metric in designing a barrier that would absorb that energy, and allows other types of comparisons. Plugging in your numbers, 100 metric tons at 8 mph, you get about 650 million joules, which is 300 lbs. of TNT equivalent, or the amount of energy of a large diesel locomotive (150 tons) traveling at about 100 mph. You need a lot of "crumple zone" to absorb that much energy, but of course, it could be done at some significant effort and cost.
https://www.nytimes.com/shared/comment/3u1k9a?rsrc=cshare&sm...
Momentum, forces, and energy are all important.
> You need a lot of "crumple zone" to absorb that much energy, but of course, it could be done at some significant effort and cost.
Or you focus on deflecting ships.