# Motion in One Dimension Concise ICSE Class-9 Selina Publishers

## Revised Concise Selina Publishers Physsics Chapter-2 Motion in One Dimentions

**Motion in One Dimension Concise** ICSE Class-9 Selina Publishers Chapter-2 .Step By Step ICSE Selina Concise Solutions of Chapter-2 **Motion in One Dimension** with Exercise-2(A), Exercise-2(B) and Exercise-2(C) including Numerical and MCQ Questions Solved **. **Visit official Website CISCE for detail information about ICSE Board Class-9.

Board | ICSE |

Publications | Selina Publication |

Subject | Physics |

Class | 9th |

Chapter-2 | Motion in One Dimension Exe-2(A) |

Book Name | Concise |

Topics | Solution of Exe-2(A), MCQ 2(A), Numericals 2(A), Exercise-2(B), MCQ-2(B), Numericals-2(B), Exercise-2(C), MCQ-2(C) , Numericals-2(C), |

Academic Session | 2021-2022 |

## Motion in One Dimension Concise ICSE Class-9 Selina Publishers

**Select Topics **

** Exercise-2(A),**** ****MCQ-2(A), **** ****Numericals-2(A),**

** Exercise-2(B), **** MCQ-2(B)****, ****Numericals-2(B),**

** Exercise-2(C), ****MCQ-2(C)**** , ****Numericals-2(C),**

Note :- Before Viewing **Selina** Concise Physics Solutions of Chapter-2 **Motion in One Dimension **Class-9 . Read the whole chapter carefully and Solved all example of Exercise-2 **Motion in One Dimension**Class-9**. **

Focus on Chapter-2 **Motion in One Dimension** in Scalar and vector quantities, distance, speed, velocity acceleration. **Motion in One Dimension** in graph of distance – time and speed- time. Equations of uniformly accelerated motion with derivations in **Motion in One Dimension **

**Exercise-2(A) Motion in One Dimension Selina Solutions ICSE Class-9 Physics Chapter-2**

**Page 35**

**Question 1**

Differentiate between the scalar and vector quantities, giving two examples of each.

**Answer** **1**

** Differentiate between the scalar and vector quantities**

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 |

** **

**Question 2( Motion in One Dimension)**

State whether the following quantity is a scalar or vector?

a) pressure | d) force |

b) momentum | e) energy |

c) weight | f) speed |

** **

**Answer 2**

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.

**Question 3**

When is a body said to be at rest?

**Answer 3**

A body is said to be at rest if it does not change its position with respect to its immediate surroundings.

**Question 4**

When is a body said to be in motion?

**Answer 4**

A body is said to be in motion if it changes its position with respect to its immediate surroundings.

**Question 5**

What do you mean by motion in one direction?

**Answer 5**

When a body moves along a straight line path, its motion is said to be one-dimensional motion.

**Question 6 **

Define displacement. State its unit.

**Answer 6**

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)

**Question 7**

Differentiate between distance and displacement.

**Answer 7**

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 traveled 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.

**Question 8 **

Can displacement be zero even if the distance is not zero? Give one example to explain your answer.

**Answer 8 **

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).

**Question 9**

When is the magnitude of displacement equal to the distance?

**Answer 9**

The magnitude of displacement is equal to distance if the motion of the body is one-dimensional.

**Question 10**

Define velocity. State its unit.

**Answer 10**

The velocity of a body is the distance traveled per second by the body in a specified direction.

Its SI unit is metre/second (m/s).

**Question 11**

Define speed. What is its S.I. unit?

**Answer 11**

The speed of a body is the rate of change of distance with time.

Its SI unit is metre/second (m/s).

**Question 12**

Distinguish between speed and velocity.

**Answer 12**

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.

**Question 13**

Which quantity-speed or velocity-gives the direction of motion of a body?

**Answer 13**

Velocity gives the direction of motion of the body.

**Question 14**

When is instantaneous speed the same as the average speed?

**Answer 14**

Instantaneous velocity is equal to average velocity if the body is in uniform motion

**Question 15**

Distinguish between uniform velocity and variable velocity.

**Answer 15**

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).

**Question 16**

Distinguish between average speed and average velocity.

**Answer 16**

**Distinguish between average speed and average velocity**

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.

**Question 17**

Give an example of the motion of a body moving with a constant speed but with a variable velocity. Draw a diagram to represent such a motion.

**Answer 17**

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.

**Question 18 Motion in One Dimension Concise**

Give an example of motion in which the average speed is not zero but the average velocity is zero.

**Answer 18**

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 traveled is not zero, and therefore, the average speed in not zero.

**Question 19**

Define acceleration. State its unit.

**Answer 19**

Acceleration is the rate of change of velocity with time.

Its SI unit is metre/second^{2} (m/s^{2}).

**Question 20**

Distinguish between acceleration and retardation.

**Answer 20**

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.

**Question 21**

Differentiate between uniform acceleration and variable acceleration.

**Answer 21**

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.

