Class 9 Motion (Science)
Ncert Cbse Solutions for Science
Extra Model Questions  Sample Questions
(Important for Examinations)
Q.1: Describe the terms ‘Rest’ and ‘Motion’.
Ans:
Rest  A body is said to be at rest if it does not change its position
with respect to its surroundings. For example, a table lying in a room is at
rest with respect to the walls of the room.
Motion A body is said
to be in motion if it changes its position with respect to its surroundings.
For example, a car running on a road is in motion with respect to trees on
roadside.
Q.2: Describe the various types of motions observed in bodies.
Ans:
1. Translatory
motion  When a body moves as a whole along a straight or curved path, it
is said to be in translatory motion. Translatory
motion is of two types:
(a) Rectilinear
motion: Here a body moves as a whole along a straight path. For example, a
train moving on straight rails has translatory rectilinear motion.
(b) Curvilinear
motion: Here a body moves as a whole along a curved path. For example, a
bicycle taking a turn along a curved path.
2. Rotatory / Rotational motion  When
a body rotates about a fixed point or axis, it has a rotatory motion. For example, motion of flywheel about a shaft.
3. Vibratory
or Oscillatory motion  When a body moves to and fro about a mean position
again and again, it has vibratory or
oscillatory motion. For example, the motion of the pendulum of a
wallclock.
4. Complex
motion  Sometimes, the motion of a body may be a combination of more than
one type of motion. For example, a ball rolling down an inclined plane has both
translatory and rotatory motions.
Q.3: What are scalar quantities? Give examples.
Ans:
The physical quantities which require only magnitude and not the direction for
their complete description are called scalars
or scalar quantities. Distance,
speed, time, area, mass, volume, density, work, energy etc are all scalar
quantities.
Q.4: What are vector quantities? Give examples.
Ans:
The physical quantities which need both magnitude and direction for their
complete description are called vectors or
vector quantities. Displacement,
velocity, force, acceleration, momentum, weight etc. are all vector quantities.
Q.5: What are the differences between the terms ‘distance’ and
‘displacement’?
Ans:
The following table shows some differences between Distance and Displacement

Distance

Displacement

1.
Distance is the length of actual path travelled by a body, irrespective of
its direction of motion.
2.
Distance between two given points may be the same or different for different
paths chosen.
3. It
is a scalar quantity.
4. Distance
covered is always positive or zero.

1. Displacement
is the shortest distance between the initial and final positions of a body in
a given direction.
2.
Displacement between two given points is always same.
3. It
is a vector quantity.
4. Displacement
covered may be positive, negative or zero.

