Science Class 9 Chapter 8 Motion Notes
Basic Terminology
Motion: When an object changes its position respect to its surrounding with time, it is said to be the object is in motion.
Example motion of flying birds, motion of car, motion of blood in arteriesand veins etc.
Origin: We need to specify a reference point that describes the position of an object is called the origin.
Distance: This is total path covered by the body between its initial and final position is called distance.
Displacement: The shortest distance measured from the initial to the final position of an object is known as the displacement.
Magnitude: The numerical value of a physical quantity is its magnitude.
Physical Quantities: Quantities in physics which can be measured are called physical quantities like mass, time, area, velocity, area, density, volume, force etc.
Scalars and Vectors Quantities
Scalars quantities: The quantities that require magnitudes only to specify them are called scalars quantities or scalars. e.g Mass, time, area, speed, distance, temperature, density, volume etc.
Vector quantities: Quantities that require both magnitudes and direction to specify them are called vector quantities or vectors. e.g Displacement, velocity, force, momentum, weight etc.
Differences between Displacement and Distance
Distance:-
- It is scalar quantity.
- It cannot be negative.
- It cannot be zero.
- It is total path covered between to point.
Displacement:-
- It is vector quantity.
- It can be negative and positive.
- It can be zero.
- It is the shortest distance between two points.
Use of distance and displacement:
- The displacement and distance are used to describe the overall motion of an object.
- They are used to locate its final position with reference to its initial position at a given time.
Motion along a straight line
The simplest type of motion is the motion along a straight line.
When an object moves on a straight path, such a motion is called motion along a straight line or Linear motion.
Examples: A bus moving on a straight road, A ball rolling along a straight path.
Type of Motion Along Straight Line:-
- Uniform Motion: When an object covers equal distance in equal intervals of time, it is said that the object is in uniform motion. Example: Motion of car, Flying of a bird, Motion of a sparing.
- Non-uniform Motion: When an object does not covers equal distance in equal intervals of time, it is said to be in non-uniform motion.
Object A covers equal distance in equal time interval, while object B does not cover equal distance in equal time interval. Therefore it is said that object A is in uniform motion and object B is in non-uniform motion.
Finding Rate of Motion
Rate of motion is found by the speed of an object.
Speed: The distance travelled by the object in per unit time is called speed.
The SI unit of speed is metre per second. symbolically it is written as m/s or m s-1 . Other representation of speed are cm/s and km/h which is mostly used.
Avarage speed: The average speed of an object is obtained by dividing the total distance travelled by the total time taken.
If an object travels a distance s in time t then its speed vis,
Speed with Direction
Velocity: Velocity is the speed of an object moving in a definite direction. It is called velocity.
SI unit of velocity is m/s or m s-1.
Type of Velocity:-
Uniform velocity: When an object covers equal displacement in equal time of interval. It is called uniform velocity.
Non-uniform velocity: When a object covers unequal displacement in equal time of interval. It is called non-uniform velocity.
Initial velocity: Initial velocity of an object means the velocity when the object starts displacing where time(t) may be zeto or t ≠ 0.
It is denrted by 'u'.
When object starts moving from rest then u = 0.
When an object is in motion then u ≠ 0. The value will be any integer.
Final velocity: Final velocity of an object is given by time t. It is the value of final position in velocity of the object.
Initial and final velocity of an object can be zero.
When an object starts from rest its initial velocity is zero.
If it stops after applying break,its final velocity will be zero. As object is in rest.
Differences between Speed and Velocity
Speed:-
- It is a distance covered in unit time.
- It is a scalar quantity.
- Average speed of moving object can not be zero.
- Speed of an object can never be zero.
Velocity:-
- It is a displacement covered in unit time with direction.
- It is a vector quantity.
- Average velocity of moving object can be zero.
- Velocity of an object can be positive, negative or zero.
Note: When the object is moving in a straight line along positive direction and the distance and displacement of an two different objects are equal in same time then the value of speed and velocity will be equal.
Changing the velocity of an object
- Change its magnitude.
- To change its the direction.
- To change its both magnitude and direction.
Acceleration: Acceleration is changing in the velocity of an object per unit time is called acceleration.
In other words: The rate of changing in the velocity is known as acceleration.
The SI unit ofacceleration is m s-2.
It is denoted by a.
Acceleration is a vector quantity.
