物理 2002 年 DSE、CE 選擇題

1.

Distance-time graphs of cars P and Q meeting at t = 5 s, both reaching about 20 m

The figure above shows the distance-time graphs of two toy cars $P$ and $Q$ moving along a linear track. Which of the following statements is/are correct ?

(1)Car

P

will reach the

20

m-mark first.

(2)Car

P

is overtaking car

Q

at

t = 5

s.

(3)The average speed of car

P

in the first

5

s is smaller than that of car

Q

.

2.

A piece of stone is hung from a balloon, which is rising vertically upward. If the string connecting the stone and the balloon suddenly breaks, which of the following velocity-time graphs represents the subsequent motion of the stone ? (Note : Velocity pointing upward is taken to be positive.)

3.

The following are statements written by three students about Newton's first law of motion.

(1)A stationary object will remain in a state of rest unless acted on by an unbalanced force.

(2)An object undergoing uniform motion will maintain its motion unless acted on by an unbalanced force.

(3)An unbalanced force is required to maintain the motion of an object at uniform velocity.

Which of the above statements is/are correct ?

4.

Which of the following objects are under the action of an unbalanced force at the instant shown in the diagrams ? (1) a football being kicked by a player

(2) a parachutist falling with uniform velocity

(3) a swinging pendulum bob

A pendulum bob swinging on a string hung from the ceiling

5.

A block of mass $0.5$ kg slides down a rough inclined plane with an acceleration of $3\ \mathrm{m\,s^{-2}}$ . If the plane is inclined at $30^\circ$ to the horizontal, find the friction between the block and the plane.

Block sliding down a rough plane inclined at 30 degrees

6.

John, of mass $80$ kg, is standing on a weighing scale in a lift. At a certain instant, the reading of the weighing scale is $600$ N. Which of the following statements about John at this instant is/are correct ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

(1)The gravitational force acting on John is

600

N.

(2)The force exerted by the weighing scale on John is

200

N.

(3)John is accelerating downward at a rate of

2.5\ \mathrm{m\,s^{-2}}

.

7.

Block projected up a smooth inclined plane

A block is projected up a smooth inclined plane. Which of the graphs below shows the variation of the kinetic energy (K.E.) of the block with its potential energy (P.E.) during the upward motion ?

8.

Block on a smooth horizontal table with a 60 N vertical force upward and a 30 N horizontal force to the right

A block of weight $100$ N is placed on a smooth horizontal table. A vertical force of $60$ N and a horizontal force of $30$ N are applied to the block as shown. Find the magnitude of the resultant force acting on the block.

9.

Ferris wheel with eight cages, one passenger in the bottom cage

The figure above shows a flying wheel in an amusement park. The wheel is of diameter $18$ m and carries eight cages. There is only one passenger of mass $60$ kg inside one of the cages. The wheel rotates with uniform speed and it takes $80$ s for the passenger to travel from the bottom to the top of the wheel. Find the average useful power output of the motor of the wheel.

10.

A block is initially at rest on smooth horizontal ground. Two forces of equal magnitude $F$ act on the block. In which of the below cases will the block remain at rest ?

Three cases of a rectangular block seen from above with two forces F: (1) forces along parallel lines in opposite directions, (2) forces along the diagonal in opposite directions, (3) forces along parallel lines in the same direction