物理 1991 年 DSE、CE 選擇題

1.

Which of the following is/are vectors ?

(1)momentum

(2)power

(3)voltage

2.

Blocks A and B in contact on a smooth horizontal surface, 12 N force pushing A to the right

Two blocks of equal mass are placed on a smooth horizontal surface as shown above. A constant force of $12$ N is applied to block $A$ so that the two blocks move towards the right together. The force acting on $A$ by $B$ is

3.

Graph of Y against displacement, decreasing linearly from a positive value to zero

For a body falling freely under gravity, $Y$ has a linear relationship with the displacement of the body as shown in the above figure. $Y$ may represent

4.

Strobe photograph of a ball rolling down a slope with successive separations 4 cm, 8 cm and 12 cm

The above figure shows the strobe photograph of a ball rolling down a slope. The stroboscope is flashing at a frequency of $5$ Hz. Find the acceleration of the ball.

5.

The figure shows the velocity-time graph of an object. Which of the following statements about the object is/are true ?

(1)Its acceleration in the first

10

s is

2

m s

^{-2}

.

(2)The total distance travelled is

250

m.

(3)It returns to its starting point after

25

s.

Velocity-time graph: rises from 0 to 20 m/s at 10 s, constant until 20 s, then falls to zero at about 25 s

6.

A block is pulled along a horizontal bench at a constant velocity of $10\ \mathrm{m\,s^{-1}}$ by a force of $5$ N. What is the work done against friction in $4$ s ?

7.

Graph of net force against time: zero until t1, then a constant positive value

The diagram above shows the variation of the net force acting on an object which is initially at rest. Which of the following velocity-time graphs correctly describes the motion of the object ?

8.

A ball collides with a fixed wall and bounces back with the same speed. Which of the following quantities of the ball remain(s) unchanged before and after the collision ?

(1)kinetic energy

(2)velocity

(3)momentum

9.

A force $F$ is applied to a block of mass $1$ kg as shown below. The greatest value of $F$ for the block to remain at rest is $11$ N. What would be the motion of the block if $F$ is not applied ? (Take the acceleration due to gravity to be $10\ \mathrm{m\,s^{-2}}$ .)

Block on a 30 degree incline pulled up the slope by force F

10.

A man of weight $W$ stands inside a lift which is moving upwards with a constant speed. If the force exerted by the floor on the man is $R$ , which of the below statements is/are correct ?

(1)

R

is greater than

W

in magnitude.

(2)

R

and

W

are in opposite directions.

(3)

R

and

W

form an action and reaction pair according to Newton's third law.