# Important Questions Class 9 Science Chapter 8 Motion

Please refer to Motion Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams

## Motion Class 9 Science Notes and Questions

The below Class 9 Motion notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook. Refer to Chapter 8 Motion Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.

• Motion occurs when an object changes its position with time.
• When a body covers equal distance in equal interval of time, it is moving with uniform motion.
• When a body covers unequal distance in equal interval of time. it is moving with non-uniform motion.

Both Distance and Time are important in describing motion.

• Sometimes you know motion has occurred even if you didn’t see it happen.
• Relative motion: when two objects are moving in a plane (either in same direction or opposite) each have relative motion with respect to second. e.g. a person sitting in a train and watching a tree, in this case tree is stable but is assumed to be moving but with respect to train.

Distance vs. Displacement

• Distance: How far an object has moved. It has only magnitude without direction.
• Displacement: How far and in what direction an object has moved from its start position. i.e. the direct distance between intial and final points.

Speed

• Speed = the distance an object travels in a given amount of time
• Speed=distancetimeSpeed=distan⁡cetime
• SI unit of speed is m/s

Types of Speed

• Constant speed: speed doesn’t change
• Changing speed: Riding a bike for 5 km. Take off and increase speed, slow down up hill, speed up down hill, stop for stop sign. The trip took you 15 min (.25 h)
• Average speed = totaldistancetotaltimetotaldistan⁡cetotaltime

Instantaneous speed: speed at any given time.

Velocity

• Velocity: includes speed and direction.
• Storm is moving at 20km/hr.
• Suppose two trains are going with the same speed in opposite direction so they are having different velocities.
• Race car going around an oval track might have constant speed, but different velocities at each point.

Acceleration

• Any change in velocity over a period of time is called acceleration.
• The sign (+ or -) of indicates its direction. + sign shows the acceleration and – sign shows de-acceleration.
• Uniform (constant) acceleration equation

a = v/t

The acceleration equals to zero

• Images of car become farther apart as time increases
• Velocity and acceleration are in the same direction
• Acceleration is uniform (Arrows below the car maintain the same length)
• Velocity is increasing (Arrows above the car are getting longer)

This shows positive acceleration and positive velocity

The instant speed at points of equal elevations is the same.
The velocities are different because they are in opposite free fall & air resistance.

Galileo Galilei Italian physicist and astronomer Formulated laws of motion for objects in free fall

• freely falling object is any object moving freely under the influence of gravity alone.
• It does not depend upon the initial motion of the object
• Dropped – released from rest
• Thrown downward
• Thrown upward
• The acceleration of an object in free fall is directed downward, regardless of the initial motion
• The magnitude of free fall acceleration (gravitational acceleration) is g = 9.80 m/s2m/s2
• g decreases with increasing altitude
• g varies with latitude, height and depth from earth surface.
• 9.80 m/s2m/s2is the average at the Earth’s surface
• The italicized g will be used for the acceleration due to gravity

Not to be confused with g for grams

• With negligible air resistance, falling objects can be considered freely falling. objects of different shapes accelerate differently (stone vs feather)

• Speed both upward and downward

• The path is symmetrical.
• Acceleration is constant.
• The magnitude of the velocities is the same at equal heights.
• Images become closer together as time increases
• Acceleration and velocity are in opposite directions when ball goes upward.
• Acceleration is uniform (violet arrows maintain the same length)
• Velocity is decreasing in upward motion (red arrows are getting shorter)
• Positive velocity and negative acceleration
• Velocity becomes zero at maximum height.
• Time duration flight in going upward and coming back is always same.

2 Graphical Representation Of Motion & Graphs

3. Equation of motion

(1) When object is moving in straight line-

v=v0+atv=v0+at
x=x0+v0t+1/2at2x=x0+v0t+1/2at2
v2=v20+2a(Δx)v2=v02+2a(Δx)

• Average acceleration describes how fast the velocity is changing with respecttime.

aave=ΔvΔt=Δ(ΔxΔt)Δtaave=ΔvΔt=Δ(ΔxΔt)Δt

• where:
• = average acceleration

ΔΔv = change in velocity

ΔΔx = displacement

ΔΔt = elapsed time

(2) when object is coming vertically downward-

v=v0+gtv=v0+gt
h=v0t+1/2gt2h=v0t+1/2gt2
v2=v20+2ahv2=v02+2ah

(3) when object is coming vertically upward-

v=v0+gtv=v0+gt
x=v0t+1/2gt2x=v0t+1/2gt2
v2=v20−2ghv2=v02−2gh

• The SI unit of velocity is the m/s.

Average acceleration is+ or – depending on direction.

