物理功、能和功率 DSE、CE 選擇題（第 2 頁）

11.

An object is released from rest at the top of a building of height $h$ . At time $t_1$ , the object is at the half of the height of the building as shown in the figure. At time $t_2$ , the object just reaches the ground. Which of the following is/are correct ? (Neglect air resistance.)

(1)velocity of the object at

t_2 = 2\times

velocity of the object at

t_1

(2)

t_2 = 2\times t_1

(3)K.E. of the object at

t_2 = 2\times

K.E. of the object at

t_1

Building of height h with the object at t = 0 at the top, t = t1 at h/2 and t = t2 at the ground

12.

A fish jumps up vertically to a maximum height of $0.5$ m above the water surface. What is the speed when it just leaves the surface ?

13.

A car is travelling at a constant speed of $15\ \mathrm{m\,s^{-1}}$ along a horizontal straight road. The total resisting force acting on the car is $500$ N. Which of the following statements are correct ?

(1)The car travels a distance

120

m in

8

s.

(2)The work done by the car in overcoming the resisting force in

8

s is

60

kJ.

(3)The output power of the car is

7.5

kW.

14.

Velocity-time graph of a car: rises slightly to t1, constant until t1, then falls linearly to zero

The figure above shows the velocity - time graph of a car travelling on a horizontal straight road. Which of the following statements is/are correct ?

(1)Area under the graph is equal to the total displacement of the car.

(2)The acceleration of the car is maximum at time

t_1

.

(3)The kinetic energy of the car remains unchanged throughout the whole journey.

15.

An object is released from rest. Which of the following graphs best describes the variation of the kinetic energy of the object with time during falling ? (Neglect air resistance.)

16.

When a skydiver falls steadily in air under no net force, which of the following descriptions about his gravitational potential energy, kinetic energy and power in overcoming air resistance is correct ?

Gravitational potential energy	Kinetic energy	Power in overcoming air resistance
decreases	increases	increases
decreases	increases	remains unchanged
decreases	remains unchanged	remains unchanged
remains unchanged	increases	increases

17.

John performs a bungee jump from a bridge above a river. He (assumed to be a particle) is tied to the bridge at $A$ with an elastic cord. He falls from rest at $A$ . When he reaches $B$ , the elastic cord starts to stretch. John is momentarily at rest at $C$ and then bounces up. Which of the following descriptions about the motion of John is/are correct ? (Neglect the air resistance.)

(1)From

A

to

B

, John is under free falling.

(2)From

B

to

C

, the gravitational potential energy of John increases.

(3)At

C

, there is no net force acting on John.

Vertical line from the bridge at A down through B and C to the river

18.

A skier slides down a slope as shown in the diagram above. Assume constant friction along the slope, which of the following graphs best describes the change of energy of the skier with distance down the slope ?

Four graphs A to D of P.E. and K.E. against distance

19.

An electrical toy car of mass $m$ goes up an inclined plane of inclination $30^\circ$ with constant speed $v$ . The air resistance acting on the car is half of the weight of the car. What is the average power of the car ?

20.

Block sliding down an incline of 30 degrees

A block of mass $1$ kg is sliding down with constant speed along an inclined plane of inclination $30^\circ$ to the horizontal. What are the gain in kinetic energy and the work done against friction by the block after travelling a distance of $2$ m along the plane ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

	Gain in kinetic energy / J	Work done against friction / J
	0	10
	10	10
	0	20
	10	30