Q.6: An object has moved through a distance, can it have zero
displacement? If yes, support your answer with an example.
Ans:
Yes, an object which has moved through a distance can have zero displacement.
Suppose a person throws a ball upwards through a height h and catches back the ball. Then,
distance
covered by the ball = h + h = 2h.
displacement
of the ball = 0.
Q.7: A farmer moves along the boundary of a square field of side 10
m in 40 s. what will be the magnitude of displacement of the farmer at the end
of 2 minutes 20 seconds?
Ans:
If
the farmer starts from point A, then at the end of 2 minutes and 20 seconds
i.e. total 140 seconds, he will reach the diagonally opposite corner C. The
magnitude of displacement of the farmer is 
AC = √(AB^{2} + BC^{2}) =
√(10^{2} + 10^{2}) = 14.14 m Ans.
Q.8: Which of the following is true for displacement?
(a) It can not be zero.
(b) Its magnitude is greater than the distance travelled by the
object.
Ans:
Nether of the above are true.
Q.9: What is meant by ‘Uniform Motion’? Give example.
Ans:
If an object covers equal distances in equal intervals of time, however small
the time interval may be, then the motion of the object is said to be uniform motion. For example, say a car
covers 10 km in first 15 min, 10 km in second 15 min, 10 km in third 15 min and
so on. Then one can say that the car is in uniform motion.
Q.10: What is nonuniform motion? Give example.
Ans:
If an object covers unequal distances in equal intervals of time, then the
object is said to be in nonuniform
motion. Most of the motions seen in our daily life are nonuniform. For
example, if we drop a ball from the roof of a building, we will note that the
ball covers 4.9 m in the first second, 14.7 m in the next second, 24.5 m in the
third second and so on. That is the ball is covering increasingly larger
distances in successive seconds as it falls down. Hence the motion of a freely
falling ball or object is nonuniform.
Q.11: Define the term ‘Speed’. What are its various units?
Ans:
Speed is defined as the distance
travelled by a body per unit time. Thus,
Speed = Distance travelled ÷ Time taken
The
SI unit of speed is m s^{1}, a smaller unit of speed is cm s^{1}
and a larger unit is km h^{1}.
Q.12: Define the terms ‘Uniform Speed’ and ‘Nonuniform Speed’.
Ans:
Uniform speed  when an object covers equal distances in equal intervals
of time, however the small these time intervals may be, it is said to be in ‘uniform speed’. For example, if somebody
is driving a car at a uniform speed of 40 km/h, then the car will go 20 km
every halfhour, 10 km for every quarter of an hour and . . . . 11 m for every
second.
Nonuniform
speed
 when an object covers unequal distances in equal intervals of time then it is
said to be in ‘nonuniform speed’.
For example, when we start a motorbike, we press its accelerator to increase
its speed and at many times we apply brakes to slow down the bike. In such
situations, the speed of the bike is nonuniform.
Q.13: What is ‘average speed’?
Ans:
When the speed of a body varies with time, we need to define its average speed. ‘Average speed’ is the
total distance travelled by a body, divided by the total time taken to cover
the distance.
For
example, if a car travels a distance of 20 km in 2 hours, then its 
Average
speed = 100 km ÷ 2 hour = 50 km/h
Q.14:
Are rest and motion absolute or relative terms?
Ans: Both these
are relative terms.
Q.15: Can an object be at rest as well as in motion at the same
time?
Ans:
Yes, an object can be at rest as well as in motion at the same time. Because an
object may at rest relative one object and at the same time it may be in motion
relative to another object.
Q.16: Can the displacement be greater than the distance travelled
by an object?
Ans:
No, the displacement of an object can be either equal to or less than the
distance travelled by an object.
Q.17: Define the term ‘Velocity’. What is its SI unit?
Ans:
Velocity is a physical quantity that gives both the speed and direction of
motion of the body. Velocity of a body is defined as the displacement produced
per unit time. It is also defined as the speed of a body in a given direction.
Velocity
= Displacement ÷ Time
The
SI unit of velocity is ms^{1}.
Q.18:
Distinguish between the terms ‘Speed’ and ‘Velocity’.
Ans:
Speed

Velocity

1. It
is the distance traversed by a body per unit time in any direction.
2. It
is a scalar quantity.
3. It
is always positive or zero but never negative.

1. It
is the distance traversed by a body per unit time in a given direction.
2. A
vector quantity.
3. It
may be positive or negative or zero.

