Kinematics is the study of the motion of a body without taking into account the causes that produce it. It is the branch of physics belonging to the classical mechanics. The study of this branch of mechanics is commonly referred to as the geometry of motion.
In the study of kinematics, we do not take into account the chemical and physical properties of moving bodies nor the forces acting on them.
Velocity and acceleration are two fundamental concepts in studying the kinematics of a rigid body.
It should only be noted that the equations of kinematics do not take Einstein's theory of relativity into account, so they are only valid for low speeds compared with the speed of light.
What Is a Rigid Body?
A rigid body is a body that cannot be deformed. That is, at any instant, the distance between two points belonging to it remains constant.
In the study of kinematics, the motion of a rigid body is considered because it does not absorb energy from the system to deform the material. Of course, this way of calculating the movement is inaccurate, but it is a very approximate way of the reality that simplifies the calculations.
The Relative Velocity Between Two Rigid Bodies
When we define the movement of bodies, we need a reference system. However, this reference system can change its position. Therefore, it is necessary to establish a fixed coordinate system and describe the movement based on this system.
To give an example of the relative motion of a system of bodies, let's imagine a person moving inside a train car at a speed of 4 km/h concerning the train car. In turn, the train moves at a constant pace of 100 km/h relative to the ground. At this moment, the relative speed of the passenger concerning the ground is that of the individual walking plus that of the train: 104 km.
After some time, the distance traveled will be the sum of the distance traveled by the train plus the distance traveled by the person.
What Are the Types of Movements?
In the two-dimensional representation of kinematics, the following types of movement exist:
Uniform rectilinear motion: The solid moves in a straight line at a constant speed and without changing direction.
Uniformly accelerated motion: the solid moves in a straight line without changing its direction, but the speed gradually decreases until it changes sign. At this moment, the object changes direction and goes to its initial position.
Simple harmonic motion: The direction of the force under which the object moves is always opposite to the displacement, and the magnitude is proportional to the displacement.
Circular Motion: Motion whose path is a circle or part of a circle.
The parabolic throw is the movement and trajectory of an object affected by the gravity force.
Examples of Kinematics
The fall of a parachutist from an airplane would be an example of uniformly accelerated rectilinear motion.
The motion of celestial bodies orbiting around the Sun is carried out through a circular movement.
The swing of a simple pendulum is an example of simple harmonic motion.
The trajectory of a soccer ball after a free kick is an example of a parabolic shot. We can calculate the distance and displacement of the ball using kinematics.
What Is the Difference Between Kinematics and Dynamics?
The difference between kinematics and dynamics is that dynamics studies the relationship between forces and motion, while kinematics deals only with movement.