物理 1999 年 DSE、CE 選擇題

1.

A $2$ kg steel sphere and a $1$ kg wooden sphere are initially held at the same level above the ground and then released from rest simultaneously. Assume air resistance is negligible. Which of the following statements about the two spheres at any instant before they reach the ground is/are correct ?

(1)The speeds of the two spheres are equal.

(2)The accelerations of the two spheres are equal.

(3)The gravitational forces acting on the two spheres are equal.

2.

Block sliding down a friction-compensated runway

A block is sliding down a friction compensated runway as shown. Which of the following statements is/are correct ?

(1)The speed of the block is increasing.

(2)The normal reaction acting by the runway on the block is increasing.

(3)The net force acting on the block is zero.

3.

A light rope is fixed at two poles with the ends $A$ and $B$ at the same level. A T-shirt of weight $2$ N is hung at the midpoint $C$ of the rope. The rope depresses such that $\angle ACB = 150^\circ$ . Find the tension in the rope.

T-shirt hung from the midpoint C of a rope tied between poles A and B, angle ACB 150 degrees

4.

A motor car of mass $2000$ kg accelerates from rest at $3\ \mathrm{m\,s^{-2}}$ for $4$ s on a straight road. Find the average useful output power of the engine of the car.

5.

A rocket is initially at rest in space. It then explodes and breaks into two parts which move in opposite directions. If the mass of the rear part is larger than that of the front part, which of the following statements is correct ?

6.

Uniform rod of length L pivoted at a point L/4 from one end, with a 14 N downward force at the near end and 3 N downward at the far end

A uniform rod of length $L$ is pivoted at a point $\frac14L$ from one of its ends. Two forces $14$ N and $3$ N act on its two ends as shown above. If the rod is in equilibrium, find the weight of the rod. (Assume the weight of the rod acts through its midpoint.)

7.

Questions 11 and 12 : An illuminated letter 'R' is placed in front of a lens as shown below and an image is formed on a translucent screen. The object distance is $30$ cm and the image distance is $20$ cm.

Illuminated letter R, lens and screen with object distance 30 cm and image distance 20 cm, and the observer's eye

Which of the following statements is/are correct ?

(1)The lens is a converging lens.

(2)The image is diminished.

(3)The shape of the image seen by the observer is 'R' reversed.

8.

Questions 11 and 12 : An illuminated letter 'R' is placed in front of a lens as shown below and an image is formed on a translucent screen. The object distance is $30$ cm and the image distance is $20$ cm.

If a piece of paper is used to cover one-half of the lens, which of the following describes the change in the image as seen by the observer ?

9.

Ray incident on a rectangular glass block at 50 degrees to the normal

A ray of light travels in air and strikes a rectangular glass block at an angle of incidence $50^\circ$ . The critical angle of the glass is $42^\circ$ . Which of the following diagrams best shows the path of the ray ?

Four ray diagrams A to D through a rectangular glass block

10.

Graph of energy absorbed against temperature for liquids X, Y and Z (straight lines; X steepest, Y and Z parallel)

Equal masses of liquids $X$ , $Y$ and $Z$ are separately heated. The graph shows the variation of the energies absorbed by the liquids with their temperatures. Let $c_X$ , $c_Y$ and $c_Z$ be the specific heat capacities of $X$ , $Y$ and $Z$ respectively. Which of the following relations is correct ?