物理牛頓運動定律 DSE、CE 選擇題（第 2 頁）

11.

An object is projected vertically upwards. $F$ denotes the magnitude of the net force acting on the object and $W$ denotes the magnitude of the weight of the object. If air resistance is not negligible, which of the following descriptions are correct ?

(1)When the object is moving up,

F

is greater than

W

.

(2)When the object is at the highest point,

F

is equal to

W

.

(3)When the object is moving down,

F

is smaller than

W

.

12.

A ball hanging by a string inside a car, the string making 10 degrees with the vertical

In the figure above, a ball is hanging inside a car by a string. When the car accelerates on a horizontal road, the string makes an angle of $10^\circ$ with the vertical. Find the magnitude of the acceleration of the car. (Take $g$ to be $10\ \mathrm{m\,s^{-2}}$ .)

13.

A block of mass $1$ kg rests on a rough horizontal plane. When it is pulled by a horizontal force of $5$ N, it moves with an acceleration of $2\ \mathrm{m\,s^{-2}}$ . Now, the block is at rest and is pulled by a horizontal force of $2$ N, what is the resultant force acting on the block ?

14.

Square grid with the resultant force F and force F1 drawn from a point

Force $F$ is the resultant of two forces $F_1$ and $F_2$ on a horizontal plane. Which of the following diagrams best represents the missing force $F_2$ ?

Four grids A to D each showing a candidate force F2

15.

An object of weight $120$ N on the Earth's surface is taken to the Moon's surface where the acceleration due to gravity is $\dfrac{1}{6}$ of that at the Earth's surface. What are the mass and the weight of the object at the Moon's surface ? (Take the acceleration due to gravity on the Earth's surface to be $10\ \mathrm{m\,s^{-2}}$ .)

	mass	weight
	$2$ kg	$20$ N
	$2$ kg	$120$ N
	$12$ kg	$20$ N
	$12$ kg	$120$ N

16.

A 50 N block hung from two threads: S1 at 30 degrees to the horizontal and S2 horizontal

A $50$ N block is suspended by two threads as shown in the figure above. Thread $S_1$ makes an angle $30^\circ$ with the horizontal and thread $S_2$ is horizontal. What is the tension in $S_2$ ?

17.

Blocks A and B joined by a string on a smooth surface, 10 N force pulling A to the left and 4 N force pulling B to the right

Two blocks $A$ and $B$ of the same mass on a smooth horizontal surface are connected by a string as shown above. Two horizontal forces of $10$ N and $4$ N are acting on $A$ and $B$ respectively. Assume all contact surfaces are smooth. What is the magnitude of the tension in the string ?

18.

Figure (a): a block on a table on the ground. Figure (b): forces F3 and F1 on the block and F2 on the table

A block is placed on a table resting on the ground as shown in Figure (a) above. Figure (b) shows the forces acting on the block and the table respectively.

Let $F_1$ = weight of the block

$F_2$ = force exerted by the block on the table

$F_3$ = force exerted by the table on the block

Which of the following statements are correct ?

(1)

F_1

and

F_2

represent the same force.

(2)

F_1

and

F_3

balance each other.

(3)

F_2

and

F_3

form an action-reaction pair.

19.

A train (west on the left, east on the right) with a bob hung from the ceiling of a carriage, inclined to the east

A bob is hung by a string from the ceiling of a train. At a certain instant, the bob is inclined to the east as shown in the figure. Which of the following descriptions about the motion of the train at this instant is/are possible ?

(1)It is moving to the east and decelerating.

(2)It is moving to the west and accelerating.

(3)It starts to move to the east from rest.

20.

Block sliding down a rough inclined plane at angle theta with acceleration a

A block of mass $m$ slides down along a rough inclined plane with constant acceleration $a$ as shown above. What are the friction and the normal reaction acting on the block ?

	Friction	Normal Reaction
	$mg\sin\theta+ma$	$mg\sin\theta$
	$mg\sin\theta-ma$	$mg\cos\theta$
	$mg\cos\theta+ma$	$mg\sin\theta$
	$mg\cos\theta-ma$	$mg\cos\theta$