Average velocity =
(distance and direction of their change in location)
divided by
(time for the change) .
Distance and direction of their change in location = 2.0 km east (their "displacement")
Time for the change = 30 minutes
Average velocity = (2.0 km east) / (30 minutes)
Average velocity = (2/30) (km east/minute)
Average velocity = 0.067 km/min east
or
Average velocity = 4.0 km/hr east
Just to be difficult, the question specifies that the answer must be given in units of m/s. Is no problema for nosotros:
Average velocity = (4.0 km-east/hour) x (1,000 m/km) x (1 hour/3600 sec)
Average velocity = (4 x 1,000 / 3,600) (km-east-meter-hr / hr-km-sec)
Average velocity = 1.111... m/s East
