The Collaborative International Dictionary of English v.0.48:

Force \Force\, n. [F. force, LL. forcia, fortia, fr. L. fortis
   strong. See Fort, n.]
   1. Capacity of exercising an influence or producing an
      effect; strength or energy of body or mind; active power;
      vigor; might; often, an unusual degree of strength or
      energy; especially, power to persuade, or convince, or
      impose obligation; pertinency; validity; special
      signification; as, the force of an appeal, an argument, a
      contract, or a term.
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            He was, in the full force of the words, a good man.
                                                  --Macaulay.
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   2. Power exerted against will or consent; compulsory power;
      violence; coercion; as, by force of arms; to take by
      force.
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            Which now they hold by force, and not by right.
                                                  --Shak.
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   3. Strength or power for war; hence, a body of land or naval
      combatants, with their appurtenances, ready for action; --
      an armament; troops; warlike array; -- often in the
      plural; hence, a body of men prepared for action in other
      ways; as, the laboring force of a plantation; the armed
      forces.
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            Is Lucius general of the forces?      --Shak.
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   4. (Law)
      (a) Strength or power exercised without law, or contrary
          to law, upon persons or things; violence.
      (b) Validity; efficacy. --Burrill.
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   5. (Physics) Any action between two bodies which changes, or
      tends to change, their relative condition as to rest or
      motion; or, more generally, which changes, or tends to
      change, any physical relation between them, whether
      mechanical, thermal, chemical, electrical, magnetic, or of
      any other kind; as, the force of gravity; cohesive force;
      centrifugal force.
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   Animal force (Physiol.), muscular force or energy.

   Catabiotic force [Gr. ? down (intens.) + ? life.] (Biol.),
      the influence exerted by living structures on adjoining
      cells, by which the latter are developed in harmony with
      the primary structures.

   Centrifugal force, Centripetal force, Coercive force,
      etc. See under Centrifugal, Centripetal, etc.

   Composition of forces, Correlation of forces, etc. See
      under Composition, Correlation, etc.

   Force and arms [trans. of L. vi et armis] (Law), an
      expression in old indictments, signifying violence.

   In force, or Of force, of unimpaired efficacy; valid; of
      full virtue; not suspended or reversed. "A testament is of
      force after men are dead." --Heb. ix. 17.

   Metabolic force (Physiol.), the influence which causes and
      controls the metabolism of the body.

   No force, no matter of urgency or consequence; no account;
      hence, to do no force, to make no account of; not to heed.
      [Obs.] --Chaucer.

   Of force, of necessity; unavoidably; imperatively. "Good
      reasons must, of force, give place to better." --Shak.

   Plastic force (Physiol.), the force which presumably acts
      in the growth and repair of the tissues.

   Vital force (Physiol.), that force or power which is
      inherent in organization; that form of energy which is the
      cause of the vital phenomena of the body, as distinguished
      from the physical forces generally known.

   Syn: Strength; vigor; might; energy; stress; vehemence;
        violence; compulsion; coaction; constraint; coercion.

   Usage: Force, Strength. Strength looks rather to power as
          an inward capability or energy. Thus we speak of the
          strength of timber, bodily strength, mental strength,
          strength of emotion, etc. Force, on the other hand,
          looks more to the outward; as, the force of
          gravitation, force of circumstances, force of habit,
          etc. We do, indeed, speak of strength of will and
          force of will; but even here the former may lean
          toward the internal tenacity of purpose, and the
          latter toward the outward expression of it in action.
          But, though the two words do in a few cases touch thus
          closely on each other, there is, on the whole, a
          marked distinction in our use of force and strength.
          "Force is the name given, in mechanical science, to
          whatever produces, or can produce, motion." --Nichol.
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                Thy tears are of no force to mollify
                This flinty man.                  --Heywood.
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                More huge in strength than wise in works he was.
                                                  --Spenser.
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                Adam and first matron Eve
                Had ended now their orisons, and found
                Strength added from above, new hope to spring
                Out of despair.                   --Milton.
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WordNet (r) 3.0 (2006):

in force
    adj 1: exerting force or influence; "the law is effective
           immediately"; "a warranty good for two years"; "the law
           is already in effect (or in force)" [syn: effective,
           good, in effect(p), in force(p)]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "in force":
   acting, active, armipotent, at work, authoritative, being, cogent,
   current, dynamic, effective, energetic, existent, existing, extant,
   forceful, forcible, functional, functioning, going, going on,
   high-potency, high-powered, high-pressure, high-tension, in being,
   in commission, in effect, in exercise, in existence, in hand,
   in operation, in play, in power, in practice, in process,
   in service, in the works, in use, inaction, irresistible, living,
   mighty, mighty in battle, on foot, on the fire, ongoing, operating,
   operational, operative, potent, powerful, prepotent, present,
   prevalent, puissant, ruling, running, striking, strong, subsistent,
   subsisting, telling, under the sun, valid, vigorous, vital,
   working