物理功、能和功率 DSE、CE 選擇題（第 5 頁）

41.

Two blocks $X$ and $Y$ are connected by a light string passing over a smooth pulley as shown. The mass of $X$ is greater than that of $Y$ . The two blocks are released from rest. Which of the following statements is/are correct ?

(1)The potential energy lost by

X

is equal to the potential energy gained by

Y

.

(2)The kinetic energy gained by

X

is greater than the kinetic energy gained by

Y

.

(3)The potential energy lost by

X

is equal to the sum of kinetic energies gained by

X

and

Y

.

Blocks X and Y hanging on either side of a smooth pulley

42.

Force against displacement graph with a shaded rectangular area

The figure shows the variation of the force acting on an object. What physical quantity does the area of the shaded portion represent ?

43.

Pendulum bob released from point P, 0.8 m above its lowest position Q

A pendulum bob is released from rest from a point $P$ $0.8$ m above its lowest position $Q$ , as shown in the diagram. Neglecting air resistance, find the speed of the bob when it reaches $Q$ . (Take $g = 10\ \mathrm{m\,s^{-2}}$ .)

44.

Smooth curved rail with points A, B, C and D; ball released from rest at the left end at the level of D

A ball is released from rest from one end of a smooth curved rail as shown in the above diagram. Neglecting air resistance and friction, which of the following statements about the motion of the ball is/are correct ?

(1)The ball has maximum kinetic energy at point

B

.

(2)The speed of the ball at point

C

is the same as that at point

A

.

(3)The ball would not rise to a level higher than point

D

.

45.

Which of the following involve(s) an energy transfer of $100$ J ?

(1)A mass of

10

kg is raised vertically by

10

m.

(2)A mass of

2

kg gains a speed of

10\ \mathrm{m\,s^{-1}}

from rest.

(3)A

10

W heater is switched on for

10

s.

46.

Graph of Y against displacement, decreasing linearly from a positive value to zero

For a body falling freely under gravity, $Y$ has a linear relationship with the displacement of the body as shown in the above figure. $Y$ may represent

47.

A block is pulled along a horizontal bench at a constant velocity of $10\ \mathrm{m\,s^{-1}}$ by a force of $5$ N. What is the work done against friction in $4$ s ?

48.

A block of mass $5$ kg is moving horizontally on a smooth surface along a straight line at $6\ \mathrm{m\,s^{-1}}$ . In order to change its speed to $10\ \mathrm{m\,s^{-1}}$ , the work done would need to be

49.

Two objects $P$ and $Q$ , of mass $0.1$ kg and $0.5$ kg respectively, are thrown vertically upwards with the same speed from the same level. Neglecting air resistance, which of the following is true ?

50.

An object slides down a smooth plane from rest as shown in the figure above. Which of the following graphs best shows the variation of kinetic energy with time ?