What is motion in Physics Class 9 Tnpsc

What is motion and its types?

Motion definition physics

In Physics, the objects which change their position are said to be motion. Similarly, an object which does not change its position is said to be rest. Even stationary object moves along with the speed of the earth.

Any object under the influence of force is said to be in rest or in motion.

Example of Motion

  • Car, bus moving road.

  • Train running on rails.

  • Blades rotating in fans.

  • Ball hit by a batsmen in cricket.

  • Flying bird, kite, jet etc.

  • Kinematics is a branch of mechanics that deals with the motion of objects without force.

Types of Motion

Based on the direction of movement the motion is classified into different types. There are various types of motion in physics are Linear, Circular, Rotational and Vibratory motion.

Linear Motion

A thing is said to be linear motion if it moves in a straight line. Example – A person running in a straight line, A object falling towards the ground. The movement of coins on a carrom board is an example of rectilinear motion.

Circular Motion

A movement described by an object traveling in a circular path is called Circular motion. Example: The rotation of a stone attached to a thread, rotation of the earth around the sun.

Rotational Motion

Anything that moves in a rotation about an axis is called rotation. An example is the Rotation of Disc, Spinning of the Earth on its axis.

Vibratory Motion

If an object undergoes to and fro movement in a fixed point it is said to be in vibratory or oscillatory motion. An example is the Vibration of string on a guitar, Movement of a swing.

Also, there is Elliptical and helical motion.

Motion in One, Two and Three Dimension

Motion in One Dimension

“1-D motion is the movement of a particle moving in a straight line”.

Example- running of the train in a straight track, an apple falling from a tree in a straight line.

Motion in Two Dimension

If a particle is moving along a curved path in a plane, then it is said to be in two-dimensional motion”.

An example of 2-d motion is the Movement of the coin in carrom board, insect crawling in the wall

Motion in Three Dimension

A particle moving in a common three-dimensional space has three-dimensional motion.

Example: Flying bird, the movement of gas molecules, etc.

Distance and Displacement

Distance is the actual path length traveled by an object in a given interval of time during the motion. Distance is a scalar quantity. Displacement is the difference between the final and initial positions of the object in a given interval of time.

Displacement is a vector quantity.


Momentum is simply the product of mass with velocity. Momentum is a vector quantity. The direction of momentum is also in the direction of velocity and the magnitude of momentum is equal to the product of mass and speed of the particle.

P = mv.

The momentum of the particle plays a vital role in Newton’s laws. The unit of momentum is kg ms-1.

Accelerated Motion

The velocity of an object is not constant but changes from time to time and such movement is called accelerated motion. In accelerated motion, if the change in velocity of an object per unit is the same then the object is said to be moving with uniformly accelerated motion.

If the change in velocity per unit time is different at different times, then the object is said to be moving with non-uniform accelerated motion. Acceleration is a vector quantity.

Its SI unit is ms-2. Acceleration is positive if velocity is increasing and is negative if the velocity is decreasing. The negative acceleration is called Retardation or deceleration.

Movement Under Gravity

A straight line motion with constant acceleration is the motion of an object near the surface of the earth. The acceleration due to gravity ‘g’ is constant.

“The motion of a body falling towards the Earth from a small altitude (h << R), purely under the force of gravity is called free fall”. Where R is the radius of Earth.

Galileo concluded that in a vacuum all objects fall with the same acceleration g and reach the ground at the same time.


When an object is thrown in the air with some initial velocity and then allowed to move under the action of gravity alone, the object is known as a projectile. The path followed by the particle is called a trajectory.

Examples of Projectile are:

  • A thing or object dropped from moving car.
  • A Bullet fired from a gun.
  • A ball is thrown to the keeper by a fielder.
  • A batsman hitting a six.

The projectile moves under the combined effect of two velocities:

A uniform velocity in the horizontal direction, will not change provided there is no air resistance. A uniformly changing velocity (i.e increasing or decreasing) in the vertical direction.

There are two types of projectile:

The projectile is given an initial velocity in the horizontal direction (horizontal projection). The projectile is given an initial velocity at an angle to the horizontal (angular projection).

To study projection:

Air resistance is neglected. The effect due to the rotation of Earth and the Curvature of Earth is negligible. The acceleration due to gravity is constant in magnitude and direction at all points of the motion of the projectile.

Time of Flight

“The time taken for the projectile to complete its trajectory or time taken by the projectile to hit the ground is called Time of flight”.

Angular Displacement

“The angle described by the particle about the axis of rotation in a given time is called Angular Displacement”.

Angular Velocity (ω)

“The rate of change of angular displacement is called Angular Velocity”.

Angular Acceleration (𝜶)

“The rate of change of angular velocity is called Angular Acceleration”.

Circular Motion

When a point object is moving on a circular path with constant speed, it covers equal distances on the circumference of the circle in equal intervals of time. Then the object is said to be a uniform circular motion.

In a uniform circular motion, the velocity always changes with speed remains the same. If the velocity changes in both speed and direction during the circular motion, we get non-uniform circular motion.

Centripetal Acceleration

The acceleration is acting towards the centre of the circle is called Centripetal acceleration.

Physics laws of motion

Aristotle’s Idea of Motion – To maintain motion, force is required.

Galileo’s concept of Motion – To maintain motion, force is not required.

Newton’s First Law – every object continuously has its state of rest or state of motion, until the external force acts on it.

Newton’s Second Law – Force is equal to the rate of change of momentum.

Both newton’s first law and second law, are valid only in inertial frames.

Inertial frames are the ones in which if there is no force acting on the object, the object moves at a constant velocity.

Newtons Third Law – For every action, there is an equal and opposite reaction.

Lami’s theorem – If an object is in equilibrium under the concurrent force then the ratio of each force with the sine of the corresponding opposite angle is the same.

An impulse acting on a body is equal to the change in momentum of the body. Whenever a force acts on the object for short time, it gets difficult to calculate the force but the impulse can be measured.

Friction – It is a resistance offered by one surface when rubbing against another surface.

Static Friction – It is a force that opposes the movement of the object from rest.

If the object starts to move, kinetic friction comes into effect. To move an object with constant velocity, the external force must be applied to overcome kinetic friction. Rolling friction is less than static and kinetic friction. An example is Rolling Suitcase.

The origin of friction is electromagnetic interaction between the atoms of two surfaces that touch each other. In a uniform circular motion, the centripetal force acts at the centre of the circle.

In sun’s gravitational force acts as centripetal force. When the moon orbits the earth, it encounters a gravitational force as a centripetal force. When a stone is attached to the thread, and rotated, the centripetal force is given by the string.

Centrifugal force arises when the motion is measured from a rotation frame. Centrifugal force is pseudo force. The inertial motion of the object appears to the centrifugal force in the rotating frame. The centripetal force acts towards the centre of the circular motion and centrifugal force appears to acts in the opposite direction to centripetal force.


Tamil Nadu Text Book Corporation. Physics. vol. Volume-I, Department of School Education, 2018. 2 vols. Department of School Education Government of Tamilnadu, http://tnschools.gov.in/textbooks.

0 0 votes
Article Rating
Notify of
Inline Feedbacks
View all comments
* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.
Would love your thoughts, please comment.x