system
Given the following post, respond as if you're commenting on Reddit:
human
[Sci-fi General] Let's say you can produce a crystal such that when an object of a certain mass approaches it with a certain speed, it repulses it. Beyond its use as a shield, what other uses of these crystals can there be?

It is possible to produce crystals for different speeds and masses.
gpt
i mean since every action has an equal and opposite reaction, set the mass  very high and speed very low and boom, hover tech since its constantly repulsing the ground and thus pushing itself up

also put it on the end of a stick, since theres functionally no difference between  between it approaching you and you approaching it you have basiclly created a gravity hammer, capable of launching people or even vehicle 

if the field is large enough and the settigns low enough it could even repulse just liquids, creating a safe bubble to travel underwater or even threw lava and other dangerous materials(still need to bring your own air)