物理動量 DSE、CE 選擇題（第 2 頁）

11.

Three displacement-time graphs: car P (straight line), car Q (curve flattening), car R (curve steepening)

Three cars $P$ , $Q$ and $R$ move along a straight horizontal road. Their displacement-time graphs are shown above. Which of the cars experience a change in momentum during the motion ?

12.

A trolley moves along a smooth horizontal surface. A lump of plasticine is released from a height slightly above the trolley and sticks to it. Which of the following graphs shows the variation of the total horizontal momentum $p$ of the trolley and plasticine with time $t$ ?

Four graphs A to D of total horizontal momentum p against time t

13.

Photograph of an air-cushioned shoe with the air-cushion labelled

The photograph shows an air-cushioned shoe. Which of the following statements about the air-cushion is/are correct ?

(1)It reduces the time of impact between the foot and the ground during running.

(2)It reduces the impact force acting on the foot during running.

(3)It reduces the friction between the shoe and the ground during running.

14.

Wooden block hanging from a ceiling by a string with a bullet approaching it horizontally

A wooden block of mass $M$ is hanging freely in the air from a light string of length $L$ . A bullet, of mass $m$ travelling at a speed $v$ , hits the block and becomes embedded in it. The block then swings upwards. Which of the following are employed in determining the maximum height reached by the block ?

(1)Newton's first law

(2)Law of conservation of energy

(3)Law of conservation of momentum

15.

Grace and Kitty, of masses $50$ kg and $40$ kg respectively, stand on light rollers on a smooth horizontal floor. They are initially at rest with Grace holding a $2$ kg ball. Grace throws the ball to Kitty and moves backward with a speed $0.8\ \mathrm{m\,s^{-1}}$ afterwards. After catching the ball, Kitty moves in the opposite direction as shown. Which of the following statements is/are correct ?

(1)The final speed of Kitty is

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

.

(2)The horizontal momentum of the ball is conserved in this process.

(3)The total kinetic energy of Kitty and the ball decreases when Kitty catches the ball.

Grace moving left at 0.8 m/s and Kitty (holding the ball) moving right, both on rollers on a smooth floor

16.

Force against time graph with a single hump

The figure above shows the variation of the force acting on a car driver with time when the car hits a wall. The driver is not wearing a seat-belt. Which of the following graphs (in dotted lines) best shows the force acting on the driver if he is wearing a seat-belt ?

Four graphs A to D comparing the original force-time curve (solid) with a dotted curve

17.

A rocket of mass $5000$ kg is at rest in space. It then explodes and breaks into two parts $P_1$ and $P_2$ of mass $1000$ kg and $4000$ kg respectively. Find the ratio of the kinetic energy of $P_1$ to $P_2$ .

18.

The graph shows the variation of the kinetic energy $E$ of an object with the square of its velocity $v^2$ moving on a straight line. What is the momentum of the object when it is moving at a velocity $4\ \mathrm{m\,s^{-1}}$ ?

Graph of E in joules against v squared: a straight line through the origin reaching 50 J at v squared = 25

19.

A rocket is initially at rest in space. It then explodes and breaks into two parts which move in opposite directions. If the mass of the rear part is larger than that of the front part, which of the following statements is correct ?

20.

A block is pulled by a constant force and moves along a smooth horizontal surface. Which of the following describes the variations of the acceleration and momentum of the block during the time when the force is acting ?

	Acceleration	Momentum
	remains unchanged	remains unchanged
	remains unchanged	increases
	increases	remains unchanged
	increases	increases