Rotational Motion
Rotational Motion
Rotational motion occurs when an object spins or revolves around a fixed point or axis. It involves the motion of an object around its own axis without changing its position in space. The rate of rotation is measured in radians per second or revolutions per minute (RPM). Rotational motion is characterized by angular displacement, angular velocity, and angular acceleration. Angular displacement is the angle through which an object rotates, angular velocity is the rate at which an object rotates, and angular acceleration is the rate at which the angular velocity changes. Rotational motion is a fundamental concept in physics and engineering, with applications in various fields such as mechanics, astronomy, and robotics.
Physics of Rotational Motion
Rotational Motion
Rotational motion is the motion of an object around a fixed axis of rotation. The axis of rotation can be either internal or external to the object. For example, when a wheel rolls, the axis of rotation is the center of the wheel. When a person spins around, the axis of rotation is the person’s spinal cord.
Angular Displacement
The angular displacement of an object is the measure of how far the object has rotated around the axis of rotation. The angular displacement is measured in radians. One radian is the angle formed by two radii of a circle that intersect at the center of the circle and are separated by an arc length equal to the radius of the circle.
Angular Velocity
The angular velocity of an object is the rate at which the object is rotating around the axis of rotation. The angular velocity is measured in radians per second. The angular velocity of an object is constant if the object is rotating at a constant speed.
Angular Acceleration
The angular acceleration of an object is the rate at which the angular velocity of the object is changing. The angular acceleration is measured in radians per second squared. The angular acceleration of an object is constant if the object is rotating with a constant angular acceleration.
Torque
Torque is the force that causes an object to rotate around an axis. The torque is measured in newtonmeters. The torque is equal to the product of the force and the perpendicular distance from the axis of rotation to the point where the force is applied.
Moment of Inertia
The moment of inertia of an object is a measure of the object’s resistance to rotational motion. The moment of inertia is measured in kilogrammeters squared. The moment of inertia of an object depends on the mass of the object and the distribution of the mass around the axis of rotation.
Equations of Rotational Motion
The equations of rotational motion are similar to the equations of linear motion. The following are the equations of rotational motion:
 Angular displacement: θ = ωt
 Angular velocity: ω = dθ/dt
 Angular acceleration: α = dω/dt
 Torque: τ = Iα
where:
 θ is the angular displacement in radians
 ω is the angular velocity in radians per second
 α is the angular acceleration in radians per second squared
 τ is the torque in newtonmeters
 I is the moment of inertia in kilogrammeters squared
Examples of Rotational Motion
There are many examples of rotational motion in everyday life. Some examples include:
 A wheel rolling
 A person spinning around
 A fan rotating
 A car engine running
 A planet orbiting the sun
Applications of Rotational Motion
Rotational motion is used in a variety of applications, including:
 Transportation: Wheels are used to transport people and goods.
 Power generation: Turbines are used to generate electricity.
 Manufacturing: Robots are used to perform tasks in factories.
 Sports: Rotational motion is used in a variety of sports, such as baseball, basketball, and golf.
Rotational Kinematics
Rotational kinematics is the study of the motion of rotating objects. It is a branch of classical mechanics that deals with the description of the motion of objects around a fixed axis of rotation. Rotational kinematics is analogous to linear kinematics, which deals with the motion of objects in a straight line.
The basic quantities used in rotational kinematics are:
 Angular displacement: The angular displacement of an object is the measure of how much it has rotated about an axis. It is measured in radians (rad). One radian is the angle subtended by an arc of a circle that is equal in length to the radius of the circle.
 Angular velocity: The angular velocity of an object is the rate at which it is rotating about an axis. It is measured in radians per second (rad/s).
 Angular acceleration: The angular acceleration of an object is the rate at which its angular velocity is changing. It is measured in radians per second squared (rad/s²).
The following equations relate these quantities:
 Angular displacement: θ = ωt
 Angular velocity: ω = dθ/dt
 Angular acceleration: α = dω/dt
where:
 θ is the angular displacement (in radians)
 ω is the angular velocity (in radians per second)
 t is the time (in seconds)
 α is the angular acceleration (in radians per second squared)
Examples of rotational kinematics:
 A child on a playground swing is rotating about a fixed axis. The angular displacement of the child is the angle through which they have swung. The angular velocity of the child is the rate at which they are swinging. The angular acceleration of the child is the rate at which their angular velocity is changing.
 A car driving around a curve is rotating about a fixed axis. The angular displacement of the car is the angle through which it has turned. The angular velocity of the car is the rate at which it is turning. The angular acceleration of the car is the rate at which its angular velocity is changing.
 A spinning top is rotating about a fixed axis. The angular displacement of the top is the angle through which it has spun. The angular velocity of the top is the rate at which it is spinning. The angular acceleration of the top is the rate at which its angular velocity is changing.
Rotational kinematics is used in a variety of applications, such as:
 Designing and analyzing machines
 Controlling robots
 Simulating physical systems
 Studying the motion of celestial bodies
Axis of Rotation
The axis of rotation is an imaginary line around which an object rotates. It is a fundamental concept in physics and engineering, used to describe the motion of objects such as wheels, gears, and planets.
Examples of Axis of Rotation:

