Motion in a Straight Line
THE PHENOMENON OF MOTION
Mechanics is the branch of physics which deals with the causes and effects of motion. Kinematics part of mechanics deals with the description of motion without reference to the forces which either cause its motion or are generated as a result of its motion. It is a very difficult task to discuss the motion of a material body, as a material body consists of a large number of smaller constituents (atoms or molecules, we can say), which itself is in the state of motion. To simplify our discussion of motion of objects, we start with objects, whose position can be described by locating one point i.e., the size of the object is considerably very small as compared to the distance for which its motion is considered and its internal motion and rotational motion are of no interest to us. Such an object we call a particle. The motion of particles will be discussed in terms of velocity, displacement, acceleration etc. Note: There is no limit on the size of the object for it to be treated as a particle, it totally depends on the situation whether an object can be treated as a particle or not. Even earth can be treated as a particle when we are interested in its nearly circular motion around the sun, while it can’t be treated as a particle when we are interested in its rotational motion about its own axis.
MOTION & FRAME OF REFERENCE
When a person is riding a horse, can you say he is in motion! No, because he is in motion wrt earth but he is stationary wrt horse. Let us say after some time he stops riding but is still sitting on a horse, then can you say he is at rest! Again, no, because he is at rest w.r.t horse and earth, but he is the motion wrt sun because the earth revolves around the sun. So whenever we say that a particle is in motion or at rest, we have to specify the frame of reference wrt which we are talking to make the statement means. In other words, we can say that rest and motion are relative terms, absolute rest or absolute motion are meaningless. Any object can be in a state of rest or of motion depending upon the chosen frame of reference.
Note: We can also say that the frame of reference is some array of physical objects that remain at rest relative to one another. Within any such frame, we set up a coordinate system of some kind so that we can have an idea about the motion of object wrt this frame of reference. While doing so, we are free to choose any point as origin according to our convenience. The local fixed surroundings wrt in which the position of the other objects changes can be defined as a frame of reference. It is entirely a matter of convenience which frame of reference we choose, but it is always advantageous to use a frame of reference in which the description of the motion is simplest.