Science > Physics > Motion in a Straight Line > Classification of Motion
Motion is an important part of our life. Our daily activities involve motion of different kinds. In this topic, we shall study the terminology of Mechanics, concept of motion and classification of motion.
- Notes
- Very Short Answer Type Questions
- Short Answer Type Questions
- Long Answer Type Questions
- Essay Type Answer Questions
- Concept Application
Terminology:
- Mechanics: The branch of physics which deals with the effects of forces on object is called mechanics. Mechanics is further classified into dynamics and statics.
- A Point Object: In the study of Mechanics we consider bodies or objects as particles or point objects. An object is said to be a point object if its dimensions are negligible as compared to the distance travelled by it. For example distance between stars is so large that for practical purpose those stars can be considered as particles or point objects.
- Body in Motion: A body is said to be in motion if it changes its position with respect to its immediate surroundings. It is to be noted that the motion of a body is a relative concept.
- Body at Rest: A body is said to be at rest if it does not change its position with respect to its immediate surroundings.
- Dynamics: The branch of physics (mechanics) which deals with the motion of the bodies and the forces causing it is called dynamics. It is further classified into kinematics and kinetics. Newton contributed a lot to dynamics by postulating his famous laws of motion.
- Kinematics: The branch of physics (mechanics) which deals with the motion of the bodies without considering the forces causing it is called kinematics. i kinematics measurement of time is essential.
- Kinetics: The branch of physics which deals with the motion of bodies considering cause of their motion.
- Statics: It is the branch of physics which deals with objects at rest or in equilibrium under the action of forces. the measurement of time is not essential in statics.
Motion: Meaning
The moving object is either a particle (by which we mean a point-like object such as an electron) or an object that moves like a particle (such that every portion moves in the same direction and at the same rate).
Motion of Living and Non Living Yhings:
Motion is characteristics of all living things. The movement of animals is called locomotion. Locomotion enables them to obtain food and to escape away from danger. Plants are fixed to substratum, hence they cannot locomate but they show movement. For example roots are geotropic while shoots are phototropic. The energy for movement of living things is derived from food which is metabolized in the body of living organism and energy is released.
Non-living things cannot move on their own, they are made to move. In this series of study of motion, we shall be considering motion of non-living things only.
A body is said to be in motion if it changes its position with respect to its immediate surroundings.
Motion is a Relative Concept:
The motion of a body is a relative concept. When we are specifying the motion it is with respect to some observer. Let us consider to persons say A and B in a lift moving upward. There is another person C standing outside the lift. Now though the lift is going up for A and B there is no change in positions with respect to each other thus they are at rest with respect to eachother. Thus for A and B, there is no motion with respect to each other but for both of them, C is moving downward. Now for C both A and B are moving upward. Hence motion is relative.
With respect to the earth surface, we may be at rest but we are moving about 100,000 km hr-1 relative to sun.
Nothing in the universe is in absolute motion or is at absolute rest.
In order to know whether the position of an object changes with or not, a point absolutely fixed in space has to be chosen as reference point. All human bodies are moving in a space with respect to eachother. Hence they cannot be considered as reference point. Thus no such fixed reference point is available in the space. It means no object in universe is in state of absolute rest. As no object in universe is at absolute rest, so the absolute motion cannot be realized. Due to absence of absolute rest and absolute motion, only relative rest and relative motion can be realized.
Note: If two objects are moving in same direction on same straight line or two parallel straight lines then they are in relative motion only when they have different velocities.
Concept of Reference Frames:
A frame of reference is the frame in which the observer sits and maks observations. There is no rule or restriction on the choice of frame. We can choose a frame of reference to descibe the situation under study according to our convinience. For example, when we are travelling in a train then the compartment of the train in which we are travelling and bearth on you are sitting can be chosen as frame of reference.
There are two type of reference frames:
- Inertial Reference Frame: The frame of reference which is either at rest or moving with constant velocity is known as inertial frame of reference.