• Instantaneous Acceleration

a=t−→−lim0(ΔvΔt)a=t→lim0(ΔvΔt)

• Instantaneous acceleration is the limit of Δv/ΔtasΔtΔv/ΔtasΔtapproaches zero
• Instantaneous acceleration is zero where slope is constant
• Instantaneous acceleration is positive where curve is concave up
• Instantaneous acceleration is negative where curve is concave down

Uniform Circular Motion In this kind of motion the object moves on circle with fix speed but the direction changed by the time so the velocity of the change so its called acceleration motionacceleration is called centrifugal acceleration.It is directed toward the centre.

Questions for Class 9 Science Chapter 8 Motion

Question. Divya walked 2 km on a straight road and then walked back 1 km. Which of the two quantities involved in her walking is greater- the scalar or vector?
Ans :
Distance travelled by Divya = 2 km + 1 km = 3 km
Displacement                     = 2 km – 1 km = 1 km
Hence, distance which is a scalar quantity is greater than the displacement which is a vector quantity.

Question. Two satellites A and B revolve around a planet C. The time taken by satellite B to go around the planet is twice the time taken by A. Which of the two satellites will have a greater magnitude of velocity?
Ans : Satellite A will have greater magnitude of velocity since velocity is inversely proportional to time. (v = 2p r/t)

Question. What is motion? Give some examples of directly perceivable motion in daily life.
Ans : (i) A body is said to be in motion if it changes its position in relation to a reference point (origin).
(ii) Some examples of directly perceivable motion in daily life are :
(a) Birds and animals moving from one place to another.
(b) Cars moving on the roads.
(c) Aeroplanes flying in the sky.
(d) Blades of a moving fan.
(e) Fish swimming in water.

Question. In a long distance race, the athletes were expected to take four rounds of the track such that the line of finish was same as the track was 200 m.
(i) What is the total distance to be covered by the athletes?
(ii) What is the displacement of the athletes when they touch the finish line?
(iii) Is the motion of the athletes uniform or nonuniform?
(iv) Is the distance moved by and displacement of athletes at the end of the race equal?
Ans : (i) Total distance covered by the athletes = 4 × 200 = 800 m.
(ii) The line of start and the line of finish are the same so, Displacement = 0
(iii) The motion of the athletes is non-uniform.
(iv) The distance and displacement of an athlete at the end of the race are not equal.

Question. (i) Give an example of a body which may appear to be moving for one person and stationary for the other.
(ii) What can we tell about motion from the above example?
Ans : (i) The man standing on the roadside observe that a bus along with its passenger is moving.
At the same time, a passenger sitting in the bus observes his fellow passenger to be at rest.
(ii) We can tell that motion is relative.

Question. Define uniform acceleration. Give one example.
Ans : Uniform acceleration : An object has a uniform acceleration if it travels in a straight line and its velocity increases or decreases by equal amounts in equal intervals of time.
For example : The motion of a freely falling body has uniform acceleration.

Question. (i) Give some examples of erratic and uncontrolled motion.
(ii) Give an example of controlled motion which can be a service to human beings.
(iii) Is there a need to study about the erratic which of some objects and learn to control them?
Ans :
(i) Flooded river, a hurricane, a tsunami, etc.
(ii) Controlled motion of a river in a dam is useful to generate hydroelectric power.
(iii) Yes, we should study about the erratic motion of some objects because it will help us in disaster management.

Question. Define a scalar quantity.
Ans : A physical quantity which has only magnitude and no direction is called a scalar quantity.

Question. Give two examples of a scalar quantity.
Ans : Two examples of a scalar quantity are distance and speed.

Question. Define the following :
(i) Translatory motion
(ii) Complex motion
Ans : (i) When a body moves along a straight line, its motion is called rectilinear or translatory motion.When a body has two or more types of motion,
it is said to possess complex motion. For example
: A ball rolling down on inclined planes has both rectilinear as well as circular motion.

Question. How does the velocity change with time in nonuniform rectilinear motion of an object?
Ans : The velocity has different values at different instant of time and at different points of its path, i.e. change in velocity of the object during any time interval is not zero.

Question. Why is the motion in a circle at constant speed called  accelerated motion?
Ans : When a body moves along a circular path with a constant speed, its direction of motion at any point is along the tangent to the circle at that point, this motion called accelerated motion. The direction of motion changes as the body moves in a circle and causes a change in the velocity. Therefore, the motion of an object along a circular path is an accelerated motion.

Question. Name the different types of graph.
Ans : Bar graphs, straight line graphs, histograms, etc.

Question. Which type of graph is used to describe motion?
Ans : Line graph are used to describe the motion of an object.

Question. Define the term velocity. What is its unit? Is it a scalar or vector quantity?
Ans : Velocity is a physical quantity that gives the speed and direction of motion of the body
Velocity = Displacement/ Time
The S.I. unit of velocity is ms–1. Velocity is a vector quantity because its description requires both magnitude and direction.

