Selina Textbook Solutions: Motion In One Dimension | Physics Class 9 ICSE PDF Download

 Page 1


Chapter 2. Motion in One Dimension
Exercise 2(A)
Solution 1S.
Scalar Vector
They are expressed only by 
their magnitudes.
They are expressed by 
magnitude as well as 
direction.
They can be added, 
subtracted, multiplied or 
divided by simple arithmetic 
methods.
They can be added, 
subtracted or multiplied 
following a different algebra.
They are symbolically written 
by English letter.
They are symbolically written 
by their English letter with an 
arrow on top of the letter.
Example: mass, speed Example: force, velocity
Solution 2S.
a) Pressure is a scalar quantity. 
b) Momentum is a vector quantity. 
c) Weight is a vector quantity. 
d) Force is a vector quantity. 
e) Energy is a scalar quantity. 
f) Speed is a scalar quantity.
Solution 3S.
A body is said to be at rest if it does not change its position with respect to its immediate 
surroundings.
Solution 4S.
A body is said to be in motion if it changes its position with respect to its immediate 
surroundings.
Solution 5S.
When a body moves along a straight line path, its motion is said to be one-dimensional 
motion.
Solution 6S.
The shortest distance from the initial to the final position of the body is called the 
magnitude of displacement. It is in the direction from the initial position to the final 
Page 2


Chapter 2. Motion in One Dimension
Exercise 2(A)
Solution 1S.
Scalar Vector
They are expressed only by 
their magnitudes.
They are expressed by 
magnitude as well as 
direction.
They can be added, 
subtracted, multiplied or 
divided by simple arithmetic 
methods.
They can be added, 
subtracted or multiplied 
following a different algebra.
They are symbolically written 
by English letter.
They are symbolically written 
by their English letter with an 
arrow on top of the letter.
Example: mass, speed Example: force, velocity
Solution 2S.
a) Pressure is a scalar quantity. 
b) Momentum is a vector quantity. 
c) Weight is a vector quantity. 
d) Force is a vector quantity. 
e) Energy is a scalar quantity. 
f) Speed is a scalar quantity.
Solution 3S.
A body is said to be at rest if it does not change its position with respect to its immediate 
surroundings.
Solution 4S.
A body is said to be in motion if it changes its position with respect to its immediate 
surroundings.
Solution 5S.
When a body moves along a straight line path, its motion is said to be one-dimensional 
motion.
Solution 6S.
The shortest distance from the initial to the final position of the body is called the 
magnitude of displacement. It is in the direction from the initial position to the final 
position. 
Its SI unit is metre (m).
Solution 7S.
Distance is a scalar quantity, while displacement is a vector quantity. The magnitude of 
displacement is either equal to or less than the distance. The distance is the length of 
path travelled by the body so it is always positive, but the displacement is the shortest 
length in direction from initial to the final position so it can be positive or negative 
depending on its direction. The displacement can be zero even if the distance is not zero.
Solution 8S.
Yes, displacement can be zero even if the distance is not zero.
For example, when a body is thrown vertically upwards from a point A on the ground, 
after sometime it comes back to the same point A. Then, the displacement is zero, but 
the distance travelled by the body is not zero (it is 2h; h is the maximum height attained 
by the body).
Solution 9S.
The magnitude of displacement is equal to distance if the motion of the body is one-
dimensional.
Solution 10S.
The velocity of a body is the distance travelled per second by the body in a specified 
direction. 
Its SI unit is metre/second (m/s).
Solution 11S.
The speed of a body is the rate of change of distance with time. 
Its SI unit is metre/second (m/s).
Solution 12S.
Speed is a scalar quantity, while velocity is a vector quantity. The speed is always 
positive-it is the magnitude of velocity, but the velocity is given a positive or negative 
sign depending upon its direction of motion. The average velocity can be zero but the 
average speed is never zero.
Solution 13S.
