力學一題(5)

Let the distance between point A and point B be s m.


Since the girl walks from A to B with an average speed of 1 ms^-1

Therefore, time needed to walk from A to B

= distance / velocity

= s / 1

= s sec


Since the girl walks from B to A with an average speed of 2 ms^-1

Therefore, time needed to walk from A to B

= distance / time

= s/2 m


Total distance travelled by the girl

= 2s m

Therefore, the average speed of the girl for the whole journey

= total distance / total time elapsed

= 2s / (s + s/2)

= 2s / (3s/2)

= 4/3 ms^-1

= 1.33 ms^-1

So, the average speed of the girl for the whole journey is 4/3 ms^-1 (or 1.33 ms^-1).

Let d be the distance between A and B
d/t1= 1 , t1= d
d/t2 = 2 , t2 = d/2

average speed= 2d/(t1+t2)
= 2d/(d+d/2)
= 2d/ (3d/2)
= 4/3 m/s