Rigid Bodies Translational Motion and Rotational Motion
Translational motion is one of the fundamental types of motion in physics that helps us understand how objects move from one place to another without rotation. Along with this, rotational motion describes how objects spin about an axis. Understanding the difference between translational and rotational motion is essential for building strong concepts in mechanics. Students are often asked to distinguish between translational and rotational motion, especially in school exams and competitive exams like JEE and NEET. In this article, we will explore the concept of translational motion, types such as rectilinear and curvilinear motion, and clearly explain the difference between translational and rotational motion with examples and applications in a simple and student-friendly manner.
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- Rigid body
- What is Translational motion
- Rotational motion :
- Difference Between Translational Motion and Rotational Motion :
- Applications of Translational and Rotational Motion:
Rigid body
A rigid body is defined as a body in which the distance between any two particles remains constant, even when external forces are applied. In other words, a rigid body does not undergo any change in shape or size, i.e., it has negligible or zero deformation.
There are two types of rigid body motion
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Translational motion
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Rotational motion
What is Translational motion
Translational motion is the motion of a body in which all its particles move through equal distances in the same direction in a given time. In such motion, every point of the body follows a path parallel to that of any other point.
If a body moves along a straight line, it is called rectilinear translational motion, and if it moves along a curved path, it is called curvilinear translational motion. Thus, translational motion is not limited to one-dimensional motion; it can be one, two, or three-dimensional depending on the path.
Explanation of examples of translatory motion :
Translational motion of the box on a straight line
Translational motion of the box in an inclined plane
When a box is placed on an inclined plane and pushed or allowed to slide down, it undergoes translational motion. In this motion, the entire body of the box moves along the plane, and every particle of the box travels in the same direction. The distance between any two particles of the box remains constant throughout the motion, indicating that the body does not deform.
If we consider any two points inside the box, their relative positions do not change during motion. Hence, all particles experience the same displacement and acceleration. This shows that the motion of each particle is identical to the motion of the whole body.
In translational motion, the body behaves like a single particle, and its orientation remains unchanged. If all parts of a rigid body move parallel to each other, the motion is called pure translational motion. Thus, a box moving on an inclined plane is a clear example of translational motion.
NCERT Physics Notes:
Types of translational motion :
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Rectilinear motion
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Curvilinear motion
| Rectilinear Motion | Curvilinear Motion |
| Motion along a straight line | Motion along a curved path |
| Direction remains constant | Direction continuously changes |
| Path is straight | Path is curved |
| Usually one-dimensional | Two or three-dimensional |
| Velocity direction remains same | Velocity direction changes at every point |
| Example: Car on straight road, lift motion |
Example: Projectile motion, car on curved road, planet motion |
Rectilinear motion can be of two types:
- Uniform rectilinear motion – when an object moves in a straight line with a constant speed.
- Non-uniform rectilinear motion – when the speed of the object changes while moving in a straight line.
Rotational motion :
When a body moves in a circular path about an axis, the motion is called rotational motion. The axis is called axis of rotation.
Example of rotational motion –motion of a wheel, motion of a ball.
Explanation of rotational motion :
Rotational motion of a wheel around its axis passing through the centre.
When a wheel rotates around the axis passing normally through its centre then the motion is rotational motion. The whole body as well as the constituent particles of the wheel, rotate along the axis . The wheel traces a circular path on its axis so it is a rotational motion.
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The motion of a ball when it spins around its axis that passes normally through the centre of the ball then the ball and its constituent particles show rotational motion.
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In the same ball when the axis is shifted to the circumference, then the whole ball will rotate around the axis . In this case also the ball traces a circular path. This is also rotational motion.
In pure rotational motion, every constituent particle shows rotational motion along with the whole body. The rotation of a rigid body about a fixed axis is known as axis of rotation. The pure rotation is that during any time interval, every constituent particle of the rigid body experiences the same angular displacement. The whole rigid body behaves as a particle in case of pure rotation.
Difference Between Translational Motion and Rotational Motion :
| Translational Motion | Rotational Motion |
| The whole object moves in a straight line or along a path from one place to another. | The object spins or rotates about a fixed axis. |
| Every point of the object moves the same distance in the same time. | Different points of the object move through different distances in the same time. |
| Example: A car moving on a straight road. | Example: A spinning fan or a rotating wheel. |
| It is measured in terms of displacement, velocity, and acceleration. | It is measured in terms of angular displacement, angular velocity, and angular acceleration. |
| The motion is linear. | The motion is angular. |
Applications of Translational and Rotational Motion:
Translational Motion
1. A car moving on a straight road.
2. A train moving along the tracks.
3. A ball rolling in a straight line (its center shows translational motion).
4. Movement of an elevator up and down.
Rotational Motion
1. The rotation of a ceiling fan.
2. The spinning of the Earth on its axis.
3. The rotation of a wheel or turbine.
4. The blades of a mixer or washing machine drum.
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Frequently Asked Questions (FAQs)
A system of particles in which the distance between any two particles is constant ,this type of system or body is called rigid body.
At any time interval when a rigid body and its constituent particles moves in uniformly in the same direction ,then the motion is called translational motion.
When a body undergoes translational and rotational motion simultaneously , then its motion is called mixed motion.
Example – A wheel rotates on its axis at its centre, that is rotational motion . While moving on its axis when it moves (translates) along the road then it shows mixed motion.
Two equal , parallel and opposite forces,having different lines of action , acting simultaneously on a body ,this type of force is called coupled force.
Moment of inertia of a body about an axis of rotation can be defined as the torque acting on the body divided by the respective angular acceleration which is generated about the same axis of rotation
Translational motion occurs when all the particles of a rigid body move the same distance in the same direction. Example – a car moving straight on a road.
Rotational motion occurs when a rigid body spins about a fixed axis, and every point in the body moves in a circular path. Example – a spinning wheel or fan.
A rigid body is one whose shape and size do not change even when forces are applied.
A rotating body is a rigid body that spins around a fixed axis.
Translational equilibrium happens when the net force on a body is zero, so it does not move in a straight line.
Rotational equilibrium happens when the net torque on a body is zero, so it does not rotate or change its angular motion.
