Which statement best describes what is required to specify velocity?

• A. Only the speed.
• B. Only the direction.
• C. Only the time.
• D. Both speed and direction.

Answer: (D)

Velocity is a vector, so it has both how fast something is moving and the direction it’s moving in. To specify velocity you need the speed (the magnitude) and the direction. If you only have speed, you know how fast but not where it’s headed; if you only have direction, you know where it’s pointing but not how fast it’s going. Time isn’t required to define the velocity itself, though time appears in related ideas like average velocity. For example, 20 m/s to the east specifies the velocity clearly.