- Non-Inertial Reference Frame: A frame of reference moving with some acceleration is known as non-inertial reference frame.
Classification of Motion:
Classification of Motion on the Basis of Motion of Particles of a Body:
Motion can be classified into random motion, translational motion, rotational motion, and vibrational or oscillatory motion.
Random Motion:
In this motion particles move randomly in any possible direction and in any possible velocity. Thus, the path and the direction are not definite.
Example: Motion of gas molecules. Such random motion of molecules of gas is called molecular chaos.
Translational Motion:
In this motion every particle of the body has the same displacement.
The translational motion can be along straight line or along a curved path. Motion along a straight line is called a rectilinear motion (e.g. motion of car in a straight line) and the motion of a body along a curved path is called curvilinear motion (e.g. the motion of a ball thrown in air, the motion of the earth around the Sun).
It is to be noted that in translatory motion, body does not change its orientation.
Rotational Motion:
In this motion the particles of body revolve in a circle about the same axis.
Examples: Motion of a fan, the motion of a wheel of moving vehicle, the motion of merry go round, etc.
Oscillatory or Vibrational Motion:
In this motion the body moves to and fro about a fixed point along the same path.
Examples: The motion of the bob of a pendulum, vibrating string of guitar, etc.
Classification of Motion on the Basis of Positional Reference Coordinates of Particles:
One Dimensional Motion:
The motion of an object is said to be one-dimensional if only one of the three coordinates specifying the position object changes with a time. In such motion the object moves in a straight line. This motion is also known as rectililnear motion or linear motion.
Example: Motion of a train along a straight track
Two Dimensional Motion:
The motion of an object is said to be two-dimensional if only two of the three coordinates specifying the position object changes with a time. In such motion the object moves along a plane.
Examples: Motion of planets around the sun, Motion of a car on curved flat road, circular motion, projectile motion, etc.
Three Dimensional Motion:
The motion of an object is said to be tree-dimensional if all the three coordinates specifying the position object changes with a time. In such motion the object moves in a space.
Example: A bir flying freely in the sky, a kite flying on windy day
Rectilinear Motion or One Dimensional Motion:
When a body moves along a straight-line path, its motion is called the one-dimensional motion or motion in a straight line or rectilinear motion. Example: the motion of a car along a straight road.
The Position of a Body or Particle:
Assuming the direction of the motion along the x-axis, the path of one-dimensional motion can be represented by a straight line parallel to the x-axis then each point on the straight line represents the position of the particle at a different instant of time. The position of the particle at any instant can be specified by its x-coordinate. The x-coordinate changes with time.
Concepts:
Very Short Answer Type Questions
Q1. What is mechanics?
The branch of physics which deals with the effects of forces on object is called mechanics.
Q2. What are the different branches of mechanics?
Dynamics and statics are the different branches of physics.
Q3. What is kinematics?
The branch of physics (mechanics) which deals with the motion of the bodies without considering the forces causing it is called kinematics.
Q4. What is dynamics?
The branch of physics (mechanics) which deals with the motion of the bodies and the forces causing it is called dynamics.
Q5, What is kinetics?
The branch of physics which deals with the motion of bodies considering cause of their motion.
Q6. What is kinematics?
The branch of physics (mechanics) which deals with the motion of the bodies without considering the forces causing it is called kinematics.
Q7. What is statics?
It is the branch of physics which deals with objects at rest under the action of forces.
Q8. What is meant by “a body at rest”?
A body is said to be at rest if it does not change its position with respect to its immediate surroundings.
Short Answer Type Questions:
Q1. Explain what is meant by mechanics and what are its different branches?
The branch of physics which deals with the effects of forces on object is called mechanics. Mechanics is further classified into dynamics and statics.
Q2. Explain what is meant by dynamics and what are its different branches?
The branch of physics (mechanics) which deals with the motion of the bodies and the forces causing it is called dynamics. It is further classified into kinematics and kinetics.
Q3. Explain the concept of a ‘point object’.