Spinning Top: When a spinning top is in motion, it rotates around its axis of rotation, which is the vertical line passing through the center of the top.

Earth’s Rotation: The Earth rotates around its axis of rotation, which is the imaginary line passing through the North and South Poles. This rotation causes day and night.

Gears: In a gear system, the gears rotate around their respective axes of rotation. The teeth of the gears mesh together, causing them to rotate in a synchronized manner.

Bicycle Wheel: The bicycle wheel rotates around its axis of rotation, which is the imaginary line passing through the center of the wheel.

Electric Motor: In an electric motor, the rotor rotates around its axis of rotation, which is the imaginary line passing through the center of the motor.
Importance of Axis of Rotation:
The axis of rotation is important because it determines the direction and speed of rotation of an object. It also plays a crucial role in balancing and stability. For example, in the case of a spinning top, the axis of rotation determines the stability of the top. If the axis of rotation is not aligned properly, the top will wobble and eventually fall over.
In conclusion, the axis of rotation is a fundamental concept in physics and engineering that describes the motion of rotating objects. It is an imaginary line around which an object rotates and plays a crucial role in determining the direction, speed, and stability of the rotation.
What Is Torque
What is Torque?
Torque is a force that causes an object to rotate about an axis. It is measured in newtonmeters (N·m) or poundfeet (lb·ft). The amount of torque depends on the force applied and the distance from the axis of rotation.
Examples of Torque
 When you turn a screw, you are applying torque to the screw. The force you apply is the force you use to turn the screwdriver, and the distance from the axis of rotation is the distance from the center of the screw to the point where you are applying the force.
 When you open a door, you are applying torque to the door. The force you apply is the force you use to push or pull the door, and the distance from the axis of rotation is the distance from the hinges to the point where you are applying the force.
 When a car engine is running, the pistons are applying torque to the crankshaft. The force is the force of the pistons pushing against the cylinder walls, and the distance from the axis of rotation is the distance from the center of the crankshaft to the center of the pistons.
Applications of Torque
Torque is used in a wide variety of applications, including:
 Automotive: Torque is used to turn the wheels of a car. The engine produces torque, which is then transmitted to the wheels through the transmission and driveline.
 Industrial: Torque is used to power a variety of machines, such as conveyor belts, pumps, and compressors.
 Construction: Torque is used to lift heavy objects, such as beams and concrete slabs.
 Sports: Torque is used to generate power in a variety of sports, such as baseball, golf, and tennis.
Conclusion
Torque is a fundamental concept in physics that has a wide range of applications. By understanding torque, you can better understand how the world around you works.
Angular Momentum
Angular momentum is a fundamental concept in physics that describes the rotational motion of an object. It is defined as the product of the moment of inertia of the object and its angular velocity. The moment of inertia is a measure of the object’s resistance to angular acceleration, and the angular velocity is the rate at which the object is rotating.
Angular momentum is a conserved quantity, which means that it cannot be created or destroyed. This means that the total angular momentum of a closed system remains constant, even if the individual components of the system change. For example, if a spinning top slows down, its angular momentum will be transferred to the surrounding air.
Angular momentum is important in many areas of physics, including mechanics, electromagnetism, and quantum mechanics. In mechanics, angular momentum is used to describe the motion of rotating objects, such as planets, stars, and galaxies. In electromagnetism, angular momentum is used to describe the behavior of charged particles in magnetic fields. In quantum mechanics, angular momentum is used to describe the spin of electrons and other subatomic particles.
Here are some examples of angular momentum in everyday life:
 A spinning top has angular momentum.
 A planet orbiting the sun has angular momentum.
 A car driving around a curve has angular momentum.
 A person spinning in a chair has angular momentum.
The angular momentum of an object can be calculated using the following formula:
L = Iω
where:
 L is the angular momentum (in kilogrammeters squared per second)
 I is the moment of inertia (in kilogrammeters squared)
 ω is the angular velocity (in radians per second)
The moment of inertia of an object can be calculated using the following formula:
I = Σmr²
where:
 I is the moment of inertia (in kilogrammeters squared)
