Motion

 1. Motion of an object is perceived:-

*when the position of an object changes with time. 

* through indirect evidences, for example motion of air is observed by the movement of leaves.

2. What causes the phenomenon of sunrise,  sunset and changing of season ?

As the Earth rotates toward the east, it looks like the sun is moving west.

Rotation and phenomen of day & night: Sunrise, Sunset 

The sun stays in its position at the center of our solar system. It doesn't rise and set. But it appears to rise and set because of the Earth's rotation on its axis. It makes one complete turn every 24 hours. It turns toward the east.As the Earth rotates, different locations on Earth pass through the sun's light. 

The animation shows how that looks hour by hour (for 4 hours). As your town turns toward the sun and begins to enter its light, the sun seems to rise in the east. As your town begins to leave the sun's light and enter darkness, it appears to set in the west.

Tilt and Revolution and change of season

The Earth's axis is not oriented vertically, but is tilted by 23.5 degrees. The north end of the axis is always pointed toward the North Star as the Earth revolves around the sun. This tilt, combined with its revolution around the sun, causes seasonal changes. (When it’s summer in the northern hemisphere, it’s winter in the southern, and vice versa.) If the axis was not tilted, our year-round climate would be rather boring and many places on earth wouldn’t receive much light.

3. Why don't we directly perceive the motion of the earth? 

The earth moves very fast. It spins (rotates) at a speed of about1,000 miles (1600 km) an hour and orbits / revolves around the Sun at a speed of about 67,000 miles (107,000 km) an hour. We do not feel any of this motion because these speeds are constant. The rotation and revolution of the earth stay the same so we do not feel any acceleration or deceleration. 

We can only feel motion if our speed changes. For example, if we are in a car which is moving at a constant speed on a smooth surface, we will not feel much motion. However, when the car accelerates or when the brakes are applied, we do feel motion.

4. While travelling in a train, it appears that the trees, poles near the track are moving whereas co-passengers appear to be stationary. Why?

The trees, poles etc appear to be in moving opposite direction while travelling in a train due to a condition called Relative motion. All motion is relative to the observer or to the objects which are fixed and the line of sight changes very rapidly. 

Whereas, our co-passenger in a train seem to be at stationary position because their position is not changing with respect to us.

5. Reference point and 0rigin:- 

Reference Point is used to describe the position/ location of an object. 

Origin - The reference point that is used to describe the location / position of an object is called Origin .

For Example, a cafe is at a distance of 3 km south from my school. Here, the school is the reference point that is used for describing where the cafe is located.

6. Motion :- An object is said to be in motion if its position changes with time. 

7. Rest  :- An object is said to be in rest if its position  does not change with time. 

8. Distance :- 

lt is the actual length of path covered by an object.

It does not depend upon the direction of motion.      lt is a scalar quantity. 

It is a numerical quantity. 

Distance covered by a vehicle is measured by odometer. 

Distance is always positive. 

9. Displacement :- 

It is the shortest possible distance between the initial and final position of an object.

It depends upon the direction in which an object is travelling.

It is a vector quantity. 

Displacement is the vector difference between the initial and final position.

It can be positive , negative or even zero.

10. Magnitude of displacement is less than the distance if direction of motion is not the same.

11. Magnitude of the displacement is equal to the distance travelled by an object , if it travels along a straight line in a given direction. 

12. Displacement of an object can never be more than the distance covered by it. 

13. Zero displacement:- An object can have zero displacement if the final position of the object coincides with its initial position. 

14. Scalar and Vector Quantities

A scalar quantity describes a magnitude or a numerical value.

A vector quantity describes the magnitude as well as the direction.

15. Type of motions:-

Linear motion ( motion along a straight line)

Curvilinear motion or circular motion 

Rotatory motion 

Vibratory or oscillatory motion 

16. Uniform motion :- 

If an object covers equal distances in equal interval of time the object said to be in uniform motion. 

17. Non uniform motion :- 

If an object covers unequal distances in equal intervals of time or equal distance in unequal intervals of time , the object said to be in non-uniform motion. 

18. Speed :- Speed of an object is defined as the distance traveled by the object per unit time. 

    Symbols  

Speed       :- v

 Distance  :- S

Time        :- t

19. Constant speed / Uniform Speed :- 

If an object covers equal distance in equal intervals of time then the speed of the object said be uniform / constant. 

20. Variable Speed / non-uniform speed :- 

If an object covers unequal distance in equal interval of time or equal distance in unequal intervals of time then the speed of the object is said to be non-uniform. 

21. Average speed ( v_av ) :-   It is the ratio of total distance travelled by an object to the total time take by the object to cover that distance . 

22. Instantaneous Speed :- It is the speed of an object at any given instant in time. 

Speedometer indicates instantaneous speed of a vehicle.

23.Average speed is the average of all instantaneous speed found simply by a distance / time ratio.  

24. Velocity :- It is the displacement of an object per unit time. 

It is a vector quantity because it specifies both the magnitude and the direction. 

In simple language: velocity is displacement over time.

25. Uniform velocity :- If an object covers equal displacements in equal intervals of time then the velocity of that object said to be uniform. 

Magnitude and direction both remain the same in constant velocity. 

26. Non-uniform velocity :- If an object covers unequal displacements in equal intervals of time or equal displacements in unequal intervals of time , the velocity of that object is said to be non-uniform.  

