物理牛頓運動定律 DSE、CE 選擇題（第 5 頁）

41.

Amusement park ride: a seat raised to the top of a vertical pole, with point P near the ground

The figure shows a ride in an amusement park. A passenger is fastened to a seat which is then raised to the top of a vertical pole. At time $t=0$ , the seat is released from rest and falls freely. After $3$ s, the seat is brought to rest at a point $P$ near the ground. Which of the below graphs best shows the relation between the gravitational force acting on the passenger $W$ and $t$ ?

42.

Uniform cube held by four cables, each making 20 degrees with the vertical

A uniform cube of weight $600$ N is held in equilibrium in the air by four identical cables as shown above. If each cable makes an equal angle of $20^\circ$ with the vertical, find the tension in each cable.

43.

Photograph of a child sitting in a safety seat

A car accelerates at $3\ \mathrm{m\,s^{-2}}$ along a straight horizontal road. A child of mass $10$ kg is sitting on a safety seat inside a car. Find the magnitude of the resultant force exerted by the safety seat on the child. (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

44.

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

(1)The gravitational force exerted by the earth on the child. and The normal reaction exerted by the chair on the child.

(2)The force exerted by the child on the chair. and The normal reaction exerted by the chair on the child.

(3)The force exerted by the chair on the ground. and The gravitational force exerted by the earth on the chair.

45.

The following are statements written by three students about Newton's first law of motion.

(1)A stationary object will remain in a state of rest unless acted on by an unbalanced force.

(2)An object undergoing uniform motion will maintain its motion unless acted on by an unbalanced force.

(3)An unbalanced force is required to maintain the motion of an object at uniform velocity.

Which of the above statements is/are correct ?

46.

Which of the following objects are under the action of an unbalanced force at the instant shown in the diagrams ? (1) a football being kicked by a player

(2) a parachutist falling with uniform velocity

(3) a swinging pendulum bob

A pendulum bob swinging on a string hung from the ceiling

47.

A block of mass $0.5$ kg slides down a rough inclined plane with an acceleration of $3\ \mathrm{m\,s^{-2}}$ . If the plane is inclined at $30^\circ$ to the horizontal, find the friction between the block and the plane.

Block sliding down a rough plane inclined at 30 degrees

48.

John, of mass $80$ kg, is standing on a weighing scale in a lift. At a certain instant, the reading of the weighing scale is $600$ N. Which of the following statements about John at this instant is/are correct ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

(1)The gravitational force acting on John is

600

N.

(2)The force exerted by the weighing scale on John is

200

N.

(3)John is accelerating downward at a rate of

2.5\ \mathrm{m\,s^{-2}}

.

49.

Block on a smooth horizontal table with a 60 N vertical force upward and a 30 N horizontal force to the right

A block of weight $100$ N is placed on a smooth horizontal table. A vertical force of $60$ N and a horizontal force of $30$ N are applied to the block as shown. Find the magnitude of the resultant force acting on the block.

50.

Photograph of a finger flicking a cardboard with a coin on it resting on a glass

A coin is placed on a piece of cardboard resting on a glass as shown above. If the cardboard is flickered with a finger, the coin will drop into the glass. What does this experiment demonstrate ?