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However, these methods are no longer valid for the motion of objects of tiny sizes (quantum mechanics) and motions with speeds close to the light’s speed (relativistic mechanics).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
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F is the force measured in newtons (N)
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m is the constant object’s mass (kg)
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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.
Fields of study of dynamics in physics
Dynamics is a field of physics that is divided into different disciplines. Below are the most prominent:
Dynamics of a point
The dynamics of a point deals with studying the interaction of material points, which are bodies whose dimensions can be neglected in comparison with the characteristic dimensions of the phenomenon under study. It focuses on analyzing how forces act on these material points and how these forces affect their movement.
Rigid body dynamics
Rigid body dynamics is dedicated to the study of the interaction of bodies that are absolutely rigid.
It is considered that these bodies do not suffer deformations, which allows analyzing their movement and the forces that act on them in a simplified way.
Dynamics of deformable bodies
The dynamics of deformable bodies covers the study of bodies that can suffer deformations under the action of external forces.
This field analyzes how applied forces affect the shape and mechanical behavior of deformable bodies, considering aspects such as elasticity, plasticity and resistance of materials.
Fluid dynamics
Fluid dynamics is dedicated to the study of the movement of fluids (liquids and gases) and how they interact with each other and with solid bodies. Phenomena such as laminar and turbulent flow, conservation of mass and energy in a flow, viscosity and pressure are investigated.
This field of physics is also known as fluid mechanics.
Examples of dynamics
Below are some examples of everyday situations in which the concepts of dynamics are involved:
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A car accelerating: When you press the accelerator pedal in a car, a forward force is applied that propels the vehicle. The net force will determine the acceleration of the car, according to Newton's second law.
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An object in free fall: When you drop an object from a height, it experiences an acceleration due to the gravitational force. The gravitational force acts as the net force that accelerates the object downward.
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Billiard Shocks: When hitting a billiard ball with the cue, a force is applied that transfers momentum to it. During the collision, the transfer of linear momentum and the interaction of forces between the balls involved follow the principles of Newton's third law.
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Pushing an object on a rough surface: When you apply a force to push an object on a rough surface, the friction force acts in the opposite direction of the movement and depends on the characteristics of the surface and the normal force.
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The drive of a steam turbine in a nuclear power plant: In the nuclear power generation process, fluid dynamics plays a fundamental role in the process of converting thermal energy into mechanical energy.
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A hot air balloon uses the principles of fluid dynamics but can also be calculated as a rigid solid subjected to the force of gravity.