物理 2008 年 DSE、CE 選擇題（第 4 頁）

31.

John performs a bungee jump from a bridge above a river. He (assumed to be a particle) is tied to the bridge at $A$ with an elastic cord. He falls from rest at $A$ . When he reaches $B$ , the elastic cord starts to stretch. John is momentarily at rest at $C$ and then bounces up. Which of the following descriptions about the motion of John is/are correct ? (Neglect the air resistance.)

(1)From

A

to

B

, John is under free falling.

(2)From

B

to

C

, the gravitational potential energy of John increases.

(3)At

C

, there is no net force acting on John.

Vertical line from the bridge at A down through B and C to the river

32.

Block sliding down a rough inclined plane at angle theta with acceleration a

A block of mass $m$ slides down along a rough inclined plane with constant acceleration $a$ as shown above. What are the friction and the normal reaction acting on the block ?

	Friction	Normal Reaction
	$mg\sin\theta+ma$	$mg\sin\theta$
	$mg\sin\theta-ma$	$mg\cos\theta$
	$mg\cos\theta+ma$	$mg\sin\theta$
	$mg\cos\theta-ma$	$mg\cos\theta$

33.

Which of the following has the highest average speed of the molecules ?

34.

What physical properties does the temperature of an object represent ?

(1)A measure of the degree of hotness of the object.

(2)A measure of the internal energy of the object.

(3)A measure of the average kinetic energy of the molecules of the object.

35.

In the figure, $S_1$ and $S_2$ are two identical loudspeakers connected in parallel to a signal generator. The circles represent the wavefronts of the sound wave produced. What are the changes to the loudness of the sound at $X$ and $Y$ if loudspeaker $S_1$ is turned off ?

Wavefront circles around loudspeakers S1 and S2 with points X and Y where the circles overlap

	$X$	$Y$
	increases	increases
	decreases	decreases
	increases	decreases
	decreases	increases

36.

Figure (a): particles a to k evenly spaced. Figure (b): particle positions at a certain instant

Figure (a) shows a series of particles which is uniformly distributed along a slinky spring. Figure (b) shows their positions at a certain instant when a travelling wave passes through the slinky spring from left to right. Which of the following diagrams shows the correct positions of the particles after half a period from the instant shown in Figure (b) ?

Four diagrams A to D showing positions of particles a to k

37.

Which of following statements about total internal reflection is/are correct ?

(1)The angle of incidence is less than the critical angle.

(2)Both reflected and refracted rays appear.

(3)The ray is travelling from an optically denser medium to an optically less dense medium.

38.

Coil A connected to a cell and switch and coil B connected to an ammeter, both wound on a soft iron rod

An experiment about electromagnetism is designed as shown above. Coil $B$ has a greater number of turns than coil $A$ . Coil $B$ is connected to a sensitive ammeter. Which of the following statements is/are correct ?

(1)When the switch is closed, a steady current flows through the ammeter.

(2)At the moment when the switch is opened, a current flows through the ammeter momentarily.

(3)If the soft iron rod is replaced by a glass rod, the ammeter will have a greater deflection at the moment the switch is opened.

39.

Rectangular coil partly in a uniform magnetic field into the paper, pulled to the right

In the figure, a rectangular coil is pulled to the right with uniform speed in a uniform magnetic field pointing into the paper. Which of the following descriptions about the current induced in the coil and the magnetic force acting on the coil is correct ?

40.

Specification :
Input	a.c. $220$ V
Output	a.c. $12$ V, $100$ mA

The above figure shows a transformer. Assume the transformer is ideal, what is the current drawn from the mains supply if the transformer delivers currents at the rated value ?