Physics HKCEE 2001 (2)

2007-02-22 11:51 pm

A girl walks along a straight road from a point A to a point B with an average
speed 1ms-1. She then returns from B to A along the same road with an average speed 2ms-1. Find the average speed of the girl for the whole journey.

Answer: 1.33ms-1

回答 (1)

2007-02-23 12:58 am

✔ 最佳答案

Let the distance between point A and point B be d (metre)

When the girl walks from A to B with an average speed 1ms-1,
the time required = distance/speed = d/1 = d (second)

When the girl returns from B to A with an average speed 2ms-1,
the time required = distance/speed = d/2 = d/2 (second)

therefore the total time required for the whole journey = d + d/2 = 3d/2 (second)
the total distance for the whole journey = d + d = 2d

As the average speed of the girl fro the whole journey
= [ total distance travelled from point A to B and from B to A ] / [ total time required for the whole journey ]
= 2d / (3d/2)
= 4/3
= 1.33 ms-1

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