Dynamics is a branch of mechanics that studies the causes of changes in mechanical motion. This science studies the relationship between the forces that act on objects and the effects produced on the movement.

The term dynamic comes from Greek and means strength or power.

In classical mechanics, these causes are forces. In addition, concepts such as mass, momentum, angular momentum, energy are also involved in dynamics.

Dynamics based on Newton's laws is called classical dynamics. Classical dynamics describes the movement of bodies with speeds ranging from fractions of millimeters per second to kilometers per second.

However, these methods are no longer valid for the motion of objects of tiny sizes (quantum mechanics) and motions with speeds close to the light's speed (relativistic mechanics). Again, it is because such movements are subject to other laws.

This branch of physics also studies the movement of a continuous medium, that is, deformable bodies, liquids, and gases.

## Why Newton's Laws Are Essential in Dynamics?

Classical dynamics is based on Isaac Newton's three fundamental laws that explain the effects that will occur on a body and the results of the forces that are applied to it:

### Newton's First Law of Motion or Law of Inertia

A body remains at rest or moving with constant velocity if no other body acts on it or its action is compensated.

Sometimes, we observe an object moving at a particular speed without interacting force. It is the effect of the frictional force. In fact, friction forces appear everywhere in everyday life. Therefore, friction must be eliminated in order to study ideal free-body diagrams.

### Newton's Second Law of Motion

In physical inertial reference systems, the acceleration acquired by a material point is directly proportional to the net force that causes it, coincides with it in direction, and is inversely proportional to the mass of the material point.

According to the second law, Newton established this relationship of forces and motion with the following formula:

F = m·a

where

F is the force measured in newtons (N)

m is the constant object's mass (kg)

a is the object's acceleration (m/s).

### Newton's Third Law

The forces with which bodies act on each other are in the same straight line, have opposite directions, and equal modules.

For example, when an object is supported in a surface, the weight generates a force on the surface, and a plane exerts a reaction force called normal whit the same intensity and opposite sense. This reaction is important because it generates a friction force that depends on the surface's coefficient of friction. Therefore, this normal force must be decomposed into vertical and horizontal components in an inclined plane.

## What Does the Study of Dynamics in Physics Encompass?

The dynamics of a point study the interaction of material points, bodies whose dimensions can be neglected compared to the characteristic dimensions of the phenomenon under study. It is the study of moving objects affected by external forces such as kinetic friction or centripetal forces in a circular motion.

Rigid body dynamics studies the interaction of absolutely rigid bodies. It is an excellent approximation to the real physical situation in many cases.

The study of equilibrium conditions of mechanical systems deals with statics.

Dynamics of deformable bodies:

Hydrodynamics studies the movement of ideal and real fluids' movement and their force of interaction with solids.

Gas dynamics studies the laws of motion of a gaseous medium; in particular, aerodynamics studies the laws that govern the movement of air flows and their interaction with obstacles and moving bodies.