Conceptual Approach - One Dimensional Motion - Part I
What is motion?
The change in position of a particle or object with respect to time is defined as motion. It is a relative concept and not an absolute concept i.e. in defining motion we have to choose a point from which we can measure motion. This point from which motion of a particle or object is measured is called frame of reference or reference coordinate system.
Note: In syllabus of JEE we have to study only coordinate system not any other. Other coordinate system may be spherical or polar (not in syllabus of JEE or CBSE).
What is mechanics?
The chapter “Introduction to motion” or broadly you can say the topic “motion” comes under the branch of physics called as mechanics. We can define mechanics as the branch of physics that deals with study of motion and their relationships. Further the study of motion is divided into two parts kinematics and dynamics.
What is kinematics?
Kinematics describes the positions and motions of objects in space as a function of time but doesn’t consider the cause of motion. Kinematics doesn’t enquire into the reasons why object moves; it only describes their behavior.
What is dynamics?
Dynamics is the study of causes of motion. Dynamics is concerned with what causes and changes the course of object in motion.
Note: Here we are dealing with properties of motion only; we are in the world of kinematics. Kinematics does not consider the size, shape, mass, etc. of the body; we are restricted to properties of motion only.
How can you clarify the difference between rest and motion?
A body is said to be at rest if it doesn’t change its position w.r.t time in a given frame of reference while on the other hand if the body changes its position w.r.t time in the same frame of reference then the body is in motion. Here frame of reference is very important. For example, suppose you are driving a car at a speed of 80 m/s. Then you are at rest with respect to your car while for a person on the ground, you are travelling at 80 m/s. From this example we can conclude that at a same time a body can be at rest and can be in motion. It depends on the frame of reference.
What is the concept of particle or object?
We normally define a particle as an isolated point mass having no size and incapable of rotation. Practically in our daily life, we don’t treat every object as particle. Then why we are discussing concept of particle. This is because assumption of an object to particle depends on many factors/circumstance like; if a car is going 100 km then the size of the car is negligible against 100 km., in this case we can treat car as a particle. Or a spacecraft travelling to moon can also be considered as particle because size of spacecraft is negligible against the distance between moon and earth.
Note: Rigid bodies that move without rotation are said to undergo translational motion (linear motion). Rigid bodies will be discussed later.
What is frame of reference?
It is the place from which motions is observed and measured is called frame of reference. If you want to locate a point from a frame of reference then you have to mention its coordinates along the axes i.e. x, y and z axis.
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