Q.19: Under what conditions is the magnitude of average velocity of
an object equal to its average speed?
Ans:
When an object moves along a straight line in a given direction, its total
distance covered is equal to the magnitude of displacement. Hence only under
this condition, its average speed is equal to the magnitude of the average
velocity.
Q.20: What does the path of an object look like when it is in
uniform motion?
Ans:
Straight line path.
Q.21: When is the acceleration of a body positive?
Ans:
When the velocity of a body increases with time, its acceleration is positive.
For example, acceleration of a bus as it leaves the bus stop.
Q.22: Give two examples of uniformly accelerated motions.
Ans:
Examples of uniformly accelerated motions:
(i) An object
moving with a uniform speed along a circular path, has uniform acceleration
because the velocity of the object changes continuously due to the change in
its direction at every point.
(ii) The motion
of a ball rolling down an inclined plane is uniformly accelerated.
Q.23: A bus decreases its speed from 80 km/h to 60km/h in 5 s. Find
the acceleration of the bus.
Ans:
u = 80 km/h = {(80 x 1000) ÷ 3600} m/s =
800/36 ms^{1}
v = 60 km/h = 600/36 ms^{1}
Acceleration,
a = (v  u) / t = (600/36 
800/36) ÷ 5 = 1.11 ms^{2} Ans.
Q.24:
A train starting from the railway station and moving with a uniform
acceleration attains a speed of 40 km/h in 10 minutes. Find its acceleration.
Ans: Initial
speed, u = 0
Final speed, v = 40 km/h = {(40 x 1000) / 3600} ms^{1}
= 100/9 ms^{1}
Time, t = 10 min = 600 s
Acceleration, a = (v
 u) / t = (100/9  0) ÷ 600 =
1/54 ms^{2} Ans.
Q.25: What is distancetime graph of a body? Mention the uses of
distancetime graph.
Ans:
It is a graph obtained by plotting distance travelled along Yaxis and time
along Xaxis. The uses of distance time graph are as follows:
(a)
It tells the position of the body at any instant of time.
(b)
The distance covered by the body during a particular time interval can be seen
from this graph.
(c)
The velocity of the body at any instant of time can be determined.
Q.26: Derive the equations of motion for uniformly accelerated
motion from velocitytime graph.
Ans:
Equations of Motion by Graphical Method
Consider
an object moving along a straight line with an initial velocity u and uniform acceleration a. Suppose, it travels distance s in time t. As shown in figure its velocitytime graph is a straight line.
Here
OA = ED = u, OC = EB = v, OE = AD = t.
1. Equation for velocitytime relation:
We
know that, Acceleration = Change in velocity ÷ Time
or,
a = BD ÷ OE = (BE  ED) ÷ OE
or,
a = (v  u) ÷ t
or,
v  u = at
or,
v = u + at
This
proves the first equation of motion.
2. Equation for positiontime relation:
Distance travelled by an object in time t is s,
or, s = Area of the trapezium OABE = Area of
OADE + Area of ADB
or, s = (OA x OE) + (1/2 x DB x AD)    
   Eq. (1)
now, DB = BE 
DE = v  u = at
putting this
value for DB in Eq.(1), we get
s = ut
+ ½at^{2}
This
proves the second equation of motion.
3. Equation for positionvelocity relation:
The distance
travelled by an object in time t is
s = Area of the trapezium OABE = ½ (EB +
OA) x OE = ½ (EB + ED) x OE
or, substituting
EB, ED and OE with v, u and t we get,
s = ½ (v + u) t       Eq. (2)
But from the
first equation of motion we know that
v = u
+ at
or, t = (v
 u) / a
Substituting t
in Eq. 2 with this value we get,
s = (v
+ u) (v  u) ÷ 2a = (v^{2}
 u^{2}) ÷ 2a
or, v^{2}  u^{2} = 2as
This proves the
third equation of motion.
Q.27: Under what condition will the distance and displacement of a
moving object have the same magnitude?
Ans: When the
object moves along the same straight line in the same fixed direction.
Q.28: Can the average speed of a moving body ever be zero?
Ans:
No, speed being a scalar quantity, is always positive. So the average speed of
any moving body can never be zero.
Q.29: Why is the motion of a body in a circular path at a constant
speed called accelerated motion?
Ans:
The motion is called accelerated motion because the velocity of the rotating
body changes continuously due to the change in its direction at every point of
its motion.
Click below to read
 Motion  NCERT (CBSE) Class IX Science (Physics)  Multiple Choice Questions (MCQ)
 Class 9 CBSE Science (Physics) Guide  Chapter 8, Motion  NCERT Solutions
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