Question. When do we say that the position of body has changed?
Ans : If the distance, or direction, or both of a body relative to a reference point changes then we say that the position of the body has changed.

Question. What do graphs provide?
Ans : The graphs provide a convenient method to present pictorially the basic information about a variety of events such as motion.

Question. How many different kinds of motion are there? Name them. Is there any motion which is a combination of two or more types of motion?
Ans :(i) There are three kinds of motion. These are :
(a) Linear or translatory motion.
(b) Circular or rotational motion.
(c) Vibratory or oscillatory motion.
(ii) Complex motion means a combination of two or more types of motions.

Question. What is the difference between uniform linear motion and uniform circular motion?
Ans : When a body is moving with a uniform speed along a straight line, its motion is called linear uniform motion. The uniform linear motion is an accelerated motion.
When a body is moving with a constant speed along a circular path, the direction of motion of body changes continuously with time.
We know that a change in the direction of motion implies a change in velocity. Thus, uniform circular motion is an accelerated motion even though the speed of the body remains constant.

Question. Give the simplest type of motion.
Ans : The simplest type of motion is the motion along a straight line.

Question. What is the Si unit of acceleration?
Ans : The SI unit of acceleration is m/s2.

Question. When is the acceleration of a body positive?
Ans : The acceleration of a body is taken to be positive if it is in the direction of velocity.

Question. What is the acceleration of a body moving uniform velocity?
Ans : The acceleration of a body moving with uniform velocity is zero.

Question. Give two examples of a vector quantity?
Ans : Two examples of a vector quantity are displacement and velocity.

Question. What is the meant by retardation? Give its S.I. unit.
or
Give an example of negative acceleration.
Ans : Retardation : When the velocity of a body decreases with time, its final velocity is less than the initial velocity, i.e. its acceleration is negative. This negative acceleration is called retardation. The S.I. unit of retardation is ms 2.
Example : When brakes are applied to a moving car, its velocity gradually decreases. Under these conditions,
the car is under retardation.

Question. Define non-uniform acceleration. Give one example.
Ans : Non-uniform acceleration : An object has non-uniform acceleration if it travels in a straight line and its velocity increases or decreases by unequal amounts in equal intervals of time.
For example : The movement of a car on a busy road has non-uniform acceleration.

Question. What is the relationship between distance travelled and the time elapsed for the motion with uniform velocity?
Ans : Distance is directly proportional to time used.
In fact, distance travelled
= Uniform velocity × Time used.

Question. Define acceleration. Is it a scalar or a vector quantity?
Ans : In non-uniform motion, the velocity of a body changes with time. It has different velocities at different instants of time and at different points of its path. In such a situation, we define a physical quantity called acceleration which is a measure of the change in the velocity of a body per unit time. It is a vector quantity.

Question. Differentiate acceleration from velocity.
Ans : Velocity is the change in position of a body per unit time in a given direction and acceleration is the change in velocity of a body per unit time.

Question. “The direction in which an object moves is given by the direction of velocity of the object and not the direction of acceleration.” Give an example to justify this statement.
Ans : When an object is thrown upward, its velocity is directed upward while its acceleration is directed downwards. Thus, the velocity determines the direction of motion.

Question. Name the two types of physical quantities.
Ans : The two types of physical quantities are scalar quantity and vector quantity.

Question. What is magnitude?
Ans : The numerical value of a physical quantity is called its magnitude.

Question. Define vector quantity.
Ans : A physical quantity which has both magnitude and direction is called a vector quantity.

Question. A satellite revolves round the Earth with uniform speed. Is this motion accelerated? If so, in which direction does the acceleration act?
Ans : The velocity of the satellite revolving around the Earth changes due to change in its direction of motion. So, the motion of satellite is an accelerated one. The acceleration is directed towards the centre of the Earth.

Question. When is the acceleration of a body is negative?
Ans : The acceleration of a body is taken to be negative when it is opposite to the direction of velocity.

Question. How does the velocity change with time in uniform rectilinear motion of an object?
Ans : In uniform rectilinear motion of an object, the velocity is constant with time, i.e. change in velocity of the object during any time interval is zero.

Question. Give an example of use of bar graph and straight graph.
Ans : In telecast of one-day cricket match on T.V. vertical bar graphs show the run rate of a team in over. Straight line graph helps in solving linear equation in two variables.

Question. (i) Give an example of motion in human body which cannot be perceived directly.
(ii) Give an example of motion in nature which cannot be perceived directly.
(iii) Give an example of motion of the Earth which cannot be perceived directly.
Ans : (i) The motion of blood through veins and arteries in human body cannot be perceived directly.
(ii) The motion of air is not perceived directly. We perceive it by observing the movement of dust or leaves of trees, etc.
(iii) The phenomena of sunrise, sunset and change of seasons is due to rotation and revolution of the Earth which cannot be perceived directly.