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# Practice Test - Laws of motion, Work, Power & Energy Class 9 Notes | EduRev

## Class 9 : Practice Test - Laws of motion, Work, Power & Energy Class 9 Notes | EduRev

``` Page 1



Test -03  (Laws of motion, Work, Power & Energy)

1. An object of mass 10 kg. moves at a constant
speed of 10 ms
-1
. A constant force, that acts for 4
sec on the object. Gives it is speed of 2 ms
-1
in
opposite direction. The force acting on the object
is :
(a) -3N (b) -30 N
(c) 3 N (d) 30 N
2. A block of mass M is pulled along horizontal
frictionless surface by a rope of mass m. Force P is
applied at one end of rope. The force which the
rope exerts on the block is :
(a)
m M
P
+
(b)
) M m ( M
P
+

(c)
) M m (
PM
+
(d)
m M
P
-

3. A uniform rope of length L resting on a
frictionless horizontal surface is pulled at one end
by a force F. What is the tension in the rope at a
distance ? from the end where the force is
applied :
(a) F   (b) ) L / 1 ( F ? +
(c) F/2  (d) ) L / 1 ( F ? -
4. A machine gun has a mass 5 kg. It fires 50 gram
bullets at the rate of 30 bullets per minute at a
speed of 400 ms
-1
. What force is required to keep
the gun in position :
(a) 10 N  (b) 5 N
(c) 15 N (d) 30 N
5. A man weighing 80 kg is standing on a trolley
weighing 320 kg. The trolley is resting on
frictionless horizontal rails. If the man starts
walking on the trolley along the rails at a speed of
one metre per second, then after 4 seconds, his
displacement relative to the ground will be :
(a) 5 metres  (b) 4.8 metres
(c) 3.2 metres  (d) 3.0 metres
6. The elevator shown in fig. is descending with an
acceleration of 2 m/s
2
. The
mass of the block A = 0.50
kg. the force exerted by the
block A on block B  is :
(a) 2 N
(b) 4 N
(c) 6 N
(d) 8 N
7. A particle of mass 10 kg is
moving in a straight line if its displacement x with
time t is given by           x = (t
3
- 2t - 10)m, then the
force acting on it at the end of 4 seconds is :
(a) 24 N  (b) 240 N
(c) 300 N  (d) 1200 N
8. When force F
1
, F
2
, F
3
are acting on  a particle of
mass m such that F
2
and F
3
are mutually
perpendicular. Then the particle remains
stationary. If the force F
1
is now removed then the
acceleration of the particle is :
(a)
m
F
1
(b)
1
3 2
mF
F F

(c)
m
F F
3 2
-
(d)
m
F
2

9.   A body moves with uniform velocity on a rough
surface, under the action of a force. The force F of
dynamic friction is plotted against time as shown
below. The graph will be :

10. A particle moves in a circle of radius 25 cm at two
revolutions per seconds. The acceleration of the
particle in metre per second is :
(a) p
2
(b)  8 p
2

(c) 4 p
2
(d) 2 p
2

11. An automobile of mass m is crossing over a
convex upwards over bridge with a speed v. If the
radius of the bridge is r, The thrust on the bridge
at the highest point will be :
(a) mg +
r
mv
2
(b)
r
mv
mg
2
-
(c)
r
g v m
2 2
(d)
r
g v
2

12. The ratio of angular speeds of minute’s hand and
hour’s hand of a watch is  :
(a) 1 : 12  (b) 6 : 1
(c) 12 : 1  (d) 1 : 6
13. A particle is moving along a circular path with a
uniform speed. Through what angle does its
angular velocity change when it completes half of
the circular path :
(a) 0 ° (b) 45 °
(c) 180 ° (d) 360 °
14. Three identical blocks of masses m = 2 kg are
drawn by a force F = 10.2 N on a frictionless
surface, then what is the tension (in N) in the
string between the blocks B and C ?
