I want a word that is used to mean an object involved in a collision, for example, say two tennis balls collide – ball 1 and ball 2 – what would be a word that could describe either ball, only in the context of a collision.
Similarly to how each particle involved in a reaction is a reactant.
Example sentence following the tennis balls:
The two balls, or _____ in a physics context, collided in mid-air.
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.