Download Supplementary exercise for Ch.1 to 4

Supplementary Exercise for S4 Physics (Ch.1 to 4)
1.
A small object P is suspended by a vertical light string. It is
then pulled to one side by a force equal in magnitude to the
weight of the object and held stationary in the position shown
below.
2.
The graph shows the variation with time t of the acceleration a of a body that starts
from rest at t = 0.
Which of the following is the speed of the object after 10 s?
A. 0.67 m s-1
3.
B. 1.5 m s-1
C. 75 m s-1
D. 150 m s-1
A feather is dropped from rest at a height of 9.0 m above the surface of the Moon. It
takes 3.0 s to reach the surface. Based on this observation, which of the following is the
best estimate of the acceleration of free fall at the surface of the Moon?
A. 0.50 m s-2
B. 1.0 m s-2
C. 2.0 m s-2
P.1
D. 3.0 m s-2
4.
An insect of weight W jumps to a vertical height h. The time from the start of the jump
to when the insect leaves the surface is t. Which of the following is the best estimate for
the power generated by the insect to perform the jump?
5.
Two identical metal spheres are held above the ground as shown.
The separation between them is small compared to their distance above the ground.
When the spheres are released, the separation of the spheres will
A. remain constant.
B. decrease continuously.
C. increase continuously.
D. increase initially and then remain constant.
6.
Which of the following phenomena can be explained with the Newton’s first law of
motion?
(1) When a car stops suddenly, passengers will be thrown forwards.
(2) A coin and a feather will fall with the same acceleration in vacuum.
(3) A spaceman on the moon can lift up a body heavier than on the earth.
A.
C.
7.
(1) only
(1) and (2) only
B.
D.
(3) only
(2) and (3) only
Which of the following statements about mass and weight is INCORRECT?
A.
Mass is measured in kilograms and weight in newtons.
B.
Both the mass and weight of an object vary slightly at different places on the
earth.
C.
Mass is a measure of the inertia of an object and weight is a measure of the
gravitational pull on it.
D.
The weight of an object at a particular place is proportional to its mass.
P.2
8.
Two blocks X and Y of weights 2 N and 8 N respectively are
suspended by two light strings as shown. A downward force of 4 N is
applied to X. Find the tensions T1 and T2 in the two strings.
T1
T2
A.
4N
10 N
B.
4N
14 N
C.
6N
12 N
D.
6N
14 N
9.
Two blocks of masses 5 kg and 3 kg respectively
are connected by a light string passing through a frictionless
fixed light pulley. Find the magnitude of the acceleration of the
blocks in terms of the acceleration due to gravity g when they
are released. Neglect air resistance.
A. g
10.
B.
g
2
C.
g
4
D.
g
8
Two wooden blocks P and Q of masses 20 kg and 40 kg respectively are in contact
and at rest on a smooth horizontal surface. A force of 300 N is applied on block P as
shown, what is the force acting on block P by block Q?
A. 100 N to the right.
300 N
P
B. 100 N to the left.
C. 200 N to the right.
Q
smooth horizontal surface
D. 200 N to the left.
11.
Kelvin is standing on a balance inside a lift. The table shows the readings of the
balance in three situations.
Motion of the lift
Reading of the balance
Moving upwards with a uniform speed
R1
Moving upwards with an acceleration
R2
Moving downwards with an acceleration
R3
Which of the following relationships is correct?
A. R1>R2>R3
B.
R2>R1>R3
C. R2>R3>R1
D.
R3>R2>R1
P.3
12.
A child is sitting on a chair. Which of the following pairs of forces is/are (an) action
and reaction pair(s)?
(1) The gravitational force exerted by
the earth on the child
(2) The force exerted by the child on
the chair on the child
The normal reaction exerted by
and
the chair
(3) The force exerted by the chair on
the chair on the child
The gravitational force exerted by
and
the ground
A. (1) only
C. (1) and (3) only
The normal reaction exerted by
and
the earth on the chair.