Page 2



Test -03  (Laws of motion, Work, Power & Energy)

1. An object of mass 10 kg. moves at a constant
speed of 10 ms
-1
. A constant force, that acts for 4
sec on the object. Gives it is speed of 2 ms
-1
in
opposite direction. The force acting on the object
is :
(a) -3N (b) -30 N
(c) 3 N (d) 30 N
2. A block of mass M is pulled along horizontal
frictionless surface by a rope of mass m. Force P is
applied at one end of rope. The force which the
rope exerts on the block is :
(a)
m M
P
+
(b)
) M m ( M
P
+

(c)
) M m (
PM
+
(d)
m M
P
-

3. A uniform rope of length L resting on a
frictionless horizontal surface is pulled at one end
by a force F. What is the tension in the rope at a
distance ? from the end where the force is
applied :
(a) F   (b) ) L / 1 ( F ? +
(c) F/2  (d) ) L / 1 ( F ? -
4. A machine gun has a mass 5 kg. It fires 50 gram
bullets at the rate of 30 bullets per minute at a
speed of 400 ms
-1
. What force is required to keep
the gun in position :
(a) 10 N  (b) 5 N
(c) 15 N (d) 30 N
5. A man weighing 80 kg is standing on a trolley
weighing 320 kg. The trolley is resting on
frictionless horizontal rails. If the man starts
walking on the trolley along the rails at a speed of
one metre per second, then after 4 seconds, his
displacement relative to the ground will be :
(a) 5 metres  (b) 4.8 metres
(c) 3.2 metres  (d) 3.0 metres
6. The elevator shown in fig. is descending with an
acceleration of 2 m/s
2
. The
mass of the block A = 0.50
kg. the force exerted by the
block A on block B  is :
(a) 2 N
(b) 4 N
(c) 6 N
(d) 8 N
7. A particle of mass 10 kg is
moving in a straight line if its displacement x with
time t is given by           x = (t
3
- 2t - 10)m, then the
force acting on it at the end of 4 seconds is :
(a) 24 N  (b) 240 N
(c) 300 N  (d) 1200 N
8. When force F
1
, F
2
, F
3
are acting on  a particle of
mass m such that F
2
and F
3
are mutually
perpendicular. Then the particle remains
stationary. If the force F
1
is now removed then the
acceleration of the particle is :
(a)
m
F
1
(b)
1
3 2
mF
F F

(c)
m
F F
3 2
-
(d)
m
F
2

9.   A body moves with uniform velocity on a rough
surface, under the action of a force. The force F of
dynamic friction is plotted against time as shown
below. The graph will be :

10. A particle moves in a circle of radius 25 cm at two
revolutions per seconds. The acceleration of the
particle in metre per second is :
(a) p
2
(b)  8 p
2

(c) 4 p
2
(d) 2 p
2

11. An automobile of mass m is crossing over a
convex upwards over bridge with a speed v. If the
radius of the bridge is r, The thrust on the bridge
at the highest point will be :
(a) mg +
r
mv
2
(b)
r
mv
mg
2
-
(c)
r
g v m
2 2
(d)
r
g v
2

12. The ratio of angular speeds of minute’s hand and
hour’s hand of a watch is  :
(a) 1 : 12  (b) 6 : 1
(c) 12 : 1  (d) 1 : 6
13. A particle is moving along a circular path with a
uniform speed. Through what angle does its
angular velocity change when it completes half of
the circular path :
(a) 0 ° (b) 45 °
(c) 180 ° (d) 360 °
14. Three identical blocks of masses m = 2 kg are
drawn by a force F = 10.2 N on a frictionless
surface, then what is the tension (in N) in the
string between the blocks B and C ?


(a) 9.2 (b) 8
(c) 3.4 (d) 9.8
15. Work is always done on a body when :
(a) a force acts on it
(b) it moves through a certain distance
(c) it experience an increase in energy through a
mechanical influence
(d) none of the above
16. A weight hangs freely from the end of a spring. A
body then slowly pushes the weight upwards
until the spring becomes slack. The gain in
gravitational potential energy of the weight
during this process is equal to :
(a) the workdone by the boy against the gravity
force acting on the weight
(b) the loss of stored energy by the spring minus
the work done by the tension in the spring
(c) the workdone on the weight by the boy plus
the stored energy lost by the spring
(d) the workdone on the weight by the boy
minus the workdone by the tension in the
spring plus the stored energy lost by the
spring
17. A light and a heavy body have equal momentum.
Which one has greater K.E. :
(a) The light body
(b) Both have equal K.E.
