These words all denote the urging or driving of a person to do something or the obtaining of the performance of some action, all by the use of irresistible physical or moral force. Compel may have as its agent a person, an impersonal entity, a law, an action or a set of conditions, and its object may be a person or an action. A parent may compel his child to do his lessons by threatening to suspend his allowance; the government compels young men to perform military service; the law compels one to report one’s income on pain of penalty; an attack compels one to defend oneself; a recession may compel an employer to lay off many of his employees; the government may take steps to compel compliance with the law.

Force suggests an actual physical process, the use of power, energy or strength to accomplish something or to subdue resistance: to force a confession out of someone; to force the enemy back; to force a lock; to force someone to change an opinion; to force a smile. Coerce can imply the actual use of force but very often suggests its potential in an attempt to secure the surrender of the will: A child may be coerced into obedience by physical punishment or by the threat of it.

Constrain has connotations of repression, restriction, confinement or limitation. It means to compel , but often suggests that the action being prompted or urged is in a negative direction or at least away from that which one may consider positive or pleasant: a man who was constrained by his weak heart to give up all forms of strenuous exercise.

To necessitate and to oblige , as here considered, mean to make an action necessary by the imposition of conditions that must be reacted to. There is often a suggestion of urgency about these words.

• Reduction of income necessitates a curtailment of spending; Courtesy obliges a man to respond to an invitation.