物理牛頓運動定律 DSE、CE 選擇題（第 6 頁）

51.

Block on a rough horizontal table with a 10 N force to the right and a 2 N force to the left

A block on a rough horizontal table is acted on by two horizontal forces of magnitudes $10$ N and $2$ N as shown. It remains at rest on the table. If the force of magnitude $10$ N is removed, find the resultant force acting on the block.

52.

Trolley on a friction-compensated runway pulled by force F parallel to the runway; graph of acceleration a against force F is a straight line through the origin

A student uses a friction-compensated runway to study Newton's second law of motion. The variation of the acceleration $a$ of the trolley with the force $F$ applied parallel to the runway is shown above. If the experiment is repeated with the runway making a larger angle of inclination with the horizontal, which of the graphs (in dotted lines) represents the expected result ?

Four graphs A to D, each with the original solid line and a dotted line

53.

A man is pulling a suitcase along the horizontal ground as shown below.

Photograph of a man pulling a suitcase along 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)The gravitational force exerted by the earth on the man and The gravitational force exerted by the man on the earth

(2)The pulling force exerted by the man on the suitcase and The friction exerted by the ground on the suitcase

(3)The gravitational force exerted by the earth on the suitcase and The normal reaction exerted by the ground on the suitcase

54.

It is said that Galileo Galilei (1564 - 1642), an Italian scientist, dropped a small iron ball and a large cannon ball from the top of the Leaning Tower of Pisa. He found that the two balls reached the ground at almost the same time. Which of the following is/are correct deduction(s) from this experiment ?

(1)The two balls fell with the same acceleration.

(2)A body will maintain uniform motion if there is no external force acting on it.

(3)The gravitational forces acting on the two balls were identical.

55.

An astronaut lands on the moon and finds that his weight is about one-sixth of that on the earth. Which of the following deductions is/are correct ?

(1)If he throws an object upwards on the moon, it will reach a higher level than throwing the object with the same speed on earth.

(2)If he releases an object on the moon, it will take a shorter time to reach the ground than releasing the object from the same height on earth.

(3)The maximum weight he can lift on the moon is greater than on earth.

56.

Three forces of magnitudes $F_1$ , $F_2$ and $10$ N act on an object as shown. If the object is in equilibrium, find the force $F_2$ .

Forces on an object: F1 to the left, F2 up and to the right at 30 degrees to the vertical, 10 N downward

57.

Block projected up a rough inclined plane

A block is placed on a rough inclined plane and then projected upwards along the plane. After reaching the highest point, the block slides down along the plane. Which of the following graphs shows the time variation of the velocity $v$ of the block ?

58.

Uniform steel ball at rest on horizontal ground, just touching a vertical wall

A uniform steel ball lies at rest on a horizontal ground and just touches a vertical wall as shown in the diagram. Which of the following diagrams shows all the forces acting on the ball ? (Note : $W$ = gravitational force acting on the ball, $R$ = normal reaction from the ground, $F$ = friction acting by the ground on the ball, $N$ = normal reaction from the wall.)

59.

Block on a rough horizontal surface pulled to the right by horizontal force F

A block is placed on a rough horizontal ground and a horizontal force acts on the block. If the magnitude of the force, $F$ , is increased gradually, which of the following graphs shows the relation between $F$ and the acceleration $a$ of the block ?

Four graphs of acceleration a against force F

60.

A $2$ kg steel sphere and a $1$ kg wooden sphere are initially held at the same level above the ground and then released from rest simultaneously. Assume air resistance is negligible. Which of the following statements about the two spheres at any instant before they reach the ground is/are correct ?

(1)The speeds of the two spheres are equal.

(2)The accelerations of the two spheres are equal.

(3)The gravitational forces acting on the two spheres are equal.