Velocity gives the direction of motion of the body.
Solution 14S.
Instantaneous velocity is equal to average velocity if the body is in uniform motion.
Solution 15S.
Page 3


Chapter 2. Motion in One Dimension
Exercise 2(A)
Solution 1S.
Scalar Vector
They are expressed only by 
their magnitudes.
They are expressed by 
magnitude as well as 
direction.
They can be added, 
subtracted, multiplied or 
divided by simple arithmetic 
methods.
They can be added, 
subtracted or multiplied 
following a different algebra.
They are symbolically written 
by English letter.
They are symbolically written 
by their English letter with an 
arrow on top of the letter.
Example: mass, speed Example: force, velocity
Solution 2S.
a) Pressure is a scalar quantity. 
b) Momentum is a vector quantity. 
c) Weight is a vector quantity. 
d) Force is a vector quantity. 
e) Energy is a scalar quantity. 
f) Speed is a scalar quantity.
Solution 3S.
A body is said to be at rest if it does not change its position with respect to its immediate 
surroundings.
Solution 4S.
A body is said to be in motion if it changes its position with respect to its immediate 
surroundings.
Solution 5S.
When a body moves along a straight line path, its motion is said to be one-dimensional 
motion.
Solution 6S.
The shortest distance from the initial to the final position of the body is called the 
magnitude of displacement. It is in the direction from the initial position to the final 
position. 
Its SI unit is metre (m).
Solution 7S.
Distance is a scalar quantity, while displacement is a vector quantity. The magnitude of 
displacement is either equal to or less than the distance. The distance is the length of 
path travelled by the body so it is always positive, but the displacement is the shortest 
length in direction from initial to the final position so it can be positive or negative 
depending on its direction. The displacement can be zero even if the distance is not zero.
Solution 8S.
Yes, displacement can be zero even if the distance is not zero.
For example, when a body is thrown vertically upwards from a point A on the ground, 
after sometime it comes back to the same point A. Then, the displacement is zero, but 
the distance travelled by the body is not zero (it is 2h; h is the maximum height attained 
by the body).
Solution 9S.
The magnitude of displacement is equal to distance if the motion of the body is one-
dimensional.
Solution 10S.
The velocity of a body is the distance travelled per second by the body in a specified 
direction. 
Its SI unit is metre/second (m/s).
Solution 11S.
The speed of a body is the rate of change of distance with time. 
Its SI unit is metre/second (m/s).
Solution 12S.
Speed is a scalar quantity, while velocity is a vector quantity. The speed is always 
positive-it is the magnitude of velocity, but the velocity is given a positive or negative 
sign depending upon its direction of motion. The average velocity can be zero but the 
average speed is never zero.
Solution 13S.
Velocity gives the direction of motion of the body.
Solution 14S.
Instantaneous velocity is equal to average velocity if the body is in uniform motion.
Solution 15S.
If a body travels equal distances in equal intervals of time along a particular direction, 
then the body is said to be moving with a uniform velocity. However, if a body travels 
unequal distances in a particular direction in equal intervals of time or it moves equal 
distances in equal intervals of time but its direction of motion does not remain same, 
then the velocity of the body is said to be variable (or non-uniform).
Solution 16S.
Average speed is the ratio of the total distance travelled by the body to the total time of 
journey, it is never zero. If the velocity of a body moving in a particular direction changes 
with time, then the ratio of displacement to the time taken in entire journey is called its 
average velocity. Average velocity of a body can be zero even if its average speed is not 
zero.
Solution 17S.
The motion of a body in a circular path with uniform speed has a variable velocity 
because in the circular path, the direction of motion of the body continuously changes 
with time. 
Solution 18S.
If a body starts its motion from a point and comes back to the same point after a certain 
time, then the displacement is zero, average velocity is also zero, but the total distance 
travelled is not zero, and therefore, the average speed in not zero.
Solution 19S.