(c) The heavy body
(d) Data given is incomplete
18. A shell initially at rest explodes into two equal
fragments A and B. Then :
(a) A and B have collinear velocities in the same
direction
(b) A and B have equal speeds but opposite
velocities
(c) A and B have equal speeds but their
velocities inclined to each other
(d) A and B may have unequal speeds but
opposite velocities
19. One end of a light spring of spring constant k is
fixed to a wall and the other end is tied to a block
placed on a smooth horizontal surface. In a
displacement, the work done by the spring is
2
1
k
x
2
. The possible cases are :
(a) The spring was initially compressed by a
distance x, was finally in  its natural length
(b) It was initially stretched by a distance x and
was finally in its natural length
(c) It was initially in its natural length and
finally in a compressed position
(d) Both (a) & (b) are correct
20. Choose the correct statement.  Work done by a
variable force :
(a) it defined as  S . F
? ?

(b) Is independent of path
(c) Is always dependent on the initial and final
positions
(d) None of these
21. Work done by friction :
(a) Is always negative
(b) Is always zero
(c) May be negative zero or positive
(d) Is always positive
22. The example (s) of conservative force is/are :
(a) Gravitational force
(b) Spring force
(c) Electrostatic force
(d) All the above
23. A particle describes a horizontal circle of radius
0.5 m with uniform speed. The centripetal force
acting is 10 N. The work done in describing a
semicircle is :
(a) zero  (b) 5 J
(c) 5 pJ (d) 10 pJ
24. A boy pushes a toy box 2.0 m along the floor by
means of a force of 10 N directed downward at an
angle of 60 ° to the horizontal. The work done by
the boy is :
(a) 6 J  (b) 8 J
(c) 10 J (d) 12 J
25. Two masses m 1
= 5 kg and m
2
= 4.8 kg tied to a
string are hanging over a light frictionless pulley.
What is the acceleration of the masses what left
free to move ? (g = 9.8 m/s
2
)

(a) 0.2 m/s
2
(b) 9.8 m/s
2

(c) 5 m/s
2
(d) 4.8 m/s
2

In every question a statement of ASSERTION is
followed by a statement of REASON. Mark the
correct answer out of the following choices:
If both ASSERTION and REASON are true and reason
is the correct explanation of the assertion.
(a) If both ASSERTION and REASON are true
but reason is not the correct explanation of the
assertion.
(b) If ASSERTION is true but REASON is false.
(c) If ASSERTION is false but REASON is true.
Page 3



Test -03  (Laws of motion, Work, Power & Energy)

1. An object of mass 10 kg. moves at a constant
speed of 10 ms
-1
. A constant force, that acts for 4
sec on the object. Gives it is speed of 2 ms
-1
in
opposite direction. The force acting on the object
is :
(a) -3N (b) -30 N
(c) 3 N (d) 30 N
2. A block of mass M is pulled along horizontal
frictionless surface by a rope of mass m. Force P is
applied at one end of rope. The force which the
rope exerts on the block is :
(a)
m M
P
+
(b)
) M m ( M
P
+

(c)
) M m (
PM
+
(d)
m M
P
-

3. A uniform rope of length L resting on a
frictionless horizontal surface is pulled at one end
by a force F. What is the tension in the rope at a
distance ? from the end where the force is
applied :
(a) F   (b) ) L / 1 ( F ? +
(c) F/2  (d) ) L / 1 ( F ? -
4. A machine gun has a mass 5 kg. It fires 50 gram
bullets at the rate of 30 bullets per minute at a
speed of 400 ms
-1
. What force is required to keep
the gun in position :
(a) 10 N  (b) 5 N
(c) 15 N (d) 30 N
5. A man weighing 80 kg is standing on a trolley
weighing 320 kg. The trolley is resting on
frictionless horizontal rails. If the man starts
walking on the trolley along the rails at a speed of
one metre per second, then after 4 seconds, his
displacement relative to the ground will be :
(a) 5 metres  (b) 4.8 metres
(c) 3.2 metres  (d) 3.0 metres
6. The elevator shown in fig. is descending with an
acceleration of 2 m/s
2
. The
mass of the block A = 0.50
kg. the force exerted by the
block A on block B  is :
(a) 2 N
(b) 4 N
(c) 6 N
(d) 8 N
7. A particle of mass 10 kg is
moving in a straight line if its displacement x with
time t is given by           x = (t
3
- 2t - 10)m, then the
force acting on it at the end of 4 seconds is :
(a) 24 N  (b) 240 N
(c) 300 N  (d) 1200 N