 m is the mass of the object (in kilograms)
 r is the distance from the axis of rotation to the center of mass of the object (in meters)
The angular velocity of an object can be calculated using the following formula:
ω = Δθ/Δt
where:
 ω is the angular velocity (in radians per second)
 Δθ is the change in angle (in radians)
 Δt is the change in time (in seconds)
Angular momentum is a powerful concept that can be used to describe a wide variety of phenomena. It is a fundamental property of matter and plays an important role in many areas of physics.
Rotational Motion – Important Topics
Rotational Motion – Important Topics
Rotational motion is the motion of an object around a fixed axis of rotation. It is a fundamental concept in physics and has applications in many areas, such as engineering, mechanics, and astronomy.
Important Topics in Rotational Motion
 Angular displacement: The angular displacement of an object is the measure of how far it has rotated around an axis. It is measured in radians, where one radian is the angle formed by an arc of length equal to the radius of the circle.
 Angular velocity: The angular velocity of an object is the rate at which it is rotating around an axis. It is measured in radians per second.
 Angular acceleration: The angular acceleration of an object is the rate at which its angular velocity is changing. It is measured in radians per second squared.
 Moment of inertia: The moment of inertia of an object is a measure of its resistance to rotational motion. It depends on the object’s mass and its distribution of mass around the axis of rotation.
 Torque: Torque is the force that causes an object to rotate around an axis. It is measured in newtonmeters.
 Rotational kinetic energy: The rotational kinetic energy of an object is the energy it has due to its rotation. It is measured in joules.
Examples of Rotational Motion
 A wheel rolling down a hill
 A fan spinning
 A planet orbiting the sun
 A dancer pirouette
Applications of Rotational Motion
Rotational motion has many applications in the real world, including:
 Engineering: Rotational motion is used in many engineering applications, such as gears, pulleys, and turbines.
 Mechanics: Rotational motion is used in many mechanical devices, such as engines, motors, and generators.
 Astronomy: Rotational motion is used to study the motion of planets, stars, and galaxies.
Conclusion
Rotational motion is a fundamental concept in physics with many applications in the real world. By understanding the important topics in rotational motion, you can better understand how the world around you works.
Rotational Motion – Important Questions
Rotational Motion – Important Questions
1. What is rotational motion?
Rotational motion is the motion of an object around a fixed axis. The axis can be either internal or external to the object. For example, when a wheel rolls, it undergoes rotational motion around its axle. When a person swings a bat, the bat undergoes rotational motion around the person’s hand.
2. What are the key concepts of rotational motion?
The key concepts of rotational motion include:
 Angular displacement: The angular displacement of an object is the measure of how far it has rotated around an axis. It is measured in radians.
 Angular velocity: The angular velocity of an object is the rate at which it is rotating around an axis. It is measured in radians per second.
 Angular acceleration: The angular acceleration of an object is the rate at which its angular velocity is changing. It is measured in radians per second squared.
 Moment of inertia: The moment of inertia of an object is a measure of its resistance to rotational motion. It depends on the object’s mass and its distribution of mass around the axis of rotation.
3. What are the equations of rotational motion?
The equations of rotational motion are similar to the equations of linear motion. They include:
 Angular displacement: θ = ωt + 1/2αt^2
 Angular velocity: ω = ω0 + αt
 Angular acceleration: α = (ωf  ω0)/t
where:
 θ is the angular displacement
 ω is the angular velocity
 ω0 is the initial angular velocity
 α is the angular acceleration
 t is the time
4. What are some examples of rotational motion?
Some examples of rotational motion include:
 The rotation of the Earth around its axis
 The rotation of a wheel
 The rotation of a fan
 The rotation of a planet around the Sun
 The rotation of a satellite around a planet
5. What are some applications of rotational motion?
Rotational motion has many applications, including:
 Transportation: Wheels are used to transport people and goods.
 Energy production: Wind turbines use the rotational motion of the wind to generate electricity.
 Manufacturing: Rotational motion is used in many manufacturing processes, such as drilling, milling, and turning.
 Robotics: Robots use rotational motion to move their arms and legs.
 Sports: Rotational motion is used in many sports, such as baseball, golf, and tennis.
Rotational Motion for JEE Advanced – Concepts & Questions
Rotational Motion
