The Gunnery Chief is a male human in the Systems Alliance Marines. He is giving a lecture on physics to two recruits, Serviceman Burnside and Serviceman Chung, on Zakera Ward on the Citadel.
The Gunnery Chief, in a humorous aside, explains that Sir Isaac Newton is "the deadliest son-of-a-bitch in space". This is because, as stated in Newton's First Law of Motion, an object in motion will maintain a constant velocity unless acted upon by an outside force. Considering that there is no such force to act upon anything in space, the Gunnery Chief exclaims in no uncertain terms that when one of the Servicemen fires a slug from a ship, specifically an Everest-class dreadnought, that explodes with more power than the Hiroshima bomb, they must be sure that the computer has a lock-on, because that slug will keep going until it hits something, whether it be an asteroid, another ship, a planet, or something else in deep space in a few thousand years. He also tells Serviceman Chung not to act like "a cowboy shooting from the hip". Commander Shepard can hear his lecture near the C-Sec Customs officer.