motion問題

Question

(a)Charles travels from his home in Wan Chai to Times Square in Causeway Bay to see a movie .He then goes home .Are the displacements of his first trip and his first and his returning trip the same ?Why?
ANS 點解會係NO,但是他是返回原點,不是嗎?
(b)a toy train completes a cycle along a circular track of rsdius 20cm .Find the distance travelled and the displacemet.
Ans 的 distance 為什麼會是126cm ,不明白.

Answer 1

(a) I suppose the words "...his first trip and..." have been typed twice. The question asks: are the displacement of his first trip and his return trip the same?

Displacement is a vector quantity, it has both magnitude and direction.
The two displacement are different because they are equal in magnitude, but are opposite in direction.

(b) Distance travelled (which is a scalar quantity) = circumference of the circle
= 2xpix20 cm = 126 cm
(where pi=3.14159 ..... )

Displacement = 0 cm because the initial and final positions are the same.

Answer 2

a 佢係問 ' first trip ' 同 ' his first and his returning trip '
displacement of ' the first trip ' = Displacement from Wan Chai to Causeway Bay
displacement of 'his first and his returning trip' = 0
so they are no the same

b 佢係問distance, not displacement
所以距離就係圓周
=直徑乘兀
=40 x 3.1415....
=125.66...
=126 cm (corr to nearest unit)