物理功、能和功率 DSE、CE 選擇題（第 3 頁）

21.

Marble at the top of a table with two smooth rails A (steeper) and B (gentler) down to the ground

John releases a marble from the top of a smooth rail $A$ placed at the edge of a table as shown above. He repeats the same process by using another smooth rail $B$ . Which of the following statements about the marble is/are correct ?

(1)The marble has the same velocity when it reaches the ends of both rails.

(2)The marble has the same kinetic energy when it reaches the ends of both rails.

(3)It takes the same time for the marble to travel through both rails.

22.

A car stopped after emergency braking. The skid mark left by the car was $22.3$ m long. Assume that the friction between the road and tyres was $0.65$ times that of the weight of the car. Estimate the speed of the car when it began to skid.

23.

A high-diver jumps up into the air from a spring board and then falls into a swimming pool. Which of the following graphs best shows the variation of her kinetic energy $K$ with time $t$ from the time she just leaves the board up to the moment just before she enters the water ? (Neglect air resistance.)

Four graphs A to D of kinetic energy K against time t

24.

Weight-lifter lifting a barbell from the floor

The diagram shows a weight-lifter lifting a weight of mass $80$ kg from the floor to a height of $2$ m. Find the work done by the weight-lifter.

25.

Ferris wheel with eight cages and Edmund in the cage at the right, wheel rotating anticlockwise

The diagram shows Edmund riding the 'Ferris Wheel' in an amusement park. If the wheel is rotating at a uniform speed, which of the following physical quantities of Edmund would remain unchanged ?

26.

Photograph of a mother pushing a baby in a push-chair

The photograph shows a baby sitting on a push-chair and her mother Amy is pushing the push-chair with a uniform velocity $v$ along the horizontal ground. Let $F$ = horizontal force exerted by Amy on the push-chair, and $m$ = total mass of the baby and the push-chair. Which of the following expressions denotes the average output power of Amy in pushing the push-chair ?

27.

Block projected up a smooth inclined plane

A block is projected up a smooth inclined plane. Which of the graphs below shows the variation of the kinetic energy (K.E.) of the block with its potential energy (P.E.) during the upward motion ?

28.

Ferris wheel with eight cages, one passenger in the bottom cage

The figure above shows a flying wheel in an amusement park. The wheel is of diameter $18$ m and carries eight cages. There is only one passenger of mass $60$ kg inside one of the cages. The wheel rotates with uniform speed and it takes $80$ s for the passenger to travel from the bottom to the top of the wheel. Find the average useful power output of the motor of the wheel.

29.

A stone is thrown vertically upwards. Assuming air resistance is negligible, which of the following statements is/are correct ?

(1)The acceleration of the stone decreases throughout the upward motion.

(2)The net force acting on the stone becomes zero when the stone reaches the highest point.

(3)The total energy of the stone remains unchanged throughout the motion.

30.

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

Photograph of a man pulling a suitcase along the ground

If the rope is inclined at an angle $\theta$ to the horizontal and the tension in the rope is $100$ N, find the work done by the man in pulling the suitcase for a distance of $5$ m along the ground.