B. (2) only
D. (2) and (3) only
13.
Scale: 1cm represents 1N
Two forces F1 and F2 act on a particle P shown above. If a third force F3 acts on P to
keep it in equilibrium, find the magnitude of F3.
A. 1.4 N
14.
15.
B. 4.0 N
C. 4.2 N
D. 4.5 N
In the figure, a ball of weight 20 N is suspended by a
string under the ceiling. The block is pulled by a
horizontal force F such that it makes angle of 30° to
the vertical. Find F.
A. 10 N
B. 11.5 N
C. 17.3 N
D. 20 N
Blocks P and Q of mass 1 kg and 2 kg respectively are connected by a light spring
balance and placed on a smooth horizontal surface as shown. If horizontal forces 15 N
and 6 N act on P and Q respectively and the whole system moves to the left
with constant acceleration, what is the reading of the spring balance?
A.
9N
B.
10 N
C.
11 N
D.
12 N
P.4
16.
A rod XY hinged at X is kept horizontal by a light string. M is the midpoint of XY. In
which of the following arrangements will the tension T in the string be the smallest?
17.
A uniform rod of weight 50 N is supported by two spring balances P and Q
and remains at rest as shown above. Assume the weight of the rod acts through its
mid-point. Find the readings of P and Q.
Reading of P
Reading of Q
18.
A.
17 N
33 N
B.
20 N
30 N
C.
30 N
20 N
D.
33 N
17 N
A boy of weight W exerts a downward pulling force F on a rope of weight G hung
vertically from the ceiling. He stands still on the ground as shown. Which of the
following gives the magnitude of the force exerted by
(1) the boy on the ground;
(2) the rope on the ceiling?
A.
B.
C.
D.
19.
(1)
(2)
W
W
W-F
W-F
G-F
G+F
G-F
G+F
An object of mass 0.5 kg is raised vertically from the ground by a motor. The object is
raised 2.5 m in 1.5 s with uniform speed. Estimate the output power of the motor.
Neglect air resistance. (g = 9.81 m s-2)
A.
5.5 W
B.
8.2 W
C.
11.0 W
D.
16.4 W
P.5
20. An object at a certain height falls freely from rest under gravity. Which graph correctly
shows the variation of its gravitational potential energy U with time t? Neglect air
resistance and take U = 0 at the ground.
21.
The graph above shows the velocity-time graph of an object which is thrown vertically
upwards under gravity. If the object is thrown vertically upwards with a higher initial
velocity, which of the following graphs (in dotted lines) best represents the
expected result? (Neglect air resistance.)
P.6
22.
A particle is projected vertically downward with
an initial speed of 2.0 m s-1 from the rooftop of
house. The particle reaches the ground with a
speed of 11 m s-1 as shown. Estimate the height of
the house. Neglect air resistance.
(g = 9.81 m s-2)
A. 3.3 m
C. 6.5 m
23.
B. 6.0 m
D. 12 m
A toy car travelled due east for 10 m in 5 s, then
immediately turned north and travelled 5 m for 1 s.
What was the average speed of the car?
A.
C.
1.9 m s-1
2.5 m s-1
B. 2.2 m s-1
D. 3.5 m s-1
24.
A rubber ball bounces vertically up and down from the ground. Which graph best
shows the variation of its velocity v with time t? Neglect air resistance.
25.
An electrical toy car of mass m goes up an inclined plane of inclination 30º
with constant speed v. The friction acting on the car is half of the weight of the car.
What is the average power of the car? (g = acceleration due to gravity)
A. (1/2) mgv
26.
B. mgv
C. (3/2) mgv
D. 2mgv
A block of mass 1 kg is sliding down with constant speed along an inclined plane of
inclination 30º to the horizontal. What are the gain in kinetic energy and the work done
against friction by the block after traveling a distance of 2 m along the plane?
(g = 10 m s-2)
A.
B.
C.
D.
27.