8. When force F
1
, F
2
, F
3
are acting on  a particle of
mass m such that F
2
and F
3
are mutually
perpendicular. Then the particle remains
stationary. If the force F
1
is now removed then the
acceleration of the particle is :
(a)
m
F
1
(b)
1
3 2
mF
F F

(c)
m
F F
3 2
-
(d)
m
F
2

9.   A body moves with uniform velocity on a rough
surface, under the action of a force. The force F of
dynamic friction is plotted against time as shown
below. The graph will be :

10. A particle moves in a circle of radius 25 cm at two
revolutions per seconds. The acceleration of the
particle in metre per second is :
(a) p
2
(b)  8 p
2

(c) 4 p
2
(d) 2 p
2

11. An automobile of mass m is crossing over a
convex upwards over bridge with a speed v. If the
radius of the bridge is r, The thrust on the bridge
at the highest point will be :
(a) mg +
r
mv
2
(b)
r
mv
mg
2
-
(c)
r
g v m
2 2
(d)
r
g v
2

12. The ratio of angular speeds of minute’s hand and
hour’s hand of a watch is  :
(a) 1 : 12  (b) 6 : 1
(c) 12 : 1  (d) 1 : 6
13. A particle is moving along a circular path with a
uniform speed. Through what angle does its
angular velocity change when it completes half of
the circular path :
(a) 0 ° (b) 45 °
(c) 180 ° (d) 360 °
14. Three identical blocks of masses m = 2 kg are
drawn by a force F = 10.2 N on a frictionless
surface, then what is the tension (in N) in the
string between the blocks B and C ?


(a) 9.2 (b) 8
(c) 3.4 (d) 9.8
15. Work is always done on a body when :
(a) a force acts on it
(b) it moves through a certain distance
(c) it experience an increase in energy through a
mechanical influence
(d) none of the above
16. A weight hangs freely from the end of a spring. A
body then slowly pushes the weight upwards
until the spring becomes slack. The gain in
gravitational potential energy of the weight
during this process is equal to :
(a) the workdone by the boy against the gravity
force acting on the weight
(b) the loss of stored energy by the spring minus
the work done by the tension in the spring
(c) the workdone on the weight by the boy plus
the stored energy lost by the spring
(d) the workdone on the weight by the boy
minus the workdone by the tension in the
spring plus the stored energy lost by the
spring
17. A light and a heavy body have equal momentum.
Which one has greater K.E. :
(a) The light body
(b) Both have equal K.E.
(c) The heavy body
(d) Data given is incomplete
18. A shell initially at rest explodes into two equal
fragments A and B. Then :
(a) A and B have collinear velocities in the same
direction
(b) A and B have equal speeds but opposite
velocities
(c) A and B have equal speeds but their
velocities inclined to each other
(d) A and B may have unequal speeds but
opposite velocities
19. One end of a light spring of spring constant k is
fixed to a wall and the other end is tied to a block
placed on a smooth horizontal surface. In a
displacement, the work done by the spring is
2
1
k
x
2
. The possible cases are :
(a) The spring was initially compressed by a
distance x, was finally in  its natural length
(b) It was initially stretched by a distance x and
was finally in its natural length
(c) It was initially in its natural length and
finally in a compressed position
(d) Both (a) & (b) are correct
20. Choose the correct statement.  Work done by a
variable force :
(a) it defined as  S . F
? ?

(b) Is independent of path
(c) Is always dependent on the initial and final
positions
(d) None of these
21. Work done by friction :
(a) Is always negative
(b) Is always zero
(c) May be negative zero or positive
(d) Is always positive
22. The example (s) of conservative force is/are :
(a) Gravitational force
(b) Spring force
(c) Electrostatic force
(d) All the above
23. A particle describes a horizontal circle of radius
0.5 m with uniform speed. The centripetal force
acting is 10 N. The work done in describing a
semicircle is :
(a) zero  (b) 5 J
(c) 5 pJ (d) 10 pJ
24. A boy pushes a toy box 2.0 m along the floor by
means of a force of 10 N directed downward at an
angle of 60 ° to the horizontal. The work done by
the boy is :
(a) 6 J  (b) 8 J
(c) 10 J (d) 12 J
25. Two masses m 1
= 5 kg and m
2
= 4.8 kg tied to a
string are hanging over a light frictionless pulley.