Acceleration is the rate of change of velocity with time. 
Its SI unit is metre/second
2
 (m/s
2
).
Solution 20S.
Acceleration is the increase in velocity per second, while retardation is the decrease in 
velocity per second. Thus, retardation is negative acceleration. In general, acceleration is 
Page 4


Chapter 2. Motion in One Dimension
Exercise 2(A)
Solution 1S.
Scalar Vector
They are expressed only by 
their magnitudes.
They are expressed by 
magnitude as well as 
direction.
They can be added, 
subtracted, multiplied or 
divided by simple arithmetic 
methods.
They can be added, 
subtracted or multiplied 
following a different algebra.
They are symbolically written 
by English letter.
They are symbolically written 
by their English letter with an 
arrow on top of the letter.
Example: mass, speed Example: force, velocity
Solution 2S.
a) Pressure is a scalar quantity. 
b) Momentum is a vector quantity. 
c) Weight is a vector quantity. 
d) Force is a vector quantity. 
e) Energy is a scalar quantity. 
f) Speed is a scalar quantity.
Solution 3S.
A body is said to be at rest if it does not change its position with respect to its immediate 
surroundings.
Solution 4S.
A body is said to be in motion if it changes its position with respect to its immediate 
surroundings.
Solution 5S.
When a body moves along a straight line path, its motion is said to be one-dimensional 
motion.
Solution 6S.
The shortest distance from the initial to the final position of the body is called the 
magnitude of displacement. It is in the direction from the initial position to the final 
position. 
Its SI unit is metre (m).
Solution 7S.
Distance is a scalar quantity, while displacement is a vector quantity. The magnitude of 
displacement is either equal to or less than the distance. The distance is the length of 
path travelled by the body so it is always positive, but the displacement is the shortest 
length in direction from initial to the final position so it can be positive or negative 
depending on its direction. The displacement can be zero even if the distance is not zero.
Solution 8S.
Yes, displacement can be zero even if the distance is not zero.
For example, when a body is thrown vertically upwards from a point A on the ground, 
after sometime it comes back to the same point A. Then, the displacement is zero, but 
the distance travelled by the body is not zero (it is 2h; h is the maximum height attained 
by the body).
Solution 9S.
The magnitude of displacement is equal to distance if the motion of the body is one-
dimensional.
Solution 10S.
The velocity of a body is the distance travelled per second by the body in a specified 
direction. 
Its SI unit is metre/second (m/s).
Solution 11S.
The speed of a body is the rate of change of distance with time. 
Its SI unit is metre/second (m/s).
Solution 12S.
Speed is a scalar quantity, while velocity is a vector quantity. The speed is always 
positive-it is the magnitude of velocity, but the velocity is given a positive or negative 
sign depending upon its direction of motion. The average velocity can be zero but the 
average speed is never zero.
Solution 13S.
Velocity gives the direction of motion of the body.
Solution 14S.
Instantaneous velocity is equal to average velocity if the body is in uniform motion.
Solution 15S.
If a body travels equal distances in equal intervals of time along a particular direction, 
then the body is said to be moving with a uniform velocity. However, if a body travels 
unequal distances in a particular direction in equal intervals of time or it moves equal 
distances in equal intervals of time but its direction of motion does not remain same, 
then the velocity of the body is said to be variable (or non-uniform).
Solution 16S.
Average speed is the ratio of the total distance travelled by the body to the total time of 
journey, it is never zero. If the velocity of a body moving in a particular direction changes 
with time, then the ratio of displacement to the time taken in entire journey is called its 
average velocity. Average velocity of a body can be zero even if its average speed is not 
zero.
Solution 17S.
The motion of a body in a circular path with uniform speed has a variable velocity 
because in the circular path, the direction of motion of the body continuously changes 
with time. 
Solution 18S.
If a body starts its motion from a point and comes back to the same point after a certain 
time, then the displacement is zero, average velocity is also zero, but the total distance 
travelled is not zero, and therefore, the average speed in not zero.
