Unless in this sense is a conjunction. (There is a prepositional sense too, but this is now rare).
Conjunctions of course join clauses, words or phrases.
He went to the shops and he bought some milk.
She wanted to go but she felt too ill.
It is common, and standard, with conjunctions to omit a subject common to both phrases joined:
He went to the shops and bought some milk.
She wanted to go but felt too ill.
When a copula is used, then the copula might also be omitted:
The boat was built and it was launched.
He was taken outside and he was shot.
The boat was built and launched.
He was taken outside and shot.
Unless is a subordinating conjunction (or subordinator) so it joins an independent and dependent clause:
An object will remain at rest or in motion at constant velocity unless it is acted upon by an unbalanced force.
The omission of the subject it and the copula is can happen here just as with any other conjunction:
An object will remain at rest or in motion at constant velocity unless acted upon by an unbalanced force.
As can be seen, it is just another example of this normal omission that happens with conjunctions.