What is the acceleration of the masses what left
free to move ? (g = 9.8 m/s
2
)

(a) 0.2 m/s
2
(b) 9.8 m/s
2

(c) 5 m/s
2
(d) 4.8 m/s
2

In every question a statement of ASSERTION is
followed by a statement of REASON. Mark the
correct answer out of the following choices:
If both ASSERTION and REASON are true and reason
is the correct explanation of the assertion.
(a) If both ASSERTION and REASON are true
but reason is not the correct explanation of the
assertion.
(b) If ASSERTION is true but REASON is false.
(c) If ASSERTION is false but REASON is true.


26. Assertion : When a car crashes head on into a
heavy truck, the car experiences a
greater change in momentum.
Reason     :  Newton’s second law states that the
change in momentum is equal to
impulse.
27. Assertion : When a force acts on a body, it can
change its momentum and kinetic
energy
Reason     :  A force produces acceleration
28. Assertion : No work is done when a man is
whirling a stone tied to a rope in a
circle with a constant speed.
Reason     : The centripetal force is
perpendicular to the direction of
displacement.
29. Assertion : Kilowatt hour (Kwh) is the unit of
electric power.
Reason     : Watt hour has units of energy.
30. Assertion : A body builder does no work in
holding up a weight.
Reason     : No work is done because the
displacement moved is zero.

Test -02 (Set - B) Medical     C10    (Laws of motion, Work, Power & Energy)   ANSWER KEY

1
B
6
B
11
B
16
C
21
C
26 D
2
C
7
B
12
C
17
A
22
D
27 A
3
D
8
A
13
A
18
B
23
A
28 A
4
A
9
B
14
C
19
D
24
C
29 D
5
C
10
C
15
C
20
C
25 A 30 A

SOLUTIONS
1. Here, u = 10 ms
-
1
, v = - 2ms
-
1
;   t = 4 s, a = ?
Using a =
2
s / m 3
4
10 2
t
u v
- =
- -
=
-
, Force, F = ma
= 10 × ( -3) = - 30 N
2. Acceleration of block, a =
mass total
force
=
m M
P
+
?
Force on block = mass × acceleration = Ma =
m M
MP
+
.
3. Let m be the mass per unit length of rope.
Therefore, mass of rope = mL. Acceleration in the
rope due to force F will be a = F/mL mass of rope
of length (L - ) ? will be m (L - ) ? . Therefore,
tension in the rope of length (L - ) ? , is equal to
pulling fore on it = m(L - ) ? a = m(L - ) ? × F/mL =
F(1 - ) L / ?
4. Force required =
taken time
momentum in change
=
N 10
60
400 ) 30 10 50 (
3
=
× × ×
-

5. (c) Due to conservation of momentum 80 × 1 = (80
+ 320)V ? V =
5
1
= 0.2 m/s. Velocity of man wrt
ground =- 1 – 0.2 = 0.8 m/s. ?Displacement of
man ground = (0.8 m/s) × 4s = 3.2 m
6. F = mg - R = 0.5 × 10 - 0.5 × 2 = 5 - 1 = 4N
7. m = 10 kg, x = (t
3
- 2t - 10)m  ?   2 t 3 v
dt
dx
2
- = =
and t 6 a
dt
x d
2
2
= = At the end of 4 secs, a = 6 × 4 = 24
m/s
2

F = ma = 10 × 24 = 240 N.
8. The formula for force is given by F
1
= ma.
Acceleration of the particle a =
m
F
1

9. Dynamic friction force is constant
10. (c) a = rw
2
=
4
1
× (2 p ×2)
2
=
4
1
× 4 p
2
= 4 p
2

11. Mg – N =
r
mv
2
? N = mg -
r
mv
2
.
12. Angular speed of hour hand.  ?
1
=
) 60 12 (
2
×
p
.
Angular speed of minute hand ?
2
=
60
2 p
?
1
12
60
60 12
1
2
=
×
=
?
?

