物理動量 DSE、CE 選擇題（第 4 頁）

31.

Bullets, each of mass $m$ , are fired at the rate of $n$ bullets per second. They hit a vertical wall with horizontal speed $v$ and rebound from the wall with the same horizontal speed $v$ . Which of the following statements is/are correct ?

(1)The total change in momentum of the bullets is zero.

(2)The total change in momentum of the bullets in one second is

2mnv

.

(3)The average force exerted on the wall is

2mnv

.

32.

The figure shows two trolleys moving towards each other along a smooth runway. After collision, they stick together. What is the total loss in kinetic energy by the trolleys during the collision ?

A 2 kg trolley moving right at 4 m/s and a 1 kg trolley moving left at 2 m/s on a smooth runway

33.

A ball moving in a smooth horizontal plane hits against a wall and rebounds perfectly elastically. Which of the following graphs best represents the variation of kinetic energy with time ?

34.

Which of the following is/are correct unit(s) for momentum ?

(1)

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

(2)

\mathrm{kg\,m\,s^{-2}}

(3)N s

35.

A mass of $3$ kg initially at rest explodes into two fragments $X$ and $Y$ of masses $1$ kg and $2$ kg respectively. What is the ratio of the kinetic energy of $X$ to that of $Y$ just after the explosion ?

36.

Which of the following is a vector quantity with correct unit ?

37.

Graph I: speed against time with a shaded triangle under the line. Graph II: force against distance with a shaded triangle under the line

What physical quantity does the area of the shaded portion of each of the above graphs represent ?

	I	II
	acceleration	energy
	distance	power
	acceleration	change of momentum
	distance	energy

38.

A ball of mass $2$ kg drops from rest to the ground from a height $5$ m and rebounds to the same height. If the time of duration of the impact between the ball and the ground is $0.2$ s, what is the force acting on the ball by the ground ? (Take the acceleration due to gravity be $10\ \mathrm{m\,s^{-2}}$ )

39.

A basketball falls freely from rest and hits the ground. It then rebounds to $1/4$ of its original height. Neglecting air resistance, which of the following statements about the basketball is/are correct ?

(1)Its kinetic energy just before collision is four times its kinetic energy just after collision.

(2)Its potential energy just before collision is four times its potential energy just after collision.

(3)The speed just before collision is two times the speed just after collision.

40.

A system consists of two identical masses travelling in opposite directions with equal speed. Suppose they collide elastically at time $t_0$ . Which of the following graphs correctly shows the total momentum of the system before and after the collision ?

Four graphs A to D of total momentum against time with the collision at time t0