CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

MOTION

REST AND MOTION:

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Rest: An object is said to be at rest , if object does not change its position with respect to its surrounding (reference point) and time.

e g Book lying on the table, trees etc.

Motion : An object is said to be in motion , if object changes its position with respect to its surrounding (reference point) and time.

e g A car moving on a road w r t trees , blades of moving fan.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Reference point: It is a point used to describe the position of an object. An object can be referred or defined with respect to reference point.

In the above example of rest Book lying on table, Table is used as a reference point to define the position of book. In the example of moving car, trees are the reference point.

SCALAR QUANTITIES AND VECTOR QUANTITIES:

Physical Quantities: The quantities which can be measured directly or indirectly are called physical quantities.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Types of Physical Quantities:

Scalar Physical Quantities: The quantities which can be completely described by magnitude (size) only are called scalar physical quantities.

e g Mass , Length , Time , Distance, Speed , Work , Energy etc.

Note:

• Length = 5 m, where 5 is magnitude and m is unit.
• Numerical value is called magnitude.

Vector Physical Quantities: The quantities which can be completely described by both magnitude (size) and direction are called vector physical quantities.

e g Displacement , Velocity , Acceleration , Force , Momentum etc.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Difference between Scalar Quantities and Vector quantities:

S N	Scalar Quantities	Vector Quantities
1	The quantities which can be completely described by magnitude (size) only are called scalar physical quantities.	The quantities which can be completely described by both magnitude (size) and direction are called vector physical quantities.
2	e g Mass , Length , Time , Distance, Speed , Work , Energy etc.	e g Displacement , Velocity , Acceleration , Force , Momentum etc.
3	Scalar quantities are always positive.	Vector quantities can be positive, negative and zero.
4	Represented with simple notation e g length ( l ) , Work (W).	Represented with special notation e g Acceleration ( Bold a or arrow on a)
5	Scalars are solved by simple algebra.	Vectors are solved by vector algebra.

Note:

• Vector quantity can also be written in scalar form i e only in magnitude.
• If I visit a shop and ask shopkeeper for 1kg Sugar, I would ask him 'give me 1 kg of sugar' not like 'give 1 kg of sugar in north direction' 🤣. So in this statement no need of direction.

1. But when we apply force we have to tell that in which direction the force is applied otherwise the statement is not complete.

DISTANCE AND DISPLACEMENT:

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Distance (s,d,x): Distance is the  actual total path length covered by an object .

S I unit of distance is meter (m).

Displacement: Displacement is the shortest distance between the initial point and the final point of total path length.

Mathematically 🛆x= x_f -x_i, 
where, x_f  = final position of object and x_i = initial position of object.

S I unit of displacement is also meter (m).

DISTANCE AND DISPLACEMENT

Note : 

• Both distance and displacement are length so there S I unit also same i e meter.
• Other units of Distance and displacement is Centimeter (cm), Kilometer(km).

Types of Displacement:

1. Positive Displacement:
2. Negative Displacement:
3. Zero Displacement:

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

1. Positive Displacement: When the final position is greater than initial position.

i e 🛆x= x_f -x_i= +ve , If x_f >x_i

🛆x= x_f -x_i= 8 - 5= 3 m

2. Negative Displacement: When the final position is less than initial position.

i e 🛆x= x_f -x_i= -ve , If x_f <x_i
🛆x= x_f -x_i= 5 - 8= - 3 m.
3. Zero Displacement: When the final position is same as initial position.
i e 🛆x= x_f -x_i= 0 , If x_f =x_i
🛆x= x_f -x_i= 8 - 8 = 0
e g circular motion.

Note:

• When object move in +ve axies then displacement is +ve.
• When object move in -ve axies then displacement is -ve.
• When object move in north and east displacement is +ve.
• When object move in south and west displacement is -ve.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Difference between Distance and Displacement:

S N	Distance	Displacement
1	Distance is the actual total path length covered by an object.	Displacement is the shortest distance between the initial point and the final point of total path length.
2	It is a scalar quantity.	It is a vector quantity.
3	Distance is always positive.	Displacement can be positive, negative and zero.
4	The distance between two points may have may values depending upon the path followed by the object.	Displacement between two points always has a unique value.
5	Distance is always greater or equal value to that of displacement.	Displacement is always lesser or equal value to that of distance.

UNIFORM MOTION AND NON UNIFORM MOTION:

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Uniform motion in a straight line : 

An object is said to be in uniform motion if it covers equal distance in equal intervals of time, howsoever small these intervals may be.

e g Car moving on straight road with constant speed 40 km/h.

Non-Uniform motion in a straight line : 

An object is said to be in non uniform motion if it covers unequal distance in equal intervals of time, howsoever small these intervals may be.
e g Car moving in traffic road with variable speed.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

SPEED AND VELOCITY:

Speed: Speed is defined as distance travelled by object per unit time.

Speed= total path length/time taken

v = s/t

S I unit of speed is m/s or ms^-1

Speed is a scalar quantity. 

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Uniform speed: An object is said to be moving with a uniform speed, if it covers equal distances in equal intervals of time, howsoever small these intervals may be.

Non uniform speed/Variable speed: An object is said to be moving with a variable speed if it covers equal distance in unequal intervals of time or unequal distances in equal intervals of time, howsoever small these intervals may be. 

Average speed: It is ratio of total distance travelled  by the object to the total time taken by object to cover that distance.

Average speed = Total distance/Total time

v_av = S₁ + S₂ / t₁ + t₂

Instantaneous speed: The speed of an object at a particular precise moment of time (instant) during the motion of an object.