Rotational motion is the motion of an object around a fixed axis. The axis of rotation can be either fixed in space or rotating with the object. Rotational motion is described by the following quantities:
 Angular displacement: The angular displacement of an object is the measure of how much it has rotated around its axis. It is measured in radians.
 Angular velocity: The angular velocity of an object is the rate at which it is rotating around its axis. It is measured in radians per second.
 Angular acceleration: The angular acceleration of an object is the rate at which its angular velocity is changing. It is measured in radians per second squared.
Concepts of Rotational Motion
The following are some of the key concepts of rotational motion:
 Moment of inertia: The moment of inertia of an object is a measure of its resistance to rotational motion. It depends on the mass of the object and its distribution around the axis of rotation.
 Torque: Torque is the force that causes an object to rotate around its axis. It is equal to the product of the force and the perpendicular distance from the axis of rotation to the point where the force is applied.
 Newton’s laws of motion: Newton’s laws of motion also apply to rotational motion. The first law states that an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless acted upon by an external force. The second law states that the acceleration of an object is directly proportional to the net force applied to it and inversely proportional to its mass. The third law states that for every action, there is an equal and opposite reaction.
Questions on Rotational Motion
The following are some sample questions on rotational motion:
 A wheel is rotating at a constant angular velocity of 10 rad/s. What is the angular displacement of the wheel after 5 seconds?
 A torque of 10 N·m is applied to a wheel with a moment of inertia of 2 kg·m². What is the angular acceleration of the wheel?
 A ball is thrown horizontally from the top of a building. What is the angular velocity of the ball when it hits the ground?
Examples of Rotational Motion
Rotational motion is a common occurrence in everyday life. Some examples include:
 The rotation of the Earth around its axis
 The rotation of a fan blade
 The rotation of a car wheel
 The rotation of a washing machine drum
Rotational motion is also used in many machines, such as engines, generators, and pumps.
Frequently Asked Questions on Rotational Motion
What is rotational motion? Give an example.
Rotational Motion
Rotational motion is the motion of an object around a fixed axis of rotation. The axis of rotation can be either internal or external to the object. For example, when a wheel rolls, the axis of rotation is the center of the wheel. When a person swings a bat, the axis of rotation is the point where the bat is held.
Rotational motion can be described by a number of different variables, including:
 Angular displacement: The angular displacement of an object is the measure of how far it has rotated around its axis of rotation. Angular displacement is measured in radians.
 Angular velocity: The angular velocity of an object is the rate at which it is rotating around its axis of rotation. Angular velocity is measured in radians per second.
 Angular acceleration: The angular acceleration of an object is the rate at which its angular velocity is changing. Angular acceleration is measured in radians per second squared.
Rotational motion is a common type of motion in the real world. Some examples of rotational motion include:
 The rotation of the Earth on its axis
 The rotation of a wheel
 The rotation of a fan
 The rotation of a planet around the Sun
Rotational motion can be used to do work. For example, a windmill uses the rotational motion of its blades to generate electricity. A car engine uses the rotational motion of its pistons to power the car.
Rotational motion is also used in many sports. For example, a baseball pitcher uses the rotational motion of his arm to throw the ball. A golfer uses the rotational motion of his body to swing the club.
Rotational motion is a fundamental concept in physics. It is used to describe a wide variety of phenomena, from the rotation of the Earth to the motion of a spinning top.
What is the reason for rotational motion?
Rotational motion occurs when an object spins or revolves around an axis. It is a fundamental concept in physics and has numerous applications in various fields. The primary reason for rotational motion is the application of a torque, which is a force that causes an object to rotate.
Understanding Torque:
Torque is a vector quantity that measures the twisting force applied to an object. It is defined as the product of the force (F) applied to an object and the perpendicular distance (r) from the axis of rotation to the point where the force is applied. Mathematically, torque (τ) is calculated as:
τ = F × r
The direction of torque is determined by the righthand rule. If you curl the fingers of your right hand in the direction of the applied force, your thumb will point in the direction of the torque vector.
Examples of Rotational Motion:

