物理功、能和功率 DSE、CE 選擇題（第 4 頁）

31.

Hand pulling a block upwards with a string, tension 24 N

A $2$ kg block is initially at rest on the ground. A man uses a force to pull up the block as shown above. If the tension in the string is kept at $24$ N and the block reaches a height of $4$ m in $2$ s, which of the following statements is/are correct ?

(1)The potential energy of the block is increasing when the block is rising.

(2)The kinetic energy of the block is increasing when the block is rising.

(3)The average power developed by the man during the two seconds is

40

W.

32.

A motor car of mass $2000$ kg accelerates from rest at $3\ \mathrm{m\,s^{-2}}$ for $4$ s on a straight road. Find the average useful output power of the engine of the car.

33.

Which of the following units does not represent energy ?

34.

A smooth inclined plane makes an angle $30^\circ$ with the horizontal as shown. A man pulls a $50$ kg block up the inclined plane with uniform velocity. The inclined plane is $6$ m long and the block is pulled from the bottom of the inclined plane to the top in $30$ s. Find the average power of the man. Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .

50 kg block on a smooth incline of 30 degrees, 6 m long

35.

Pendulum bob released from A, with a fixed pin below the support on the vertical through B

A pendulum bob is suspended by a light inextensible string. The bob is released from rest at point $A$ . When the bob reaches the lowest point $B$ , the string hits a fixed pin. Neglect air resistance. Which of the following statements is/are correct ?

(1)The bob has maximum kinetic energy at

B

.

(2)The highest level that the bob can reach is the level of the pin.

(3)The work done by the tension of the string is zero throughout the motion.

36.

A machine lifts up a load of $1200$ N to a height of $1.5$ m in $2$ s. Find the average output power of the machine.

37.

A motor car of mass $2500$ kg accelerates from rest to a speed of $20\ \mathrm{m\,s^{-1}}$ in $5$ s on a level road. Find the average power of the car.

38.

Figure (a): bullet at 450 m/s passing through a single wooden block. Figure (b): bullet at 450 m/s approaching a stack of identical blocks

A bullet of mass $0.02$ kg traveling at $450\ \mathrm{m\,s^{-1}}$ enters a wooden block as shown in Figure (a) and leaves it with a speed $400\ \mathrm{m\,s^{-1}}$ . In figure (b), what is the maximum number of identical blocks that the same bullet can pass through ?

39.

A trolley runs down a friction-compensated runway as shown above. Which of the following statements is true ?

40.

A stone is projected vertically upwards. Neglecting air resistance, which of the following graphs shows the variation of the kinetic energy $KE$ of the stone with the distance travelled $s$ during its upward motion ?

Four graphs A to D of KE against distance s