How to use the idiom "in force"

I'm wondering how to correctly use the idiom "in force". Often "active" can be used instead, but are there any situations in which "in force" can be used and "active" cannot, or vice versa? More generally, what are the idiomatic limitations of this expression?

To illustrate, I have the following example from a text I wrote (emphasis added):

Each event has associated with it a tension weight. The tension at a particular point in the story is obtained by adding all individual tension weights in force at that particular point. [It is not specified] how to determine which tensions are in force.

In both cases, the sentence could be rewritten to use the word "active" instead, but is it incorrect to use "in force" here?

I found the following here:

in force

1. In full strength; in large numbers: Demonstrators were out in force.
2. In effect; operative: a rule that is no longer in force.

The second meaning seems to suggest that the above use of "in force" is ok.

This is one of times where English is very, very tricky!

I believe in this particular usage, you want to use the idiom "in force" and the words "reinforce" and "enforce" in the following manners:

"Each event has associated with it a tension weight. The tension at a particular point in the story is obtained by adding all individual weights, in force [altogether, or, all at once], which reinforces [strengthens] that particular point. [It is not specified] how to determine which tensions are enforced [compelled to act on the particular point in the story]."

As far as general usage of the idiom, I most often use it to reference attendance (as in the first usage of your definition), such as "On the day of the rally, protesters were out in force on the capitol's streets." It is also frequently used, as in your example, to mean "all at once" or "all together." The "all at once" usage is more appropriate to use in reference to objects and events than to beings, though.

I'm not sure of your exact intentions, but hope this was a bit helpful. The definitions of the words "enforce" and "reinforce" (from a few different sources) are as follows:

enforce: tr.v. en·forced, en·forc·ing, en·forc·es 1. To compel observance of or obedience to: enforce a law. 2. To impose (a kind of behavior, for example): enforce military discipline. 3. To give force to; reinforce: "enforces its plea with a description of the pains of hell" (Albert C. Baugh).

1. to ensure observance of or obedience to (a law, decision, etc.)
2. to impose (obedience, loyalty, etc.) by or as by force

3. to emphasize or reinforce (an argument, demand, etc.)

4. to put or keep in force; compel obedience to: to enforce a law.

5. to obtain by force or compulsion; compel: to enforce obedience.
6. to impose (a course of action) upon a person.
7. to support by force.
8. to impress or urge forcibly.

reinforce: re·in·force also re-en·force or re·en·force (rn-fôrs, -frs) tr.v. re·in·forced also re-en·forced or re·en·forced, re·in·forc·ing also re-en·forc·ing or re·en·forc·ing, re·in·forc·es also re-en·forc·es or re·en·forc·es

1. To give more force or effectiveness to; strengthen: The news reinforced her hopes.
2. To strengthen (a military force) with additional personnel or equipment.
3. To strengthen by adding extra support or material.
4. To increase the number or amount of; augment.

5. Psychology a. To reward (an experimental subject, for example) with a reinforcer subsequent to a desired response or performance. b. To encourage (a response) by means of a reinforcer.

6. to give added strength or support to

7. to give added emphasis to; stress, support, or increase his rudeness reinforced my determination
8. (Military) to give added support to (a military force) by providing more men, supplies, etc.

9. (Psychology) Psychol to reward an action or response of (a human or animal) so that it becomes more likely to occur again

10. to strengthen with some added piece, support, or material: to reinforce a wall.

11. to make more forcible or effective: to reinforce efforts.
12. to augment; increase.
13. to strengthen (a military force) with additional personnel, ships, or aircraft.
14. to strengthen the probability of (a desired behavior) by giving or withholding a reward.

Meaning 2 is often found in legal documents. A treaty or a piece of legislation will normally have a clause naming the date from which it is in force or the date from which it comes, or enters, into force. It is difficult to comment on your example without knowing the broader context, but, because of the legal connotations of ‘in force’, an alternative might be preferable.