物理牛頓運動定律 DSE、CE 選擇題（第 9 頁）

81.

Block X on a smooth table joined by a string over a pulley to hanging block Y

Two identical blocks $X$ and $Y$ are connected by a light string passing over a smooth pulley as shown above. The two blocks are released from rest. After a while, the string breaks. Which of the following correctly describes the motion of the blocks immediately after the string breaks ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

	$X$	$Y$
	stops moving	accelerates at $5\ \mathrm{m\,s^{-2}}$
	moves with constant velocity	accelerates at $5\ \mathrm{m\,s^{-2}}$
	moves with constant velocity	accelerates at $10\ \mathrm{m\,s^{-2}}$
	decelerates at $5\ \mathrm{m\,s^{-2}}$	accelerates at $10\ \mathrm{m\,s^{-2}}$

82.

Block B (3 kg) and block A (2 kg) joined by string S on a smooth table, 2 N force pulling A to the right

In the diagram above, blocks $A$ and $B$ are connected by a light inextensible string and rest on a smooth horizontal table. The masses of $A$ and $B$ are $2$ kg and $3$ kg respectively. Block $A$ is pulled by a force of $2$ N. Find the tension in the string $S$ .

83.

Which of the following pairs of forces $F_1$ and $F_2$ is/are action and reaction pair(s) according to the Newton's third law of motion ?

Three cases: (1) block on the floor, F1 weight of the block and F2 reaction from the floor; (2) ball hanging on a string, F1 weight of the ball and F2 tension of the string; (3) negative and positive charges attracting each other, F1 and F2 electric forces

84.

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

Two blocks of equal mass are placed on a smooth horizontal surface as shown above. A constant force of $12$ N is applied to block $A$ so that the two blocks move towards the right together. The force acting on $A$ by $B$ is

85.

Graph of net force against time: zero until t1, then a constant positive value

The diagram above shows the variation of the net force acting on an object which is initially at rest. Which of the following velocity-time graphs correctly describes the motion of the object ?

86.

A force $F$ is applied to a block of mass $1$ kg as shown below. The greatest value of $F$ for the block to remain at rest is $11$ N. What would be the motion of the block if $F$ is not applied ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

Block on a 30 degree incline pulled up the slope by force F

87.

A man of weight $W$ stands inside a lift which is moving upwards with a constant speed. If the force exerted by the floor on the man is $R$ , which of the below statements is/are correct ?

(1)

R

is greater than

W

in magnitude.

(2)

R

and

W

are in opposite directions.

(3)

R

and

W

form an action and reaction pair according to Newton's third law.

88.

$Y$ has a linear relationship with time as shown. $Y$ may represent

(1)the speed of a body starting from rest under a constant force.

(2)the distance travelled by a body at constant speed.

(3)the acceleration of a body falling from rest.

Graph of Y against Time: straight line through the origin

89.

Force F pushing blocks A, B and C in contact on a smooth horizontal surface

Three blocks of equal mass are placed on a smooth horizontal surface as shown above. A constant force $F$ is applied to block $A$ so that the three blocks move towards the right with the same acceleration. The resultant force acting on block $B$ is

90.

Which of the following statements is/are true ? A man in a lift feels heavier when the lift is moving

(1)upwards with acceleration.

(2)upwards with retardation.

(3)downwards with retardation.