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In law, force means unlawful violence, or lawful compulsion. "Forced entry" is an expression falling under the category of unlawful violence; "in force" or "forced sale" would be examples of expressions in the category of lawful compulsion.
When something is said to have been done "by force", it usually implies that it was done by actual or threatened violence ("might"), not necessarily by legal authority ("right"). For example, a person forced against their will to commit an unlawful act, which they would not have committed if not threatened, would not be considered criminally culpable for those actions.
"Force of arms" is a special case that can be an example of unlawful violence or lawful compulsion dependent on who is exercising the violence (or threat thereof) and their legal right and/or responsibility to do so.
When one citizen threatens another with a weapon without being in danger from the person he or she is threatening, this would be an example of the unlawful expression of force of arms. The same threat expressed by police officer making a lawful arrest would typically be considered lawful compulsion, due to the state having a monopoly on violence.