物理 2010 年 DSE、CE 選擇題（第 3 頁）

21.

12 V battery with bulb X in series with bulbs Y and Z in parallel, points P, Q, R, S marked

Three identical bulbs $X$ , $Y$ and $Z$ are connected to a $12$ V battery in the circuit above. Which of the following statements are correct ?

(1)The voltage across

PQ

is greater than

6

V.

(2)The voltage across

QS

is zero.

(3)If

Y

burns out and becomes open circuit,

Z

becomes brighter.

22.

Wire PQ placed between two bar magnets with poles X and Z, experiencing an upward force F

A current carrying wire $Y$ is placed between two bar magnets as shown above. The wire experiences an upward force. Which of the following is/are the possible combination(s) of the direction of the current in $Y$ and the magnetic poles of $X$ and $Z$ ?

	pole $X$	current direction in $Y$	pole $Z$
(1)	N	from $P$ to $Q$	S
(2)	S	from $P$ to $Q$	N
(3)	N	from $Q$ to $P$	S

23.

The activity of a radioisotope is $960$ Bq. Its activity drops to $240$ Bq in $2$ minutes. How much more time would be required for its activity to become $30$ Bq ?

24.

Two charged metal balls P and Q hung by insulating threads from a ceiling

In the figure above, two charged metal balls $P$ and $Q$ are hung by insulating threads. $P$ is positively charged while $Q$ is negatively charged. An $\alpha$ source is put near the balls without touching them. Which of the following figures shows the situation after a period of time ?

Four figures A to D showing the positions of balls P and Q above an alpha source

25.

The diagram shows the mass number and atomic number of a radioactive nuclide $K$ . After undergoing the following decays, it becomes $Z$ . $K\xrightarrow{\alpha}X\xrightarrow{\beta}Y\xrightarrow{\gamma}Z$ Which of the following nuclides represents $Z$ ?

Grid of mass number (230 to 235) against atomic number (80 to 85) with points K, Q, S, R and P

26.

Block on a rough horizontal surface pulled by string S1 (100 N) to the left and string S2 (30 N) to the right

A block of mass $4$ kg remains at rest on a horizontal rough surface with strings $S_1$ and $S_2$ of tensions $100$ N and $30$ N respectively pulling it. If $S_1$ is broken, what is the magnitude of the net force acting on the block ?

27.

A block of mass $5$ kg is projected up along a smooth inclined plane of inclination $\theta$ as shown in the figure above. The graph shows the variation of kinetic energy of the block with the distance travelled $s$ up along the plane. Find the angle $\theta$ . Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .

Block on an incline of angle theta, and graph of kinetic energy against s falling linearly from 24 J at s = 0 to zero at s = 2.4 m

28.

An object is projected vertically upwards. $F$ denotes the magnitude of the net force acting on the object and $W$ denotes the magnitude of the weight of the object. If air resistance is not negligible, which of the following descriptions are correct ?

(1)When the object is moving up,

F

is greater than

W

.

(2)When the object is at the highest point,

F

is equal to

W

.

(3)When the object is moving down,

F

is smaller than

W

.

29.

A ball hanging by a string inside a car, the string making 10 degrees with the vertical

In the figure above, a ball is hanging inside a car by a string. When the car accelerates on a horizontal road, the string makes an angle of $10^\circ$ with the vertical. Find the magnitude of the acceleration of the car. (Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .)

30.

Ball A (6 m/s to the right) and ball B (4 m/s to the left) approaching each other

Ball $A$ and ball $B$ of masses $2$ kg and $1$ kg respectively collide head-on as shown above. Which of the following diagrams show(s) the possible result(s) after the collision ?

Three diagrams (1) to (3) showing velocities of A and B after collision