Common Words:
* arbor: a shady place formed by trees or climbing plants (often a trellis or structure for vines)
* arcade: a covered passageway with arches on either side
* arctic: relating to the region around the North Pole
* arid: extremely dry
* arm: the upper limb of the human body
* aroma: a distinctive smell
* arrange: to put in a particular order
* arrest: to seize or stop someone, usually by legal authority
* arrive: to reach a place
* arrow: a pointed projectile shot from a bow
* art: the expression of human creative skill and imagination
* article: a piece of writing in a newspaper, magazine, etc.
* artificial: not natural or real
* artist: a person who creates art
* asthma: a respiratory condition that causes wheezing and shortness of breath
* atom: the smallest unit of an element
* attack: to use violence against someone or something
Less Common, but Still Useful Words:
* arboreal: relating to trees
* archetype: a typical example of a certain person or thing
* armament: weapons and ammunition
* arousal: a state of being excited or stimulated
* array: an impressive display of a particular type of thing
* arroyo: a gully or ravine, especially in the southwestern United States
* artifact: an object made by humans, especially from an earlier time
* articulate: able to express oneself clearly and effectively
* ascend: to move upwards
* ascertain: to find out for certain
* ascribe: to attribute something to a particular cause
* aspiration: a strong desire or ambition
* assail: to attack violently
* assemble: to gather together in one place
* assert: to state something forcefully
This is not an exhaustive list, as there are many words with the prefix "ar-." These examples should give you a good starting point.