物理牛頓運動定律 DSE、CE 選擇題（第 8 頁）

71.

A block remains at rest on an inclined plane as shown above. Which of the following statements is/are true ?

(1)The frictional force acting by the plane on the block is zero.

(2)The normal reaction acting by the plane on the block is zero.

(3)The resultant force acting on the block is zero.

72.

The below diagram shows a man lifting a ball vertically upwards with uniform acceleration.

A hand lifting a ball upwards, with force arrows F1 upward on the ball, F2 and F3 downward

Let $F_1$ be the force acting on the ball by the man, $F_2$ be the force acting on the man by the ball, $F_3$ be the gravitational force acting on the ball. Which of the following correctly describes the relation between the magnitudes of the forces ?

73.

A 4 kg block and a 2 kg block joined by string S on a smooth surface, force F pulling the 2 kg block to the right

Two blocks are connected together by a light string $S$ placed on a smooth horizontal surface. They move with uniform acceleration of $2\ \mathrm{m\,s^{-2}}$ under the action of force $F$ . What will the accelerations of the blocks become if $S$ suddenly breaks ?

	$2$ kg block	$4$ kg block
	$6\ \mathrm{m\,s^{-2}}$	$0\ \mathrm{m\,s^{-2}}$
	$6\ \mathrm{m\,s^{-2}}$	$2\ \mathrm{m\,s^{-2}}$
	$2\ \mathrm{m\,s^{-2}}$	$0\ \mathrm{m\,s^{-2}}$
	$0\ \mathrm{m\,s^{-2}}$	$3\ \mathrm{m\,s^{-2}}$

74.

Two objects of different masses are released from rest at the same height. Assume air resistance is negligible. Which of the following statements is/are correct ?

(1)A greater gravitational force is acting on the object with greater mass.

(2)They take the same time to reach the ground.

(3)They have equal velocities when they reach the ground.

75.

A trolley is placed on a horizontal ground. A force $F$ inclined at an angle $\theta$ to the horizontal acts on the trolley. What is the horizontal component of $F$ that pulls the trolley towards the right ?

Trolley pulled by force F at angle theta above the horizontal

76.

A car moves with a speed $30$ km h $^{-1}$ . The driver applies the brake and the car is stopped in a distance of $12$ m. If the car is moving at $60$ km h $^{-1}$ , what is the stopping distance ? Assume that the same constant braking force is applied in both cases.

77.

Block resting on the ground with forces R, W and F marked, and the meaning of W, F and R

The above diagram shows a block resting on the ground. Which of the following pairs of forces is/are action and reaction pair(s) according to Newton's third law of motion ?

(1)

R

and

W

(2)

W

and

F

(3)

F

and

R

78.

Paper tape with evenly spaced dots, pulled to the right

A trolley is given a push and runs down a friction-compensated runway. The motion of the trolley is recorded on the paper tape as shown above. Which of the following changes can enable the trolley to produce a paper tape as shown below :

Paper tape with more widely spaced dots, pulled to the right

(1)Giving the trolley a harder initial push.

(2)Increasing the angle of inclination of the runway.

(3)Increasing the frequency of the ticker-tape timer.

79.

Block Y hung from the ceiling by string T2 and block X hung below Y by string T1, with a 4 N downward force on X

Two blocks $X$ and $Y$ of weights $2$ N and $8$ N respectively are suspended by two light strings as shown in the diagram. A downward force of $4$ N is applied to $X$ . Find the tensions $T_1$ and $T_2$ in the two strings.

	$T_1$	$T_2$
	$4$ N	$10$ N
	$4$ N	$14$ N
	$6$ N	$12$ N
	$6$ N	$14$ N

80.

A man of mass $50$ kg is standing in a lift. If the lift is falling freely, which of the following statements is/are true ?

(1)The weight of the man is

0

N.

(2)The force acting on the floor of the lift by the man is

491

N.

(3)The force acting on the man by the floor of the lift is

0

N.