Solution 19S.
Acceleration is the rate of change of velocity with time. 
Its SI unit is metre/second
2
 (m/s
2
).
Solution 20S.
Acceleration is the increase in velocity per second, while retardation is the decrease in 
velocity per second. Thus, retardation is negative acceleration. In general, acceleration is 
taken positive, while retardation is taken negative.
Solution 21S.
The acceleration is said to be uniform when equal changes in velocity take place in equal 
intervals of time, but if the change in velocity is not the same in the same intervals of 
time, the acceleration is said to be variable.
Solution 22S.
Retardation is the decrease in velocity per second. 
Its SI unit is metre/second
2
 (m/s
2
).
Solution 23S.
Velocity determines the direction of motion.
Solution 24S.
(a) Example of uniform velocity: A body, once started, moves on a frictionless surface 
with uniform velocity. 
(b) Example of variable velocity: A ball dropped from some height is an example of 
variable velocity. 
(c) Example of variable acceleration: The motion of a vehicle on a crowded road is with 
variable acceleration. 
(d) Example of uniform retardation: If a car moving with a velocity ‘v’ is brought to rest 
by applying brakes, then such a motion is an example of uniform retardation.
Solution 25S.
Initially as the drops are equidistant, we can say that the car is moving with a constant 
speed but later as the distance between the drops starts decreasing, we can say that the 
car slows down.
Solution 26S.
When a body falls freely under gravity, the acceleration produced in the body due to the 
Earth’s gravitational acceleration is called the acceleration due to gravity (g). The average 
value of g is 9.8 m/s
2
.
Solution 27S.
No. The value of ‘g’ varies from place to place. It is maximum at poles and minimum at 
the Equator on the surface of the Earth.
Solution 28S.
In vacuum, both will reach the ground simultaneously because acceleration due to gravity 
is same (=g) on both objects.
Solution 1M.
Page 5


Chapter 2. Motion in One Dimension
Exercise 2(A)
Solution 1S.
Scalar Vector
They are expressed only by 
their magnitudes.
They are expressed by 
magnitude as well as 
direction.
They can be added, 
subtracted, multiplied or 
divided by simple arithmetic 
methods.
They can be added, 
subtracted or multiplied 
following a different algebra.
They are symbolically written 
by English letter.
They are symbolically written 
by their English letter with an 
arrow on top of the letter.
Example: mass, speed Example: force, velocity
Solution 2S.
a) Pressure is a scalar quantity. 
b) Momentum is a vector quantity. 
c) Weight is a vector quantity. 
d) Force is a vector quantity. 
e) Energy is a scalar quantity. 
f) Speed is a scalar quantity.
Solution 3S.
A body is said to be at rest if it does not change its position with respect to its immediate 
surroundings.
Solution 4S.
A body is said to be in motion if it changes its position with respect to its immediate 
surroundings.
Solution 5S.
When a body moves along a straight line path, its motion is said to be one-dimensional 
motion.
Solution 6S.
The shortest distance from the initial to the final position of the body is called the 
magnitude of displacement. It is in the direction from the initial position to the final 
position. 
Its SI unit is metre (m).
Solution 7S.
Distance is a scalar quantity, while displacement is a vector quantity. The magnitude of 
displacement is either equal to or less than the distance. The distance is the length of 
path travelled by the body so it is always positive, but the displacement is the shortest 
length in direction from initial to the final position so it can be positive or negative 
depending on its direction. The displacement can be zero even if the distance is not zero.
Solution 8S.
Yes, displacement can be zero even if the distance is not zero.
For example, when a body is thrown vertically upwards from a point A on the ground, 
after sometime it comes back to the same point A. Then, the displacement is zero, but 
the distance travelled by the body is not zero (it is 2h; h is the maximum height attained 
by the body).
Solution 9S.
