Introduction to Kinematics

Kinematics is a branch of Mechanics which studies the motion of a particle without going into the cause of the motion. In Kinematics, we describe the motion of a particle using vectors to specify its position, velocity, and acceleration. Another major portion of Mechanics is Dynamics which deals with cause of motion. Dynamics will be introduced later on.

Rest and Motion

The first thing to understand is that Rest and Motion are relative terms. They are defined with respect to an observer or frame of reference. We cannot for sure say that a body is at rest or in motion because a body might be at rest wrt one observer while being in motion wrt another observer.

If the position of a body does not change over time with respect to an observer, the body is said to be at rest with respect to that observer. For example $x(t)=constant$ (x-coordinate as a function of time) represents a body at rest in that frame of reference.

If its position changes over time with respect to an observer, the body is said to be in motion with respect to that observer. For example $x(t) = 4t + 3$ represents a body in motion in that reference frame.

We now begin our study of Kinematics with a brief introduction to Position.