Physical force is sufficient.
You may also find that a person who, in fact, has the present ability to inflict bodily harm upon another and who threatens or attempts to inflict bodily harm upon such person acts forcibly. In such case, the threat must be a present one. Eleventh Circuit Offense Instructions 1.1 and 1.2. Notes on Use 1 Whether a person performing the functions delegated to the assault victim is a federal officer or employee within the meaning of section 111 is a question of law for the court. The Committee recommends that the specific title of the federal officer be used.