The magnitude of displacement is equal to distance if the motion of the body is one-
dimensional.
Solution 10S.
The velocity of a body is the distance travelled per second by the body in a specified 
direction. 
Its SI unit is metre/second (m/s).
Solution 11S.
The speed of a body is the rate of change of distance with time. 
Its SI unit is metre/second (m/s).
Solution 12S.
Speed is a scalar quantity, while velocity is a vector quantity. The speed is always 
positive-it is the magnitude of velocity, but the velocity is given a positive or negative 
sign depending upon its direction of motion. The average velocity can be zero but the 
average speed is never zero.
Solution 13S.
Velocity gives the direction of motion of the body.
Solution 14S.
Instantaneous velocity is equal to average velocity if the body is in uniform motion.
Solution 15S.
If a body travels equal distances in equal intervals of time along a particular direction, 
then the body is said to be moving with a uniform velocity. However, if a body travels 
unequal distances in a particular direction in equal intervals of time or it moves equal 
distances in equal intervals of time but its direction of motion does not remain same, 
then the velocity of the body is said to be variable (or non-uniform).
Solution 16S.
Average speed is the ratio of the total distance travelled by the body to the total time of 
journey, it is never zero. If the velocity of a body moving in a particular direction changes 
with time, then the ratio of displacement to the time taken in entire journey is called its 
average velocity. Average velocity of a body can be zero even if its average speed is not 
zero.
Solution 17S.
The motion of a body in a circular path with uniform speed has a variable velocity 
because in the circular path, the direction of motion of the body continuously changes 
with time. 
Solution 18S.
If a body starts its motion from a point and comes back to the same point after a certain 
time, then the displacement is zero, average velocity is also zero, but the total distance 
travelled is not zero, and therefore, the average speed in not zero.
Solution 19S.
Acceleration is the rate of change of velocity with time. 
Its SI unit is metre/second
2
 (m/s
2
).
Solution 20S.
Acceleration is the increase in velocity per second, while retardation is the decrease in 
velocity per second. Thus, retardation is negative acceleration. In general, acceleration is 
taken positive, while retardation is taken negative.
Solution 21S.
The acceleration is said to be uniform when equal changes in velocity take place in equal 
intervals of time, but if the change in velocity is not the same in the same intervals of 
time, the acceleration is said to be variable.
Solution 22S.
Retardation is the decrease in velocity per second. 
Its SI unit is metre/second
2
 (m/s
2
).
Solution 23S.
Velocity determines the direction of motion.
Solution 24S.
(a) Example of uniform velocity: A body, once started, moves on a frictionless surface 
with uniform velocity. 
(b) Example of variable velocity: A ball dropped from some height is an example of 
variable velocity. 
(c) Example of variable acceleration: The motion of a vehicle on a crowded road is with 
variable acceleration. 
(d) Example of uniform retardation: If a car moving with a velocity ‘v’ is brought to rest 
by applying brakes, then such a motion is an example of uniform retardation.
Solution 25S.
Initially as the drops are equidistant, we can say that the car is moving with a constant 
speed but later as the distance between the drops starts decreasing, we can say that the 
car slows down.
Solution 26S.
When a body falls freely under gravity, the acceleration produced in the body due to the 
Earth’s gravitational acceleration is called the acceleration due to gravity (g). The average 
value of g is 9.8 m/s
2
.
Solution 27S.
No. The value of ‘g’ varies from place to place. It is maximum at poles and minimum at 
the Equator on the surface of the Earth.
Solution 28S.
In vacuum, both will reach the ground simultaneously because acceleration due to gravity 
is same (=g) on both objects.
Solution 1M.
Velocity is a vector quantity. The others are all scalar quantities.
Solution 2M.
m s
-1
Solution 3M.
m s
-2
Solution 4M.
The displacement is zero.
Solution 5M.
5 m s
-1
Solution 1N.
Solution 2N.
Solution 3N.
Solution 4N.