Gain in kinetic energy/ J
0
10
0
10
Work done against friction/ J
10
10
20
30
A car is travelling at a constant speed of 15 m s-1 along a horizontal straight road. The
total resisting force acting on the car is 500 N. Which of the following statements
are correct?
(1) The car travels a distance 120 m in 8 s
(2) The work done by the car in overcoming the resisting force in 8 s is 60 kJ
(3) The output power of the car is 7.5 kW
A. (1) and (2) only
B. (1) and (3) only
C. (2) and (3) only
D. (1), (2) and (3)
P.7
28.
An object is release from rest. Which of the following graphs best describes the
variation of the kinetic energy of the object with time during falling?
29.
When a skydiver falls steadily in air under no net force, which of the following
descriptions about his gravitational potential energy (PE), kinetic energy (KE)
and power in overcoming air resistance is correct (Power)?
PE
KE
Power
30.
A.
decreases
increases
increases
B.
deceases
increases
remains unchanged
C.
D.
decreases
remains unchanged
remains unchanged
increases
remains unchanged
increases
A horse pulls a block along a rough horizontal road and moves with a uniform velocity.
Which of the following correctly describes the directions of the friction from the
ground acting on the horse and the block?
Horse
Block
A.
B.
backward
backward
forward
backward
C.
D.
forward
forward
forward
backward
P.8
Answers
1-5 A C C B A
16-20 C B D B C
6-10 A B D C D
21-25 D B C D B
11-15 B B D B D
26-30 A D B C D
Explanations to selected mc
2.
Since a = v/t
Area under graph of a against t is v.
Thus change of velocity = area of triangle = 10 x 15 / 2 = 75 m s-1
9.
Acceleration = F/m = (5g-3g) / (5+3) = g/4
11.
Constant speed => R = mg
v is upward, a is upward. This implies R>mg
v is downward, a is downward. This implies R < mg
13.
Sum of horizontal components = 2 + 2 = 4
Sum of vertical components = 3 – 1 = 2
Required resultant =
15.
42  22  4.47 N
Only Option D gives the same constant acceleration of the two blocks.
Net force on P = 15-12 = 3N, acceleration = F/m = 3/1 = 3 ms-2
Net force on Q = 12-6 = 6N, acceleration = F/m = 6/2 = 3 ms-2
Alternatively, Fnet=ma
Object P : 15  T  1a
Object Q : T  6  2a
Whole system:
15 - 6=(1+2)a
15 - 6
 3 m s-1
3
Object Q : T  6  2  3
T  12 N
a
18.
Consider the boy: his weight W is balanced by two upward forces, normal reaction
from the ground and the pulling force from the rope. Hence W=R+F, => R=W-F.
Therefore, the force acting on the ground by the boy should be R=W-F.
19.
P = W / t = mgh/t = 0.2×9.81×2.5/1.5 = 8.175 W
20.
For free falling object, h = (1/2)gt2, and the potential energy of the object is U=mgh.
Hence U is directly proportion to the square of time (t2).
21.
In both cases, the object undergoes uniformly accelerating motion, so the slope remains
unchanged, hence the new straight line must be parallel.
25.
Average power = power developed by engine = engine force x speed
= Fv = (0.5 mg + mg sin300) v = mg v
where F = engine force = friction + mgsin
26. Since the block is traveling with constant speed, there is no change in KE.
Work done against friction = friction x distance moved = mg sin 30o x 2 = 10 J
27.
Output power = engine force x speed = friction x speed since the car is traveling
with constant speed
28. KE = (1/2) mv2 = (1/2) m (gt)2, since KE varies with time2, the graph is a
parabola.
29. KE does not change because net force is zero, the velocity does not change.
Power in overcoming air resistance = air resistance x speed = mg x speed which
is unchanged.
30. The horse’s foot push backward to move forward, thus the ground presses on the
horse’s foot in forward direction. Friction opposes the motion of the block, so the
direction of friction acting on the block is backward.