In the study of Mechanics we consider bodies or objects as particles or point objects. An object is said to be a point object if its dimensions are negligible as compared to the distance travelled by it. For example distance between stars is so large that for practical purpose those stars can be considered as particles or point objects.
Long Answer Type Questions:
Q1. Explain “Motion is a relative concept”. OR Motion and rest are relative. How do you justify this?
The motion of a body is a relative concept. When we are specifying the motion it is with respect to some observer. Let us consider to persons say A and B in a lift moving upward. There is another person C standing outside the lift. Now though the lift is going up for A and B there is no change in positions with respect to each other thus they are at rest with respect to eachother. Thus for A and B, there is no motion with respect to each other but for both of them, C is moving downward. Now for C both A and B are moving upward. Hence motion is relative.
With respect to the earth surface, we may be at rest but we are moving about 100,000 km hr-1 relative to sun.
Essay Type Answer Questions
Q1. How is motion classified on the basis of the movement of particles constituting the body?
Motion can be classified into random motion, translational motion, rotational motion, and vibrational or oscillatory motion.
- Random Motion: In this motion particles move randomly in any possible direction and in any possible velocity. Thus, the path and the direction are not definite. Example: Motion of gas molecules. Such random motion of molecules of gas is called molecular chaos.
- Translational Motion: In this motion every particle of the body has the same displacement. The translational motion can be along straight line or along a curved path. Motion along a straight line is called a rectilinear motion (e.g. motion of car in a straight line) and the motion of a body along a curved path is called curvilinear motion (e.g. the motion of a ball thrown in air, the motion of the earth around the Sun).
- Rotational Motion: In this motion the particles of body revolve in a circle about the same axis. Examples: Motion of a fan, the motion of a wheel of moving vehicle, the motion of merry go round, etc.
- Oscillatory or Vibrational Motion: In this motion the body moves to and fro about a fixed point along the same path. Examples: The motion of the bob of a pendulum, vibrating string of guitar, etc.
Q2. How is motion classified on the basis of the change in representing coordinates of position of a body?
One Dimensional Motion:
The motion of an object is said to be one-dimensional if only one of the three coordinates specifying the position object changes with a time. In such motion the object moves in a straight line. This motion is also known as rectililnear motion or linear motion.
Example: Motion of a train along a straight track
Two Dimensional Motion:
The motion of an object is said to be two-dimensional if only two of the three coordinates specifying the position object changes with a time. In such motion the object moves along a plane.
Examples: Motion of planets around the sun, Motion of a car on curved flat road, circular motion, projectile motion, etc.
Three Dimensional Motion:
The motion of an object is said to be tree-dimensional if all the three coordinates specifying the position object changes with a time. In such motion the object moves in a space.
Example: A bir flying freely in the sky, a kite flying on windy day
Concept Application:
Q1. You are walking towards India Gate. Is India Gate at rest with respect to you or in motion with respect to you?
The motion of a body is a relative concept. When we are specifying the motion it is with respect to some observer. The India Gate is a structure fixed at one point and is at rest with respect to the ground (earth). Now as we start moving with repect to earth towards the India Gate, the distance between me and the India Gate decreases (position changes continuously) continuously with respect to me. Thus we can say that India Gate is in motion with respect to me.
Q2. A passenger seated in a train is at rest with respect to …….
The motion of a body is a relative concept. When we are specifying the motion it is with respect to some observer. A body is said to be at rest if it does not change its position with respect to its immediate surroundings. Thus, the passenger seated in a train is at rest with respect to other passengers seated in the train because his position at any instant is not changing with the other passengers seated in the train.
Q3. A passenger seated in a train is in motion with respect to …….
The motion of a body is a relative concept. When we are specifying the motion it is with respect to some observer. A body is said to be in motion if it changes its position with respect to its immediate surroundings. Thus, the passenger seated in a train is in motion with respect to objects outside the train (e.g. person on a platform) because his position at any instant is changing with the objects outside the train.
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