Spinning Top: When you spin a top, you apply a torque by pushing it with your hand. The force you apply creates a rotational motion around the top’s axis.

Ceiling Fan: A ceiling fan rotates due to the torque generated by an electric motor. The motor applies a force to the fan blades, causing them to spin.

Car Wheels: When you drive a car, the engine generates torque that is transmitted to the wheels. This torque causes the wheels to rotate, propelling the car forward.

Planets Orbiting the Sun: The planets in our solar system rotate around the Sun due to the gravitational torque exerted by the Sun. The Sun’s gravitational pull creates a force that causes the planets to move in circular paths.
Applications of Rotational Motion:
Rotational motion has a wide range of applications in various fields, including:

Engineering: Rotational motion is essential in machines and mechanisms, such as gears, pulleys, and wheels.

Transportation: Cars, airplanes, and other vehicles use rotational motion in their engines and wheels.

Energy Generation: Wind turbines and hydroelectric generators convert rotational motion into electrical energy.

Sports: Many sports involve rotational motion, such as throwing a ball, swinging a bat, or performing gymnastic routines.

Robotics: Robots often use rotational motion in their joints and actuators to perform various tasks.
In summary, rotational motion occurs when a torque is applied to an object, causing it to spin or revolve around an axis. It is a fundamental concept in physics with numerous applications in various fields, from engineering and transportation to energy generation and sports.
Is circular motion the same as rotational motion? Explain.
Circular motion and rotational motion are two closely related concepts in physics, but they are not exactly the same.
Circular motion refers to the motion of an object along a circular path. The object’s velocity is constantly changing as it moves around the circle, and its acceleration is always directed towards the center of the circle.
Rotational motion refers to the motion of an object around a fixed axis. The object’s angular velocity is constant, and its angular acceleration is always directed perpendicular to the axis of rotation.
Examples of circular motion:
 A planet orbiting the sun
 A car going around a curve
 A ball spinning on a string
Examples of rotational motion:
 A wheel rolling down the street
 A fan spinning
 A person doing a somersault
The main difference between circular motion and rotational motion is that circular motion involves the motion of an object along a circular path, while rotational motion involves the motion of an object around a fixed axis.
In some cases, an object can be undergoing both circular motion and rotational motion at the same time. For example, a planet orbiting the sun is also rotating on its own axis.
Here is a table summarizing the key differences between circular motion and rotational motion:
Feature  Circular Motion  Rotational Motion 

Path  Circular  Around a fixed axis 
Velocity  Constantly changing  Constant 
Acceleration  Always directed towards the center of the circle  Always directed perpendicular to the axis of rotation 
Examples  Planet orbiting the sun, car going around a curve, ball spinning on a string  Wheel rolling down the street, fan spinning, person doing a somersault 