Motion Notes-class 9th

MOTION

LEARNING OBJECTIVES       

 After a study of the chapter, the learner will be able to

• Distinguish between uniform and non-uniform motion using the graphical method.

• Calculate numerically value the displacement, velocity, or acceleration using an equation of motion. Appreciate that uniform circular motion is accelerated motion.

• Identify the given physical quantities as vector or scalar.

• Explain that motion is relative in nature.

Motion forms an integral part of our life. All microscopic and macroscopic objects in-universe show some kind of movement. All of the objects such as planets, moons, etc. Also exhibit motion.
                              All these examples can be clearly understood and discuss if we can describe motion, its property, and other related aspects such as displacement speed, etc. In this chapter, we shall study the motion and its representation.

Rest:-

If  the position of an object does not change as time passes, it is said to be at rest.

Motion:-

 If the position of an object changes as time passes it is said to be in motion.

The motion of a  car

Reference point-

The point from where the object starts its motion is said to be its reference point.

Scalar quantity

The quantity having only the magnitude it is said to be scalar quantity full stop for example mass, speed, distance, etc.

Vector quantity:-

The quantity having magnitude as well as direction is called a vector quantity. for example weight, velocity, displacement, etc.

Distance:-

Distance traveled by an object in any direction is called distance.

It is a scalar quantity its unit is km or m.

Displacement:-

The shortest straight-line path traveled by an object in a given direction is called displacement. It is a vector quantity its unit is km or m.

Difference between distance and displacement

Distance	Displacement
1. The path traveled by an object in a given direction called distance. 2. It is a scalar quantity.                      3. This is always positive. It cannot be zero or negative. 4. For example, an object travels.  5 kilometer in any direction.	1.    The shortest straight-line path traveled by an object in a given direction called displacement.        2.    It is a vector quantity.        3.    Displacement can be positive or negative.        4.    For example and object travels 5 kilometers in the north direction.

Uniform motion

A body said to be in uniform motion when it travels the equal distance in an equal interval of time.

Diagram 2

Non-uniform motion

A body said to be in non-uniform motion when it travels the unequal distances in an equal interval of time.

Diagram 3

Note:-

1. If the distance-time graph is a straight line, the motion will be uniform.

2. The distance-time graph is a curved line then the motion will be non-uniform.

Speed:-

The speed of a body is the distance traveled by it in unit time.

Formula                          Speed =distance / time

Units of speed are km / h or m/s.

For example, if the bus cover 30 kilometers into hours then the speed of the bus is given by distance/ time=30 / 2 =15 km / h.

·       Speed of any vehicle is measured by the device called speedometer.

·       Speed is a scalar quantity.

Average speed  

It is defined as the total distance travelled divided by the total time taken to cover the distance.

For example a car travels a distance of 100 km in 4 hours then its average speed can be found as 100 / 4 =25 km / h.

Uniform speed or constant speed

A body said to be in uniform speed if it covers equal distance in equal interval of time.

Velocity

          It is defined as the displacement travelled by any vehicle per unit time  in a given direction.

Or

It is defined as displacement of a vehicle per unit time.

Formula                          Velocity= displacement / time taken.

·       It has unit meter or kilometer.

·       It is a vector quantity.

Uniform velocity

A body said to be in uniform velocity if it travels in a particular direction in a straight line on move over equal distance in equal interval of time.

Note:-

1.    Velocity of a body can be changed in two ways:-

·       But changing the speed of the body.

·       By keeping speed constant but changing the direction.

2.    Speed and velocity are not always equal in magnitude.

3.    If an object travel in a straight line then speed is equal to velocity.

For example Sohan travels a distance of 30 kilometer from home to school in 2 hours. after sometime, Sohan come back to home  in 2 hours, then distance travelled by the Sohan is 60 kilometer but the displacement travel by the Sohan is zero, then the speed of the Sohan=60 / 4=15km/h.

But,  velocity of the Sohan=0/4=0 km/h.

Because, Sohan come back to his home so the distance between the starting point and ending point will be zero so its velocity will be zero.

Difference between speed and velocity

Speed	Velocity
1.    It is defined as the distance covered by the body in a given interval time. 2.    It is a scalar quantity. 3.    Speed for body can never be zero, if it covers some distance. 4.    For example, speed of vehicle is given   by 25 km/h. 5.    Formula Speed=distance/time.	1.    It is defined as displacement covered by the body in a given interval time. 2.    It is a vector quantity. 3.    Velocity for the body can be zero if its an initial and final point of destination is the same. 4.    For example, the speed of a vehicle is given by 25 km/h in the north direction. 5.    Formula       Velocity=displacement / time.

Acceleration:-

It is defined as change in velocity of a body in a given interval of time or in per unit time.

diagram 1 as shown above, an object is moving in a straight line and you is its initial velocity at point a we become the final velocity V within time so, acceleration given by

 acceleration = final velocity - initial velocity

time taken

 a = v-u

  t

 Types of acceleration

1.    uniform acceleration

2.    non uniform acceleration

 Uniform acceleration ( Definition)   

when there is equal change in velocity of a body in  equal interval of time .

 diagram 2

 Non uniform acceleration (definition)

          when there is an unequal change in velocity of a body in an equal interval of time or an equal change in velocity in equal interval of time then the object should have non-uniform acceleration.

 Cases in acceleration

 There are three cases and acceleration given below-

1.    Positive acceleration

2.    Negative acceleration

3.    Zero acceleration

Positive acceleration

 Definition:- The acceleration said to be positive if there is an increase in velocity of a body.

a=v-u/t              [v>u]. =positive

 Negative acceleration

Definition :-

The acceleration said to be positive if there is a decrease in velocity of a body.

a=v-u/t              [v<u]. = negative

 Zero acceleration

Definition:- The acceleration said to be positive if there is  no change in velocity of a body.

a=v-u/t =0         [v=u].

 Graphical representation of motion of an object

  In 2D - graph  there are two axis x and y. We always take time  on x- axis  and velocity  or displacement or  distance on y axis.

1. D-t graph(  distance-time graph)

2. V-t graph( velocity-time graph)

Uniform circular motion

 Definition:- When object moving in a circle the motion of an object is said to be  uniform circular motion.

          In circular motion there can be  a constant speed but no  constant velocity because there is only change in direction  at each point.

 Speed=Circumference of a circle / time taken

Speed = 2πr / t