9th Class Physics Unit No.2 Kinematics Notes
Rest and Motion
We observe various objects around us, some at rest, and others in motion.
The state of rest or motion of a body is relative; it depends on the observer’s frame of reference. For example, a passenger in a moving bus is at rest relative to other passengers inside the bus, but to an observer outside, the bus and its passengers are in motion.
Types of Motion
Everything in the universe is in motion, but different objects move differently.
There are three types of motion:
(i) Translatory motion (linear, circular, and random)
(ii) Rotatory motion
(iii) Vibratory motion (to and fro motion)
- Chapter No.1 Introduction to Biology
- Chapter No. 2 Solving a Biological Problem
- Chapter No.3 Biodiversity
- Chapter No.4 Cells and Tissues
- Chapter No.5 Cell Cycle
Translatory Motion
In translatory motion, a body moves along a line without rotation, which can be straight or curved. Examples include a car moving on a straight road and an airplane moving straight in the air. The motion of riders in a Ferris wheel is also translatory, moving in a circle without rotation.
Linear Motion
Linear motion refers to the straight-line motion of objects. Examples include a car moving on a straight road and objects falling vertically downward.
Circular Motion
Circular motion occurs when an object moves along a circular path.
Examples include a stone tied to a string and whirled in a circle, a toy train moving on a circular track, and the motion of the Earth around the Sun.
Random Motion
Random motion is characterized by irregular and disordered movement.
Examples include the movements of insects, birds, and dust or smoke particles in the air.
Rotatory Motion
Rotatory motion involves spinning about an axis passing through the body itself.
Examples include the spinning motion of a top and the motion of a wheel about its axis.
Vibratory Motion
Vibratory motion is to-and-fro motion about a mean position.
Examples include a baby in a swing, the pendulum of a clock, and children on a see-saw.
Scalars and Vectors
Physical quantities can be divided into scalars and vectors.
Scalars are described completely by their magnitude alone (e.g., mass, length, time).
Vectors require both magnitude and direction for a complete description (e.g., velocity, displacement, force).
Representing Vectors
Vectors can be represented graphically by a line segment with an arrowhead, where the length of the line represents the magnitude, and the direction of the line represents the direction of the vector.
Distance
Distance is the length of a path between two points.
It is a scalar quantity, represented by ‘S,’ and its unit is meters (m).
The distance traveled by an object depends on the actual path taken, which may be curved or straight.
Displacement
Displacement is the shortest distance and direction between two points.
It is a vector quantity, represented by ‘d,’ and its unit is meters (m).
Displacement takes into account the starting and ending points, ignoring the actual path taken.
Speed and Velocity
Speed is the rate at which an object covers distance. It is a scalar quantity, represented by ‘v,’ and its unit is meters per second (m/s).
Velocity is the rate of displacement of an object. It is a vector quantity, represented by ‘v,’ and its unit is meters per second (m/s).
Both speed and velocity involve the change in distance or displacement over time.
Uniform Speed and Uniform Velocity
A body has uniform speed when it covers equal distances in equal intervals of time.
A body has uniform velocity when it covers equal displacements in equal intervals of time.
Uniform speed and uniform velocity involve constant rates of change without any acceleration.
Acceleration
Acceleration is the rate of change of velocity of a body.
It is a vector quantity, represented by ‘a,’ and its unit is meters per second squared (m/s^2).
Positive acceleration occurs when velocity increases, and negative acceleration (deceleration) occurs when velocity decreases.
Graphical Analysis of Motion
- Distance-Time Graph: It represents the distance covered by an object over time. A horizontal line indicates rest (zero speed), a straight line indicates constant speed, and a curve indicates varying speed.
- Slope of the graph at any point represents the speed of the object at that instant.
- Displacement-Time Graph: It represents the displacement of an object over time. The slope of the graph represents the velocity of the object at that instant.
Remember that distance and displacement may be the same when motion occurs in a straight line, but they can differ when the motion involves curved paths or changes in direction.
There are three basic equations of motion for bodies moving with uniform acceleration.
These equations relate initial velocity, final velocity, acceleration, time, and distance covered by a moving body. The motion is assumed to be along a straight line, so only the magnitudes of displacements, velocities, and acceleration are considered.
Speed-Time Graph
The motion of a body with initial velocity v and uniform acceleration a is represented by a speed-time graph. The slope of the graph gives the acceleration (a) of the body.
First Equation of Motion
The first equation relates initial velocity (vi), final velocity (vf), acceleration (a), and time (t).
It is given by: vf = vi + at
Second Equation of Motion
The second equation relates initial velocity (vi), final velocity (vf), acceleration (a), and displacement (S).
It is given by: S = (vf + vi) * t / 2
Third Equation of Motion
The third equation relates initial velocity (vi), final velocity (vf), acceleration (a), and displacement (S).
It is given by: vf^2 = vi^2 + 2a * S
Motion of Freely Falling Bodies
All freely falling objects have the same acceleration, denoted by ‘g,’ which is approximately 10 m/s^2 on the Earth’s surface. Galileo’s experiment from the leaning tower of Pisa demonstrated that all objects reach the ground at the same time regardless of their mass. The value of ‘g’ is positive for objects falling down and negative for objects moving up.