F.4 Physics

2010-09-11 6:34 am

我而家做緊phy既練習，但係有d題目唔明點解個答案會係咁

唔該幫幫手，請詳細解釋條問題同每一個選項

謝謝各位

1. An act walks on a piece of paper. It walks for 5 cm, 3 cm, 4 cm in 3 different time intervals. Which of the following statements is/are correct?

(1) Its displacement can be zero.
(2) The magnitude of its displacement cannot be greater than 12 cm.
(3) The distance travelled can be smaller than 12 cm.

A (1) only
B (3) only
C (1) and (2) only
D (2) and (3) only

Correct answer: C

2. For the Xiangqi (象棋) board, which of the following statements is/are correct?

(1) For each move, the magnitude of displacement of the Elephant must be larger than that of the Guard.
(2) For two moves, the magnitude of displacement of the Guard can be larger than that of the Elephant.
(3) For two moves, the distance travelled by the Elephant can be zero.

A (1) only
B (3) only
C (1) and (2) only
D (2) and (3) only

Correct answer: C

回答 (1)

天同

2010-09-11 8:52 am

✔ 最佳答案

1. Statement (1) is correct.
The 3 displacements can form a right angle triangle with sides respectively 3 cm, 4 cm and 5 cm. Thus the act actually returns to its original position, hence the displacement is zero.Statement (2) is correct.If all the 3 displacements are in the same direction, the total displacement is equal to (5 + 4 + 3 ) cm = 12 cm. This is already the maximum displacement. Hence, it cannot be greater than 12 cm.

Statement (3) is wrong.Ttoal distance travelled is the arithmetic sum of the magnitudes of the 3 displacements, which is 12 cm. Thus total distance travelled is exactly equal to 12 cm.
2. Statement (1) is correct.The elephant moves across the diagonal of two squares, which is longer than only the length of a square along which the guard moves.

Statement (2) is correct.
The guard moves two square forward, whereas the elephant moves forward and retens to the original position. Thus the displacement of the elephant is zero but not the guard.

Statement (3) is wrong.
A move cannot be zero distance. Thus two moves will not give zero distance.

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