Page 4



Test -03  (Laws of motion, Work, Power & Energy)

1. An object of mass 10 kg. moves at a constant
speed of 10 ms
-1
. A constant force, that acts for 4
sec on the object. Gives it is speed of 2 ms
-1
in
opposite direction. The force acting on the object
is :
(a) -3N (b) -30 N
(c) 3 N (d) 30 N
2. A block of mass M is pulled along horizontal
frictionless surface by a rope of mass m. Force P is
applied at one end of rope. The force which the
rope exerts on the block is :
(a)
m M
P
+
(b)
) M m ( M
P
+

(c)
) M m (
PM
+
(d)
m M
P
-

3. A uniform rope of length L resting on a
frictionless horizontal surface is pulled at one end
by a force F. What is the tension in the rope at a
distance ? from the end where the force is
applied :
(a) F   (b) ) L / 1 ( F ? +
(c) F/2  (d) ) L / 1 ( F ? -
4. A machine gun has a mass 5 kg. It fires 50 gram
bullets at the rate of 30 bullets per minute at a
speed of 400 ms
-1
. What force is required to keep
the gun in position :
(a) 10 N  (b) 5 N
(c) 15 N (d) 30 N
5. A man weighing 80 kg is standing on a trolley
weighing 320 kg. The trolley is resting on
frictionless horizontal rails. If the man starts
walking on the trolley along the rails at a speed of
one metre per second, then after 4 seconds, his
displacement relative to the ground will be :
(a) 5 metres  (b) 4.8 metres
(c) 3.2 metres  (d) 3.0 metres
6. The elevator shown in fig. is descending with an
acceleration of 2 m/s
2
. The
mass of the block A = 0.50
kg. the force exerted by the
block A on block B  is :
(a) 2 N
(b) 4 N
(c) 6 N
(d) 8 N
7. A particle of mass 10 kg is
moving in a straight line if its displacement x with
time t is given by           x = (t
3
- 2t - 10)m, then the
force acting on it at the end of 4 seconds is :
(a) 24 N  (b) 240 N
(c) 300 N  (d) 1200 N
8. When force F
1
, F
2
, F
3
are acting on  a particle of
mass m such that F
2
and F
3
are mutually
perpendicular. Then the particle remains
stationary. If the force F
1
is now removed then the
acceleration of the particle is :
(a)
m
F
1
(b)
1
3 2
mF
F F

(c)
m
F F
3 2
-
(d)
m
F
2

9.   A body moves with uniform velocity on a rough
surface, under the action of a force. The force F of
dynamic friction is plotted against time as shown
below. The graph will be :

10. A particle moves in a circle of radius 25 cm at two
revolutions per seconds. The acceleration of the
particle in metre per second is :
(a) p
2
(b)  8 p
2

(c) 4 p
2
(d) 2 p
2

11. An automobile of mass m is crossing over a
convex upwards over bridge with a speed v. If the
radius of the bridge is r, The thrust on the bridge
at the highest point will be :
(a) mg +
r
mv
2
(b)
r
mv
mg
2
-
(c)
r
g v m
2 2
(d)
r
g v
2

12. The ratio of angular speeds of minute’s hand and
hour’s hand of a watch is  :
(a) 1 : 12  (b) 6 : 1
(c) 12 : 1  (d) 1 : 6
13. A particle is moving along a circular path with a
uniform speed. Through what angle does its
angular velocity change when it completes half of
the circular path :
(a) 0 ° (b) 45 °
(c) 180 ° (d) 360 °
14. Three identical blocks of masses m = 2 kg are
drawn by a force F = 10.2 N on a frictionless
surface, then what is the tension (in N) in the
string between the blocks B and C ?


(a) 9.2 (b) 8
(c) 3.4 (d) 9.8
15. Work is always done on a body when :
(a) a force acts on it
(b) it moves through a certain distance
(c) it experience an increase in energy through a
mechanical influence
(d) none of the above
16. A weight hangs freely from the end of a spring. A
body then slowly pushes the weight upwards
until the spring becomes slack. The gain in
gravitational potential energy of the weight
during this process is equal to :
(a) the workdone by the boy against the gravity
force acting on the weight
(b) the loss of stored energy by the spring minus
the work done by the tension in the spring
(c) the workdone on the weight by the boy plus
the stored energy lost by the spring
(d) the workdone on the weight by the boy
minus the workdone by the tension in the
spring plus the stored energy lost by the
spring
17. A light and a heavy body have equal momentum.
Which one has greater K.E. :
(a) The light body
(b) Both have equal K.E.
