物理牛頓運動定律 DSE、CE 選擇題（第 4 頁）

31.

A $1$ kg block is pulled by a horizontal force of $5$ N and moves with an acceleration of $2\ \mathrm{m\,s^{-2}}$ on a rough horizontal plane. Find the frictional force acting on the block.

Block on a rough horizontal plane pulled to the right by a 5 N force

32.

Questions 28 and 29 : Two blocks $X$ and $Y$ are connected by a light string passing over a smooth pulley as shown below. The mass of $X$ is greater than that of $Y$ . The blocks are released from rest.

Block X on a smooth horizontal surface connected by a string over a pulley to hanging block Y

Which of the following pairs of forces is an action and reaction pair ?

33.

Kelvin is standing on a balance inside a lift. The table shows the readings of the balance in three situations.

Motion of the lift	Reading of the balance
moving upwards with a uniform speed	$R_1$
moving downwards with a uniform speed	$R_2$
moving upwards with an acceleration	$R_3$

Which of the following relationships is correct ?

34.

Fish hung from a light string stretched between two poles, the string making 70 degrees with each pole

A fish is hung on a light string as shown above. If the tension in the string is $10$ N, find the total weight of the fish and the hook.

35.

An object weighs $60$ N on the Earth and $10$ N on the moon. Which of the following statements are correct ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

(1)The mass of the object on the Earth is

6

kg.

(2)The mass of the object on the moon is

1

kg.

(3)The acceleration due to gravity on the moon is one-sixth that on the Earth.

36.

An object is acted on by three forces $F_1$ , $F_2$ and $F_3$ in suitable directions such that it remains at rest. Which of the following combinations of the magnitudes of the forces is/are possible ?

	$F_1$	$F_2$	$F_3$
(1)	$3$ N	$4$ N	$5$ N
(2)	$3$ N	$4$ N	$7$ N
(3)	$3$ N	$5$ N	$9$ N

37.

Figure (a): masses m1 and m2 joined by a string with tension T1, force F pulling m2 to the right. Figure (b): force F pulling m1 to the left, tension T2

Two blocks, of masses $m_1$ and $m_2$ $(m_1>m_2)$ , are connected by a light inextensible string and placed on a smooth horizontal surface. Let $T_1$ and $T_2$ be the tensions in the string when a horizontal force $F$ is applied to the blocks as shown in Figures (a) and (b) respectively. Which of the following relationships are correct ?

(1)

T_1>T_2

(2)

\dfrac{T_1}{m_1}=\dfrac{T_2}{m_2}

(3)

T_1+T_2=F

38.

Graph of y against x: a straight line through the origin

A block is initially at rest on a smooth horizontal surface and is pulled by a constant horizontal force. The figure shows the relationship between the physical quantities $y$ and $x$ . Which of the following combinations of $y$ and $x$ is impossible ?

	$y$	$x$
	displacement of the block	square of time
	velocity of the block	time
	square of velocity of the block	displacement of the block
	acceleration of the block	time

39.

Photograph of a sucker sticking to a hook on a vertical wall

The photograph shows a sucker sticking to a hook on a vertical wall. Which of the following forces balances the gravitational force acting on the sucker and hook by the Earth ?

40.

Block sliding down an inclined plane at angle theta

A block of mass $m$ slides down an inclined plane with uniform velocity. What is the net force acting on the block ?