物理 1989 年 DSE、CE 選擇題

1.

Force F pushing blocks A, B and C in contact on a smooth horizontal surface

Three blocks of equal mass are placed on a smooth horizontal surface as shown above. A constant force $F$ is applied to block $A$ so that the three blocks move towards the right with the same acceleration. The resultant force acting on block $B$ is

2.

An object is falling from rest with an acceleration of $9.8$ m s $^{-2}$ . Which of the following statements is/are correct ?

(1)It falls with a constant speed of

9.8

m s

^{-1}

.

(2)It falls

9.8

m every second.

(3)It has a speed of

19.6

m s

^{-1}

after

2

s.

3.

A particle is thrown vertically upwards. When the particle is at the maximum height, its acceleration is

4.

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

.

5.

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 ?

6.

A pulse of microwave of speed $3\times10^{8}\ \mathrm{m\,s^{-1}}$ is sent out to detect the position of a stationary weather balloon. The reflected microwave was picked up in $2\times10^{-4}$ s after emission. What is the distance of the balloon from the station ?

7.

Object in front of a convex lens with four rays P, Q, R and S emerging

Which of the 4 light rays $P$ , $Q$ , $R$ and $S$ drawn above is NOT possible ?

8.

If $C$ , $C'$ are both at a distance of $2$ times the focal length from a convex lens, which of the following ray diagrams is correct ?

Four ray diagrams A to D for a convex lens with C and C'

9.

The wavelength and velocity of a sound in air are $25$ cm and $330\ \mathrm{m\,s^{-1}}$ respectively. When this sound enters a medium, its wavelength becomes $75$ cm. Find the velocity of the sound in the medium.

10.

Circular wave pattern from coherent point sources S1 and S2 with points X, Y and Z marked

The figure above shows the wave pattern in a ripple tank from coherent point sources $S_1$ and $S_2$ . What kind of interference occurs at $X$ , $Y$ and $Z$ ?

$X$	$Y$	$Z$
destructive	constructive	constructive
no interference	constructive	constructive
no interference	destructive	no interference
no interference	destructive	constructive