12. A ship travels 50 km due [N] and then turns and travels 30 km due [S] in 4.0 hours. What is its
average velocity?
a. 5.0 km/h [N]
b. 5.0 km/h [S]
c. 20 km/h [N]
d. 20 km/h [S]

a. 5.0 km/h [N]

Explanation:

The average velocity of an object is given by:

where

d is the displacement of the object

t is the time elapsed

The displacement is the change in position, so it corresponds to the vector connecting the initial position to the final position.

Here the ship travels 50 km North first, and then 30 km South: therefore, its displacement is

d = +50 + (-30) = +20 (North)

The time elapsed here is

t = 4.0 h

Therefore, the average velocity is

and the direction is the same as the displacement (North).

the part of the wave he is travelling on is the crest