**Question 22**

What is meant by the term retardation? Name its S.I. unit.

**Answer 22**

Retardation is the decrease in velocity per second.

Its SI unit is metre/second^{2} (m/s^{2}).

**Question 23**

Which of the quantity-velocity or acceleration-determines the direction of motion?

**Answer 23**

Velocity determines the direction of motion.

**Question 24**

Give one example of each type of following motion:

(a) Uniform velocity (b) Variable velocity

(c) Variable acceleration (d) Uniform retardation.

**Answer 24 Motion in One Dimension**

(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.

**Question 25 Motion in One Dimension Concise**

The diagram (Fig. 2.6) below shows the pattern of the oil dripping on the road at a constant rate from a moving car. What information do you get from it about the motion of the car.

**Answer 25**

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.

**Question 26**

Define the term acceleration due to gravity. State its average value.

**Answer 26**

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}.

**Question 27**

‘The value of g remains the same at all places on the Earth’s surface’. Is this statement true? Give reason for your answer.

**Answer 27**

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.

**Question 28 Motion in One Dimension**

If a stone and a pencil are dropped simultaneously in vacuum from the top of a tower, then which of the two will reach the ground first? Give reason.

**Answer 28**

In vacuum, both will reach the ground simultaneously because acceleration due to gravity is same (=g) on both objects.

**MULTIPLE CHOICE TYPE-2(A) Chapter-2 Motion in One Dimension Selina Physics Solutions Class-9 **

**Page 35**

**Question 1**

The vector quantity is :

(a) Work

(b) Pressure

(c) Distance

(d) velocity

**Answer 1**

Velocity is a vector quantity. The others are all scalar quantities.

**Question 2**

The S.I. unit of velocity is

(a) km h^{-1}

(b) m min^{-1}

(c) km rnin^{-1}

(d) m s^{-1}

**Answer 2**

m s^{-1}

**Question 3**

The unit of retardation is

(a) m s^{-1}

(b) m s^{-2}

(c) m

(d) m s^{2}

**Answer 3**

m s^{-2}

**Question 4 Motion in One Dimension**

A body when projected up with an initial velocity u goes to a height h in time t and then comes back at the point of projection. The correct statement is

(a) The average velocity is 2 h/t.

(b) The acceleration is zero.

(c) The final velocity on reaching the point of projection is 2 u.

(d) The displacement is zero.

**Answer 4**

(d) The displacement is zero.

**Question 5**

18 km h^{-1} is equal to

(a) 10 m s^{-1}

(b) 5 m s^{-1}

(c) 18 m s^{-1}

(d) 1.8 m s^{-1}

**Answer 5**

5 m s^{-1}

**NUMERICAL-2(A) of Chapter-2 Motion in One Dimension Selina Physics Solutions for ICSE Class-9 **

**Page 36**

**Question 1**

The speed of a car is 72 km h^{-1}. Express it in m s^{-1}.

**Answer 1**

Speed of car = 72 km h^{-1}

Speed of car in ms^{-1}

^{=72×100/3600}

^{ = 20m/s}

**Question 2**

Express 15 m s^{-1} in km h^{-1}.

**Answer 2**

**15m/s =from 15m/s multiply by 18/5 to make km/h**

**Ans 54 km/h**

** **

**Question 3**

Express each of the following in m s^{-1}.

(a) 1 km h^{-1}

(b) 18 km min^{-1}

**Answer 3**

**Question 4 Motion in One Dimension Concise**

Arrange the following speeds in increasing order.

10 m s^{-1}, 1 km min^{-1} and 18 km h^{-1}.

[Hint: 1 km min^{-1} = 16.65 m s^{-1}, 18 km h^{-1}= 5 m s^{-1}]

**Answer 4**

18 km h^{-1} < 10 m s^{-1} < 1 km min^{-1}

**Question 5**

A train takes 3 hours to travel from Agra to Delhi with a uniform speed of 65 km h^{-1}. Find the distance between the two cities.

**Answer 5**

Total time taken = 3 hours

Speed of the train = 65 km/hr

Distance travelled = speed x time

= 65 x 3 = 195 km

**Question 6**

A car travels the first 30 km with a uniform speed of 60 km h^{-1} and the next 30 km with a uniform speed of 40 km h^{-1}. Calculate: (i) The total time of journey, (ii)The average speed of the car.

**Answer 6**

For the first 30 km travelled, speed = 60 km/h.

Thus time taken (t1) = Distance / speed

= (30/60) h^{-1}

= 0.5 h^{-1} or 30 min.

For the next 30 km travelled, speed = 40 km/h

Thus time taken (t2) = Distance/speed

= (30/40) h^{-1}

= 0.75 h^{-1} or 45 min.

(i) Total time = (30 + 45) min

= 75 min or 1.25 h.

(ii) Average speed of the car = Total distance travelled/total time taken

=60km/1.25h

=48km/h

**Question 7**

A train takes 2 h to reach station B from station A, and then 3 h to return from station B to station A. The distance between the two stations is 200 km. Find: (i) The average speed, (ii) The average velocity of the train.

