Word for an object involved in a collision?
In physics, the word body is used. Form Cambridge Dictionary:
physics specialized a separate object or mass
Example sentences from Wikipedia's page on collisions (emphasis mine):
There are two types of collisions between two bodies - 1) Head-on collisions or one-dimensional collisions - where the velocity of each body just before impact is along the line of impact, and 2) Non-head-on collisions, oblique collisions or two-dimensional collisions - where the velocity of each body just before impact is not along the line of impact.