Speedometer is used to measure the instantaneous speed of vehicle.

Odometer is used to measure the distance covered by vehicle.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Velocity: 
Velocity is defined as displacement covered by the object in per unit time.

Or

Distance covered in a particular direction per unit time.

Velocity = displacement/time

v = Δ x / t

S I unit of velocity is m/s or ms^-1

Velocity is a vector quantity.

Uniform Velocity: If object covers equal displacement in equal intervals of time howsoever small these intervals may be.

Non uniform velocity and variable velocity : An object is said to be in non - uniform motion if it undergoes equal displacements in unequal intervals of time, howsoever small these intervals may be.

Average velocity: The ratio of total displacement of the object to the total time taken to cover displacement.

Average velocity = total displacement/total time

Or

Average velocity= (initial velocity + final velocity)/2

v_av = u + v/ 2

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Difference between speed and velocity:

S N	SPEED	VELOCITY
1	Speed is defined as distance travelled by object per unit time.	Velocity is defined as displacement covered by the object in per unit time.
2	Speed is a scalar quantity	Velocity is a vector quantity
3	Speed is always positive.	Velocity can be positive , negative and zero.
4	The average speed cannot be zero.	The average velocity can be zero.

Note: 

1. Speed and velocity has same units. 

2. Other units of speed and velocity are for small distances is cm/s and for large distances is Km/h.

3. Velocity can be changed by increasing or decreasing speed and by changing the direction of motion.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Conversion:

Distance:

1 km= 1000 m , 1m= 100 cm.

1m= 1/1000 km, 1cm= 1/100m.

Time:

1 hr = 60 min, 1min = 60 s, 1 hr= 3600 sec.

Example:Convert : 72 km/h into m/s.

Ans : 72 km/h= (72 x 1000)/3600 = 20 m/s

 Using 1km=1000 m and 1hr=3600 s.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

Acceleration (a) :(Rate of change of velocity):

The rate of change of velocity per unit time is called acceleration.

Acceleration = Change in velocity / time taken

Acceleration = (Final velocity - Initial velocity)/ time taken

a= (v-u)/t

S I unit of acceleration is ms^-2.

 It is a vector quantity.

Types of acceleration : 

a) Positive acceleration :

If the direction of acceleration is same that of direction of velocity then acceleration is said to be positive.

Or 

If v>u

a= (v-u)/t=+ve.

e g when we accelerated the car.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

b) Negative acceleration or Deceleration or Retardation : 

If the direction of acceleration is opposite that of direction of velocity is called retardation.

Or 

If v<u

a=(v-u)/t= -ve
e g when we applied brake on car.

Uniform Acceleration:If an object travels in a straight line and its velocity increases or decreases by equal amounts in equal intervals of time.
For example, Freely falling objects.

Non Uniform Acceleration: If its velocity changes at a non-uniform rate.
For example, if a car travelling along a straight road increases its speed by unequal amounts in equal intervals of time.

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

QUESTION BASED ON THE ABOVE TOPICS:

1 Define rest and motion give example.

2 What is displacement? Give it's S I unit.

3 Difference between scalar quantities and vector quantities.

4 Find out scalar and vectors from these physical quantities: Mass , Length , Acceleration , Time , Force , Density, Distance, Speed , Work , Energy.

5 Give difference between distance and displacement.

6 If distance covered by a particle is zero, what can be its displacement?

7 If an object move in circle and start at point A and after two revolution stop at A again , what would be the displacement of the object?

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

8 Can displacement be greater than distance travelled by an object?

9 What would be the numerical ratio of distance over displacement?

10 If a person start moving from his house toward a post office situated in south direction 1 km. What would be distance and displacement covered by person after reaching the post office ?

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

11 An object has moved through a distance. Can it have zero displacement? If yes, support your answer with an example.

12. 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 from his initial position?

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

13. Which of the following is true for displacement? (a) It cannot be zero. (b) Its magnitude is greater than the distance travelled by the object.   
14. Convert:

•    20 cm/s into m/s  

•     72 km/h into cm/s   

•    120 m/min into m/s and cm/s

•    26 cm/s into Km/h.

15. If an object covers a distance of 25 Km in 2 h. Find the speed of the object in m/s. 

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

16.  The uniform speed of an object is 5m/s and object move for a time of 3min. Find the distance covered by object.[Hint: convert 3min into sec then solve].(Ans: s = 900 m)

17. An object travels 16 m in 4 s and then another 16 m in 2 s. What is the average speed of the object?

18. Distinguish between speed and velocity.

19. Under what condition(s) is the magnitude of average velocity of an object equal to its average speed?

CBSE CLASS 9th SCIENCE CHAPTER MOTION NOTES

20. What does the odometer of an automobile measure?

21. What does the path of an object look like when it is in uniform motion?

22. Starting from a stationary position, Rahul paddles his bicycle to attain a velocity of 6 m s^–1 in 30 s. Then he applies brakes such that the velocity of the bicycle comes down to 4 m s^-1 in the next 5 s. Calculate the acceleration of the bicycle in both the cases. 

(Ans: a=0.2 m s^-2 in case first,,,, a=-0.4 m s^-2 in second case). 

23. When will you say a body is in (i) uniform acceleration? (ii) non-uniform acceleration? 

24. A bus decreases its speed from 80 km h^–1 to 60 km h^–1 in 5 s. Find the acceleration of the bus.

(Ans: a= -1.11 m s^-2 )

25. A train starting from a railway station and moving with uniform acceleration attains a speed 40 km h^–1 in 10 minutes. Find its acceleration. 

(Ans: a= 0.018 m s^-2 )