物理功、能和功率 DSE、CE 選擇題（第 7 頁）

61.

A bullet hits a target at a speed of $200\ \mathrm{m\,s^{-1}}$ . If $50\%$ of the kinetic energy has been converted into the internal energy of the bullet, what is the rise in temperature of the bullet ? (Specific heat capacity of the bullet $= 100\ \mathrm{J\,kg^{-1}\,{}^\circ C^{-1}}$ )

62.

Two balls $X$ and $Y$ , of the same mass, are released from rest simultaneously at $A$ . $X$ falls vertically downwards while $Y$ runs downwards along the smooth plane $AC$ as shown in the figure. Which of the following statements is/are correct ?

(1)

X

reaches

B

at the same time as

Y

reaches

C

.

(2)The velocity of

X

at

B

is the same as that of

Y

at

C

.

(3)The kinetic energy possessed by

X

at

B

is the same as that of

Y

at

C

.

Right-angled triangle: ball X falls vertically from A to B, ball Y runs down the smooth slope AC

63.

A bullet of mass $0.01$ kg travelling horizontally at $100\ \mathrm{m\,s^{-1}}$ is stopped by $0.1$ m of concrete. What retarding force is applied to the bullet by the concrete ?

64.

A student of weight $500$ N runs up a flight of stairs $15$ m high in $20$ s. The average power developed by the student is

65.

When $40$ kg of water falls through a certain distance, its temperature is increased by $0.2^\circ\mathrm{C}$ . What is the height through which it has fallen ? Given : Specific heat capacity of water $= 4200\ \mathrm{J\,kg^{-1}\,{}^\circ C^{-1}}$

66.

A vehicle moving with uniform speed travels up a road of constant gradient. The power developed by the vehicle's engine is

67.

Which of the following statements concerning the energy of a body falling freely under the action of gravity is/are correct ?

(1)It gains kinetic energy while falling.

(2)Its total energy at any point in the flight is equal to the work done in raising it from ground level to that point.

(3)Its kinetic energy at the end of the flight is all converted into potential energy.

68.

Masses m1 and m2 hung over a smooth pulley fixed to a ceiling

Two masses $m_1$ and $m_2$ ( $m_1 > m_2$ ) are hung over a smooth pulley as shown. If the system is released from rest, what is the kinetic energy of the system after the mass $m_1$ has descended through a distance $h$ ?

69.

A vehicle moves up a road inclined at an angle $\theta$ to the horizontal with a constant velocity of $10\ \mathrm{m\,s^{-1}}$ . If the forces resisting the movement of the vehicle are uniform, the power developed by the vehicle engine is

70.

An object is thrown vertically upwards with initial speed $u$ . Which of the following graphs represents the variation of its kinetic energy $E_k$ with time $t$ ?

Four graphs A to D of kinetic energy against time