(c) The heavy body
(d) Data given is incomplete
18. A shell initially at rest explodes into two equal
fragments A and B. Then :
(a) A and B have collinear velocities in the same
direction
(b) A and B have equal speeds but opposite
velocities
(c) A and B have equal speeds but their
velocities inclined to each other
(d) A and B may have unequal speeds but
opposite velocities
19. One end of a light spring of spring constant k is
fixed to a wall and the other end is tied to a block
placed on a smooth horizontal surface. In a
displacement, the work done by the spring is
2
1
k
x
2
. The possible cases are :
(a) The spring was initially compressed by a
distance x, was finally in  its natural length
(b) It was initially stretched by a distance x and
was finally in its natural length
(c) It was initially in its natural length and
finally in a compressed position
(d) Both (a) & (b) are correct
20. Choose the correct statement.  Work done by a
variable force :
(a) it defined as  S . F
? ?

(b) Is independent of path
(c) Is always dependent on the initial and final
positions
(d) None of these
21. Work done by friction :
(a) Is always negative
(b) Is always zero
(c) May be negative zero or positive
(d) Is always positive
22. The example (s) of conservative force is/are :
(a) Gravitational force
(b) Spring force
(c) Electrostatic force
(d) All the above
23. A particle describes a horizontal circle of radius
0.5 m with uniform speed. The centripetal force
acting is 10 N. The work done in describing a
semicircle is :
(a) zero  (b) 5 J
(c) 5 pJ (d) 10 pJ
24. A boy pushes a toy box 2.0 m along the floor by
means of a force of 10 N directed downward at an
angle of 60 ° to the horizontal. The work done by
the boy is :
(a) 6 J  (b) 8 J
(c) 10 J (d) 12 J
25. Two masses m 1
= 5 kg and m
2
= 4.8 kg tied to a
string are hanging over a light frictionless pulley.
What is the acceleration of the masses what left
free to move ? (g = 9.8 m/s
2
)

(a) 0.2 m/s
2
(b) 9.8 m/s
2

(c) 5 m/s
2
(d) 4.8 m/s
2

In every question a statement of ASSERTION is
followed by a statement of REASON. Mark the
correct answer out of the following choices:
If both ASSERTION and REASON are true and reason
is the correct explanation of the assertion.
(a) If both ASSERTION and REASON are true
but reason is not the correct explanation of the
assertion.
(b) If ASSERTION is true but REASON is false.
(c) If ASSERTION is false but REASON is true.


26. Assertion : When a car crashes head on into a
heavy truck, the car experiences a
greater change in momentum.
Reason     :  Newton’s second law states that the
change in momentum is equal to
impulse.
27. Assertion : When a force acts on a body, it can
change its momentum and kinetic
energy
Reason     :  A force produces acceleration
28. Assertion : No work is done when a man is
whirling a stone tied to a rope in a
circle with a constant speed.
Reason     : The centripetal force is
perpendicular to the direction of
displacement.
29. Assertion : Kilowatt hour (Kwh) is the unit of
electric power.
Reason     : Watt hour has units of energy.
30. Assertion : A body builder does no work in
holding up a weight.
Reason     : No work is done because the
displacement moved is zero.

Test -02 (Set - B) Medical     C10    (Laws of motion, Work, Power & Energy)   ANSWER KEY

1
B
6
B
11
B
16
C
21
C
26 D
2
C
7
B
12
C
17
A
22
D
27 A
3
D
8
A
13
A
18
B
23
A
28 A
4
A
9
B
14
C
19
D
24
C
29 D
5
C
10
C
15
C
20
C
25 A 30 A

SOLUTIONS
1. Here, u = 10 ms
-
1
, v = - 2ms
-
1
;   t = 4 s, a = ?
Using a =
2
s / m 3
4
10 2
t
u v
- =
- -
=
-
, Force, F = ma
= 10 × ( -3) = - 30 N
2. Acceleration of block, a =
mass total
force
=
m M
P
+
?
