To put it another way, every collision you are in is also a collision with the Earth, which masses a lot more than you and wins every exchange of momentum. Con

To put it another way, every collision you are in is also a collision with the Earth, which masses a lot more than you and wins every exchange of momentum. Conveniently, your delta-v with this massive body you don't want to run into is also the one conventionally reported for your speed.