**Answer 7 Motion in One Dimension Concise**

Here, total distance = (200 + 200) km = 400 km

Total time taken = (2 + 3) h = 5 h

(i) Average speed = Total distance travelled/total time taken

=400km/5h

=80km/h

(ii) Average velocity of the train is zero because the train stops at the same point from where it starts, i.e. the displacement is zero.

**Question 8**

A car moving on a straight path covers a distance of 1 km due east in 100 s. What is (i) the speed and (ii) velocity of the car?

**Answer 8**

(i) Speed of the car = Distance/time taken

=1km/100s

=1000m/100s

hence =10m/s

(ii) Velocity of car = Speed with direction

= 10 m/s due east

**Question 9 Motion in One Dimension Concise**

A body starts from rest and acquires a velocity 10m s^{-1} in 2 s. Find the acceleration.

**Answer 9**

Here, final velocity = 10 m/s

Initial velocity = 0 m/s

Time taken = 2s

Acceleration = (Final Velocity – Initial Velocity)/time

= (10/2) ms^{–}^{2}

= 5 ms^{-2}

**Question 10**

A car starting from rest acquires a velocity 180m s^{-1} in 0.05 h. Find the acceleration.

**Answer 10**

Here, final velocity = 180 m/s

Initial velocity = 0 m/s

Time taken = 0.05 h or 180 s

Acceleration = (Final Velocity – Initial Velocity)/time

= (180-0)/180 m s^{-2}

= 1 m s^{–}^{2}

**Question 11**

A body is moving vertically upwards. Its velocity changes at a constant rate from 50 m s^{-1} to 20 m s^{-1} in 3 s. What is its acceleration?

**Answer 11**

Here, final velocity = 20 m/s

Initial velocity = 50 m/s

Time taken = 3 s

Acceleration = (Final Velocity – Initial Velocity)/time

= (20 – 50)/3 m/s^{-2}

= -10 m/s

Negative sign here indicates that the velocity decreases with time, so retardation is 10 m/s.

**Question 12**

A toy car initially moving with uniform velocity of 18 km h^{-1} comes to a stop in 2 s. Find the retardation of the car in S.I. units.

**Answer 12 **

Here, final velocity = 18 km/h or 5 m/s

Initial velocity = 0 km/h

Time taken = 2 s

Acceleration = (Final Velocity – Initial Velocity)/time

= (5 – 0) / 2 m s^{-2}

= 2.5 m s^{-2}

**Question 13**

A car accelerates at a rate of 5 m s^{-2}. Find the increase in its velocity in 2 s.

**Answer 13**

Acceleration = Increase in velocity/time taken

Therefore, increase in velocity = Acceleration × time taken

= (5 × 2) m/s

= 10 m/s

**Question 14**

A car is moving with a velocity 20 m s^{-1}. The brakes are applied to retard it at a rate of 2 m s^{-2}. What will be the velocity after 5 s of applying the brakes?

**Answer 14**

Initial velocity of the car, u = 20 m/s

Retardation = 2 m/s^{2}

Given time, t = 5 s

Let ‘v’ be the final velocity.

We know that, Acceleration = Rate of change of velocity /time

= (Final velocity – Initial velocity)/time

Or, -2 = (v – 20) / 5

Or, -10 = v – 20

Or, v = -20 + 10 m/s

Or, v = -10 m/s

Negative sign indicates that the velocity is decreasing.

**Question 15 (Motion in One Dimension Concise)**

A bicycle initially moving with a velocity 5.0 m s^{-1} accelerates for 5 s at a rate of 2 m s^{-2}. What will be its final velocity?

**Answer 15**

Initial velocity of the bicycle, u = 5 m/s

Acceleration = 2 m/s^{2}

Given time, t = 5 s

Let ‘v’ be the final velocity.

We know that, acceleration = Rate of change of velocity/time

= (Final velocity – Initial velocity)/time

Or 2 = (v – 5)/5

and , 10 = (v – 5)

so, v = 5 + 10

therefore, v = 15 m/s

**Question 16 Motion in One Dimension Concise**

A car is moving in a straight line with speed 18 km h^{-1}. It is stopped in 5 s by applying the brakes. Find (i) the speed of car in m s^{-1}, (ii) the retardation and (iii) the speed of car after 2 s of applying the brakes.

**Answer 16**

Initial velocity of the bicycle, u = 18 km/hr

Time taken, t = 5 s^{-1}

Final velocity, v = 0 m/s (As the car comes to rest)

(i) Speed in m/s

=18×1000/1×3600

=5m/s

(ii) Retardation = (Final velocity – Initial velocity)/time taken

Or, Retardation

= (0-5)/5

=1m/s²

(iii) Let ‘V’ be the speed of the car after 2 s of applying the brakes.

Then, Acceleration = (V – 5)/ 2

Or, -1 = (V – 5)/2

Or, V = -2 + 5

Or, V = 3 m/s

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