Force on block = mass × acceleration = Ma =
m M
MP
+
.
3. Let m be the mass per unit length of rope.
Therefore, mass of rope = mL. Acceleration in the
rope due to force F will be a = F/mL mass of rope
of length (L - ) ? will be m (L - ) ? . Therefore,
tension in the rope of length (L - ) ? , is equal to
pulling fore on it = m(L - ) ? a = m(L - ) ? × F/mL =
F(1 - ) L / ?
4. Force required =
taken time
momentum in change
=
N 10
60
400 ) 30 10 50 (
3
=
× × ×
-

5. (c) Due to conservation of momentum 80 × 1 = (80
+ 320)V ? V =
5
1
= 0.2 m/s. Velocity of man wrt
ground =- 1 – 0.2 = 0.8 m/s. ?Displacement of
man ground = (0.8 m/s) × 4s = 3.2 m
6. F = mg - R = 0.5 × 10 - 0.5 × 2 = 5 - 1 = 4N
7. m = 10 kg, x = (t
3
- 2t - 10)m  ?   2 t 3 v
dt
dx
2
- = =
and t 6 a
dt
x d
2
2
= = At the end of 4 secs, a = 6 × 4 = 24
m/s
2

F = ma = 10 × 24 = 240 N.
8. The formula for force is given by F
1
= ma.
Acceleration of the particle a =
m
F
1

9. Dynamic friction force is constant
10. (c) a = rw
2
=
4
1
× (2 p ×2)
2
=
4
1
× 4 p
2
= 4 p
2

11. Mg – N =
r
mv
2
? N = mg -
r
mv
2
.
12. Angular speed of hour hand.  ?
1
=
) 60 12 (
2
×
p
.
Angular speed of minute hand ?
2
=
60
2 p
?
1
12
60
60 12
1
2
=
×
=
?
?



13. There is no change in the angular velocity, when
speed is constant.
14. a =
2 3
2 . 10
×
ms
-
1
=
6
2 . 10
= 1.7 ms
-
2
;

T = 2 × 1.7N =
3.4 N
15. (C)
16. (C)
17. (A)
18. Apply law of conservation of momentum.
19. Stored elastic potential energy of spring =
2
1
kx
2

where , x is compression or elongation of spring
from its natural length. In this position the spring
can do work on the block tied to it, which is equal
to
2
1
kx
2
, so both option (a) and (b) are correct.
20. The work done by a variable force is defined as W
= ds . F
?
?
. It may or may not depend on the path
followed. It is always dependent on the initial and
final positions.
21. The work done by friction may be positive,
negative or zero. Imaging a situation in which a
block of mass M is pulled horizontally by a force F
on a rough horizontal surface with coefficient of
friction µ. If F < µ Mg, then work done by friction
force is zero. If F > µ Mg, then work done by
friction force is positive.
22. All the mentioned forces are the examples of
conservation force.
23. W = F s cos 90 ° = zero
24. W = F s cos ? = 10 × 2 cos 60 ° = 10 J
25. a=
2 1
2 1
m m
m m
+
-
g =
8 . 4 5
8 . 4 5
+
-
× 9.8ms
-
2
= 0.2 ms
-
2

26. (d) Assertion is false and reason is true.
Since the car has smaller mass than the truck,
the magnitude of its acceleration will be more
than that of the truck. According to Newton’s
third law, both the vehicle experience the
same magnitude of force and the same
impulse. Therefore, the change in momentum
for the car and the truck is same.
27. (a) Both assertion and reason are true and reason
is the correct explanation.
When a force produces acceleration, it changes
the velocity of the body and hence there is
change in kinetic energy and momentum.
28. (a) Both assertion and reason are true and reason
is the correct explanation of the assertion.
? =
?
F .
?
d = Fd cos90 = 0 ? here
?
F is
towards centre of circle &
?
d is perpendicular
to
?
F .
29. (d) Assertion is false and reason is true.
Kilowatt (1000 watt) is the unit of power.
Kilowatt hour is the power consumed in one
hour = power × time = energy. One watt is
equal to Joule per second. Hence watt × time
has units of energy.
30. (d) Both assertion and reason are true and reason
is the correct explanation of the assertion.
W =
? ?
d . F =Fd cos ? = 0 because d = 0.

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