物理 2000 年 DSE、CE 選擇題

1.

It is said that Galileo Galilei (1564 - 1642), an Italian scientist, dropped a small iron ball and a large cannon ball from the top of the Leaning Tower of Pisa. He found that the two balls reached the ground at almost the same time. Which of the following is/are correct deduction(s) from this experiment ?

(1)The two balls fell with the same acceleration.

(2)A body will maintain uniform motion if there is no external force acting on it.

(3)The gravitational forces acting on the two balls were identical.

2.

A racing car accelerates from rest to a speed of $100$ km h $^{-1}$ in $3.2$ s. Find the average acceleration of the car.

3.

An astronaut lands on the moon and finds that his weight is about one-sixth of that on the earth. Which of the following deductions is/are correct ?

(1)If he throws an object upwards on the moon, it will reach a higher level than throwing the object with the same speed on earth.

(2)If he releases an object on the moon, it will take a shorter time to reach the ground than releasing the object from the same height on earth.

(3)The maximum weight he can lift on the moon is greater than on earth.

4.

Three forces of magnitudes $F_1$ , $F_2$ and $10$ N act on an object as shown. If the object is in equilibrium, find the force $F_2$ .

Forces on an object: F1 to the left, F2 up and to the right at 30 degrees to the vertical, 10 N downward

5.

Block projected up a rough inclined plane

A block is placed on a rough inclined plane and then projected upwards along the plane. After reaching the highest point, the block slides down along the plane. Which of the following graphs shows the time variation of the velocity $v$ of the block ?

6.

Uniform steel ball at rest on horizontal ground, just touching a vertical wall

A uniform steel ball lies at rest on a horizontal ground and just touches a vertical wall as shown in the diagram. Which of the following diagrams shows all the forces acting on the ball ? (Note : $W$ = gravitational force acting on the ball, $R$ = normal reaction from the ground, $F$ = friction acting by the ground on the ball, $N$ = normal reaction from the wall.)

7.

Block on a rough horizontal surface pulled to the right by horizontal force F

A block is placed on a rough horizontal ground and a horizontal force acts on the block. If the magnitude of the force, $F$ , is increased gradually, which of the following graphs shows the relation between $F$ and the acceleration $a$ of the block ?

Four graphs of acceleration a against force F

8.

The graph shows the variation of the kinetic energy $E$ of an object with the square of its velocity $v^2$ moving on a straight line. What is the momentum of the object when it is moving at a velocity $4\ \mathrm{m\,s^{-1}}$ ?

Graph of E in joules against v squared: a straight line through the origin reaching 50 J at v squared = 25

9.

Hand pulling a block upwards with a string, tension 24 N

A $2$ kg block is initially at rest on the ground. A man uses a force to pull up the block as shown above. If the tension in the string is kept at $24$ N and the block reaches a height of $4$ m in $2$ s, which of the following statements is/are correct ?

(1)The potential energy of the block is increasing when the block is rising.

(2)The kinetic energy of the block is increasing when the block is rising.

(3)The average power developed by the man during the two seconds is

40

W.

10.

Weighing device: rod AB held by a string at X, an object hanging at end A and a counter-weight hanging on the rod

A company intends to produce a kind of weighing device as shown. The device is held at point $X$ . The position of the counter-weight is adjusted until the rod $AB$ becomes horizontal. The weight of the object can be read from the scale calibrated on $AB$ . Which of the following changes can increase the maximum weight that can be measured by the device ?

(1)moving the string at

X

towards

A

(2)increasing the mass of the counter-weight

(3)increasing the length of the string from which the counter-weight hangs