物理 2011 年 DSE、CE 選擇題（第 3 頁）

21.

Which of the following statements about $\alpha$ , $\beta$ and $\gamma$ radiations is incorrect ?

22.

A radioactive source is put in front of a GM tube. The initial count rate is $1050$ counts per minute. It is known that the half-life of the source is $4$ hours and the background count rate is $50$ counts per minute. What is the most likely count rate after $8$ hours ?

23.

A ${}^{238}_{92}\mathrm{U}$ nuclide undergoes a certain number of $\alpha$ and $\beta$ decays and becomes ${}^{210}_{82}\mathrm{Pb}$ . Find the number of $\beta$ particles emitted.

24.

Three forces are acting on particle $P$ . In which of the following diagrams is the net force on $P$ not zero ?

Four grid diagrams A to D, each showing three forces of 6 N, 8 N and 10 N acting on a point P

25.

In the figure shown, horizontal force $F$ is applied to a block which is hung by a string under the ceiling. At equilibrium, the string makes an angle $\theta$ with the vertical and the tension in the string is $T$ . The weight of the block is

A block hung by a string from the ceiling, pulled by horizontal force F, the string making angle theta with the vertical and tension T

26.

Platform at P above a swimming pool; Q at the water surface and R at maximum depth

In the above figure, a mass is released from rest at $P$ from a high platform into a swimming pool. After a while, the mass reaches the water surface at $Q$ and enters the water. Finally, it reaches a maximum depth at $R$ . Which of the following descriptions about the motion of the mass is/are correct ?

(1)From

P

to

Q

, the acceleration of the mass is increasing.

(2)From

Q

to

R

, the net force acting on the mass is pointing upward.

(3)From

P

to

R

, the gravitational potential energy loss of the mass equals to its kinetic energy gain.

27.

Trolleys P and Q on a smooth track with a compressed spring between them

On a horizontal smooth track, two trolleys $P$ and $Q$ are held at rest with a light compressed spring in between as shown in the above figure. The masses of $P$ and $Q$ are $m$ and $2m$ respectively. When the trolleys are released and separate, trolley $Q$ moves to the right with speed $v$ . Which of the following statements are correct ?

(1)After separation, the total momentum of the two trolleys is

4mv

.

(2)After separation, the kinetic energy of trolley

P

is twice that of trolley

Q

.

(3)The energy initially stored in the compressed spring is at least

3mv^2

.

28.

An egg will probably break if it is released at a height and lands on a hard surface. However, the egg may not break if it is released at the same height but lands on a soft cushion. It is because, when the cushion is used,

29.

Questions 34 and 35 : The figure below shows the displacement-distance graph of a wave travelling to the right with speed $2\ \mathrm{cm\,s^{-1}}$ at a certain instant. $P$ and $Q$ are two particles at distances $x=8$ cm and $18$ cm respectively.

Displacement (cm) against x (cm) graph with P at x = 8 cm and Q at a crest at x = 18 cm

What is the period of the wave ?

30.

Questions 34 and 35 : The figure below shows the displacement-distance graph of a wave travelling to the right with speed $2\ \mathrm{cm\,s^{-1}}$ at a certain instant. $P$ and $Q$ are two particles at distances $x=8$ cm and $18$ cm respectively.

What is the shortest time for $P$ to have the same displacement as $Q$ at the instant shown ?