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I was reading Newton's first law:

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

In the part:

unless acted upon by an unbalanced force.

The meaning is clear (because I have already read this law in my native language) but I had never seen this construction:

unless + verb (pp)

Is this construction correct?

Also if you can explain all the grammar of this last part (unless....) I would appreciate it so much

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    This will not make sense unless understood as having a redundant it is elided. – tchrist Dec 28 '14 at 4:48
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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.

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In this context, the phrase

unless acted upon by an unbalanced force.

can be thought of as a short-hand notation for

unless [the object is] acted upon by an unbalanced force.

You will see a similar type of shortened speech in an American legal context. They do it for the sake of brevity.

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  • Should not be unless [it is] acted upon by an unbalanced force. instead? – Victor Castillo Torres Dec 28 '14 at 5:07
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    That's fine too. In this case, the word "it" refers to "the object". And you're right, I should have used present tense ("is" rather than "has been"). I will change my answer accordingly. – jlcgd Dec 28 '14 at 5:11
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    It's not "a shorthand notation", and certainly not legalese as the last paragraph suggests, it's perfectly standard and grammatical English. – Jon Hanna Dec 28 '14 at 17:10
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    Yes, it's a regular rule that deletes repeated subjects and auxiliary forms of be in subordinate clauses with participial predicates (acted upon, acting upon). Shorthand is a form of writing, which represents speech, not the other way around. – John Lawler Dec 28 '14 at 18:42

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