Accelerated motion: If the velocity of an object changes from an initial value u to the final value v in time t, the acceleration is a, and this kind of motion is known as accelerated motion.
Example: A bus starts from rest and attains a speed of 72 km h-1 in 5 minutes while moving in uniform motion. Find the acceleration of the bus.
Solution:
Time (t) = 5 minites = 5 × 60 = 300 seconds
Initial velocity u = 0 [∵ bus was in rest]
Final velocity v = 72 km h-1
= (20 - 0)/300
= 0.066 ms-2
Acceleration of object = 0.066 ms-2
Type of accelerations are based on direction of acceleration
Positive acceleration: If the velocity of an object increases with time in the direction of motion of the object, this is known as positive acceleration.
The direction of acceleration is same as the direction of motion.
Main points:
The direction of acceleration is same as the direction of motion.
When the velocity increases with time in the same direction.
When an object starts from rest.
The final velocity is greater than initial velocity i.e (v > u).
Negative acceleration: If the velocity of an object decreases with time in an opposite direction of motion of the object, this is known as negative acceleration.
Retardation: Negative accelaration is known as retardation.
Main points:
The direction of acceleration is opposite to the direction of motion.
Negative accelaraion produces when the velocity decreases with time.
When an object is going to rest.
The final velocity is smaller than initial velocity i.e (v < u).
Uniform accelaration: When the velocity of an object changes by an equal amount in equal intervals of time such a accelaration of the object is known as uniform accelaration.
Uniformly accelarated motion: The motion of an object having uniform accelaration is known as uniformly accelarated motion.
Examples:
- Motion of a rolling ball on smooth surface.
- The motion of a freely falling body.
- Motion of the earth round the sun.
- A high speed lift starts from multi-storied building.
- The motion of the moon round the Earth.
Non-uniform accelaration: When the velocity of an object changes by an unequal amount in equal intervals of time such a accelaration of the object is known as non-uniform accelaration.
Non-uniformly accelarated motion: The motion of an object having different accelaration is known as non-uniformly accelarated motion.
Examples:
- A racing car.
- The motion of vehicles moving at differents speeds.
- The ball is thrown upward at top it get retarded.
- The motion of a bus passing through rough road.
Graphical Representtion of Motion
Graphs are used to represent the motion of an object. Graphs provide a convenient method to present basic information about a variety of events.
Examples:
(i) In the telecast of a one-day cricket match.
(ii) Vertical bar graphs show the run rate of a team in each over.
(iii) As you have studied in mathematics, a straight line graph helps in solving a linear equation having two variables.
Uses of graphs:
(i) Graphs are very useful for comparing the motion of two moving bodies.
(ii) The position of the body at any instant of time can be determined from the distance – time graph.
(iii) Slope of velocity time graph gives the acceleration of an object.
(iv) Straight line graphs help in solving a linear equation in two variables.
(v) Distance time graph tell when and where one body crosses the other body.
To describe the motion of an object, There is one physical quantity, such as distance or velocity, on another quantity, such as time.
There are two conditions to representing motion.
1. The change in the position of an object with time can be represented on the distance-time graphs.
2. The variation in velocity with time for an object moving in a straight line can be represented by a velocity-time graphs.
1. Distance-time graphs:
In this graph:
Time is taken along the x–axis and distance is taken along the y-axis.
Distance-time graphs can be employed under various conditions where objects move with uniform speed, non-uniform speed, remain at rest etc.
Main concept:
The distance travelled by the object is directly proportional to time taken.
There are two types of graphs:
(I) Distance-Time Graphs for Uniform speed.
(II) Distance-Time Graphs for Non-uniform speed.
Distance-Time Graphs for Uniform speed:
Gradient: Slope of the graph.
Horizontal section of graph indicate periods of rest.
The gradient (the slope of graph) of a distance-time graph represents speed.
Uniform Circular Motion : When an object moves in a circular path with uniform speed, its motion is called uniform circular motion.
Example of uniform circular motion :
(i) Motion of the moon around the earth
(ii) Motion of the Earth around the Sun.
(iii) A satellite in a circular orbit around the earth
(iv) A cyclist on a circular track
Total Distance Covered in circular path : 2 π r
Speed of athlete in circular path : 2 π r / t
Factors due to which Changing in the velocity :
(i) Change in magnitude
(ii) Change in direction