Which of the following must always be true?

(I) If an object’s acceleration is constant, then it must move in a straight line.
(II) If an object’s acceleration is zero, then its speed must remain constant.
(III) If an object’s speed remains constant, then its acceleration must be zero.


(A) I and II only
(B) I and III only
(C) II only
(D) III only
(E) II and III only
  1. Correct Answer: C


    I is false since a projectile experiencing only the constant acceleration due to gravity can travel in a parabolic trajectory. II is true: Zero acceleration means no change in speed (or direction). III is false: An object whose speed remains constant but whose velocity vector is changing direction is accelerating.

Test ID: 1471
Source: SAT Physics Practice Test