27. The velocity of an object changes if its speed changes or its direction of motion changes or both change. 

28. Average velocity :- 

(A) It is the ratio of total displacement to the total time taken for that displacement.  

Average velocity = Total displacement ÷  Total time

(B) lf the velocity of an object in linear motion is changing at a uniform rate , the average velocity will be:-

29. Uniform Acceleration – An object is said to have a uniform acceleration if it travels along a straight path  and its velocity changes (increases or decreases) by equal amounts in equal intervals of time.

30. Non-Uniform Acceleration :-  An object is said to have a non-uniform acceleration if its velocity changes (increases or decreases) by unequal amounts in unequal intervals of time. 

3l. Acceleration is a vector quantity. 

32. Positive Acceleration :- lf the direction of acceleration is the same and the velocity is increasing in the same direction, Such acceleration is called Positive Acceleration.

33. Negative Acceleration / Retardation/ de-acceleration / Deceleration :- If the direction of acceleration is opposite to the direction of velocity and the velocity is decreasing in the direction. Such acceleration is called Negative Acceleration.

34. Average / Mean Acceleration :-   The average acceleration describes how rapidly the velocity changes during the time on average.

35. Graphical representation of motion:- 

*Distance  Time  Graph* 

Graph- l

Distance- time graph showing uniform speed and velocity:- 

*The graph shows that the object covers equal distances in equal intervals of time.

*Distance travelled is directly proportional to time taken.

*So for uniform speed,a graph of distance travelled against time is straight line ( here the line 0B). 0B shows that distance is increasing at a uniform speed. 

Note:- we can use the term uniform velocity in place of uniform speed if the magnitude of displacement is equal to the distance travelled along the y-axis.

Graph-2

Distance - time graph for non-uniform speed

 *This graph shows non-linear variation (  change) of the distance travelled by the object. So it represents motion with non-uniform speed. 

Velocity-Time  Graphs

Graph - 3

*This graph shows the Velocity-Time Graph for an object moving with uniform velocity of 40 km/hr.

*As the object is moving with uniform velocity,  the height of its Velocity-Time graph doesn't change with time. 

Graph- 4

This graph shows Velocity-Time graph for an object whose velocity changes by equal amounts in equal intervals of time.  

Thus Velocity-Time graph is a straight line. This happens for all uniformly accelerated motion.

Graph 5 a & b (non-uniformly accelerated motion)

* Graph 5(a) shows a Velocity-Time graph which represents the motion of an object whose velocity decreases with time.

* Graph 5(b) shows the Velocity-Time graph representing the non-uniform variation of velocity of the object with time.

36. Determination of the Distance covered using Velocity-Time Graph :- 

Distance = speed × time  

( speed is a scalar quantity)

Displacement = velocity × time 

( velocity is a vector quantity)

Displacement of an object is the product of velocity and time where the velocity is uniform. So the area enclosed  by Velocity-Time graph and the time axis gives the magnitude of the displacement. 

For an example take this Velocity-Time graph to find the displacement of an object. 

Here velocity = 5 m/ s and time = 6 s

Displacement = velocity × time=5 × 6= 30 m.....(i)

Now observe the interesting fact here. ABC0 is rectangular in shape. 

Area of rectangle = length x breadth

Area of ABC0 = 5 × 6 = 30 ............. ( ii) 

Thus from i & ii we conclude that the area enclosed  by Velocity-Time graph and the time axis gives the magnitude of the displacement. 

Example 2:- 

In this Velocity-Time graph the velocity is 40km/h and time is ( t2 - tI) so the displacement/ distance moved by the object is 

S =  AC × CD

S = (40 km/h) × (t2 - tI)h = 40(t2 - tI) km

Example 3:- 

In this Velocity-Time graph the distance travelled by the object (s) =             

area of rectangle ABCD+area of triangle ADE

   =       AB × BC  +  I/2 ( AD × DE)

This means dividing the “area” into a series of triangles and rectangles allows us to determine the overall displacement.

37. Equations of motion :- 

u: initial velocity

a: uniform acceleration

t:  time

v: final velocity

s: distance travelled  in time t

38. Uniform circular motion :- lf an object moves in a circular path with uniform speed , its motion is called uniform circular motion but there is a continuous change in its direction of motion. 

In such a case, the change in velocity is only due to the change in direction of motion of the object because in that case the magnitude of velocity or speed is constant. Therefore, this motion is accelerated without changing its speed. 

If r is the radius of a circular track and it takes t time to go one complete round, then the velocity is given by :- 

39. Centripetal Acceleration :- The direction of acceleration is towards the centre of the circular path and is known as centripetal acceleration.

 

The force associated with it, i.e. the force needed to produce circular motion is known as centripetal force.

For complete question answer of the chapter Motion of NCERT text book go to this website (copy the link and paste it in Google search) 

https://www.google.com/amp/s/www.tiwariacademy.com/ncert-solutions/class-9/science/chapter-8/

-------------NCERT  Exemplar question answer---------

Courtesy :- 

topprnation.com

 https://journeynorth.org/tm/mclass/SunriseSetAns.html

https://journeynorth.org/tm/mclass/ReasonsBack.html

https://coolcosmos.ipac.caltech.edu/ask/60-Why-don-t-we-feel-Earth-move-#:~:text=Earth%20moves%20very%20fast.,because%20these%20speeds%20are%20constant.

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NCERT text book of Science for class 9th