物理牛頓運動定律 DSE、CE 選擇題（第 3 頁）

21.

Two blocks $A$ and $B$ , of masses $1$ kg and $3$ kg respectively, are placed on a horizontal smooth surface as shown. A horizontal constant force of $12$ N is applied to block $A$ so that the two blocks move to the right with uniform acceleration. What is the magnitude of the contact force between $A$ and $B$ ?

Blocks A and B in contact on a smooth surface, 12 N force pushing A to the right

22.

A horse pulls a block along a rough horizontal road and moves with a uniform velocity. Which of the following correctly describes the directions of the friction from the ground acting on the horse and the block ?

	Horse	Block
	backward	forward
	backward	backward
	forward	forward
	forward	backward

23.

A child is standing on a weighing scale as shown above. Which of the following pairs of forces is/are (an) action and reaction pair(s) ?

(1)The force exerted by the child on the scale and The force exerted by the scale on the child

(2)The gravitational force exerted by the Earth on the child and The force exerted by the scale on the child

(3)The gravitational force exerted by the Earth on the scale and The gravitational force exerted by Earth on the child

24.

A block hung by a string from the ceiling, pulled by a 3 N horizontal force so that the string makes angle theta with the vertical

In the above figure, a $1$ kg block is suspended by a string under the ceiling. The block is pulled by a $3$ N horizontal force such that it makes angle $\theta$ to the vertical. Which of the following descriptions is correct ? (Note : The tension of the string is denoted as $T$ .)

	$\theta$	$T$
	$\theta<30^\circ$	$13\ \mathrm{N}>T>10\ \mathrm{N}$
	$\theta<30^\circ$	$T>13\ \mathrm{N}$
	$\theta>30^\circ$	$13\ \mathrm{N}>T>10\ \mathrm{N}$
	$\theta>30^\circ$	$T>13\ \mathrm{N}$

25.

Many traffic accidents on highways are associated with inertia. Which of the following traffic regulations can reduce the risk of traffic accidents due to inertia ?

(1)Passengers should wear seat belts.

(2)Vehicles should not be overloaded.

(3)People should not drive over the speed limit.

26.

Peter standing on a weighing scale and pressing down on a table with his hands with force F

If Peter stands with his feet on a weighing scale placed on level ground, his weight measured by the scale is $W$ . What would be the reading on the scale if he exerts a downward force $F$ on a table top with his hands while he is still standing on the same scale as shown above ?

27.

Acrobat standing in the middle of a cable between two posts, the cable making angle theta with each post

An acrobat is standing in the middle of a cable as shown. The angle $\theta$ is less than $60^\circ$ . Which of the following relations between the tension $T$ of the cable and the weight $W$ of the acrobat is correct ? (Neglect the weight of the cable.)

28.

A bow is pulled horizontally to create a tension of $50$ N on its string as shown in the figure. What is the net horizontal force acting on the arrow when it is released ?

A bow whose string, under 50 N tension on each side, makes 40 degrees with the arrow on each side

29.

A student performed an experiment to investigate the factors affecting the acceleration of a trolley carrying different loads. The table below shows the data recorded :

Trial	Net Force / N	Total mass of the loaded trolley / kg	Acceleration / $\mathrm{m\,s^{-2}}$
(i)	2	2	1
(ii)	2	1	2
(iii)	2	0.5	4
(iv)	4	2	2
(v)	4	4	1
(vi)	8	2	4

Which trials can the student use to deduce the relationship between the acceleration and the net force acting on the trolley ?

30.

A block remains at rest on a rough inclined plane. Which of the following diagrams shows all the forces acting on the block ? Note : $W$ = gravitational force acting on the block, $R$ = normal reaction exerted by the inclined plane on the block, and $F$ = friction acting on the block.

Four force diagrams A to D for the block