物理 2008 年 DSE、CE 選擇題（第 3 頁）

21.

A lighting system consists of $3$ bulbs, each rated ' $220$ V $30$ W'. These three bulbs should be connected in parallel to the $220$ V mains supply. However, the bulbs are wrongly connected in series to the mains supply. What is the power dissipation of the wrongly connected system ?

22.

A cell fully discharges at a constant current $225$ mA in $10$ hours. If its average voltage is $1.2$ V, what is the estimated total energy stored in the cell ?

23.

Which of the following descriptions about the half-life of a radioactive substance in a sample is correct ?

24.

Which of the following actions will maximise a person's exposure to radiation ?

25.

Which of the following nuclear reactions is a fission ?

26.

Which of the following statements about $\beta$ particles is correct ?

27.

When a skydiver falls steadily in air under no net force, which of the following descriptions about his gravitational potential energy, kinetic energy and power in overcoming air resistance is correct ?

Gravitational potential energy	Kinetic energy	Power in overcoming air resistance
decreases	increases	increases
decreases	increases	remains unchanged
decreases	remains unchanged	remains unchanged
remains unchanged	increases	increases

28.

In an explosion, an object is blown into $2$ pieces, $A$ and $B$ , which fly off in opposite directions. The mass of $A$ is $0.3$ kg. The graph shows the variation of velocity of $A$ and $B$ with time before and after the explosion. What is the mass of $B$ and the estimated magnitude of the average net force acting on $B$ during the explosion ?

Velocity-time graph: A rises from 0 to 3 m/s and B falls from 0 to -1 m/s between t = 0.05 s and t = 0.25 s

	Mass of $B$	Average net force acting on $B$
	$0.1$ kg	$0.4$ N
	$0.1$ kg	$0.5$ N
	$0.9$ kg	$3.6$ N
	$0.9$ kg	$4.5$ N

29.

A train (west on the left, east on the right) with a bob hung from the ceiling of a carriage, inclined to the east

A bob is hung by a string from the ceiling of a train. At a certain instant, the bob is inclined to the east as shown in the figure. Which of the following descriptions about the motion of the train at this instant is/are possible ?

(1)It is moving to the east and decelerating.

(2)It is moving to the west and accelerating.

(3)It starts to move to the east from rest.

30.

Trolleys $A$ and $B$ of masses $m_A$ and $m_B$ respectively travel along a horizontal road in opposite directions as shown.

Trolley A moving right and trolley B moving left on a horizontal road

Later they make a head-on inelastic collision and stick together. The graph below shows the velocity-time relationship of the two trolleys before and after the collision.

Velocity-time graph: A at 4 m/s and B at -1 m/s until 4 s, then A and B together at 2 m/s

(Ignore the collision time and the friction acting on the trolleys.) What is the ratio $m_A : m_B$ ?