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A set of three physical laws forming the basis of classical mechanics. The first law (inertia) states an object remains at rest or in uniform motion unless acted upon by a net external force. The second law quantifies force as mass times acceleration, \(\vec{F} = m\vec{a}\). The third law states that for every action, there is an equal and opposite reaction.

A Newtonian fluid's shear stress is directly proportional to the rate of shear strain. This linear relationship is defined by Newton's law of viscosity: \(\tau = \mu \frac{du}{dy}\), where \(\tau\) is the shear stress, \(\mu\) is the dynamic viscosity (a constant of proportionality), and \(\frac{du}{dy}\) is the shear rate or velocity gradient.

Bernoulli's principle states that for an inviscid flow, an increase in a fluid's speed occurs simultaneously with a decrease in pressure or a decrease in its potential energy. It is a statement of the conservation of energy for a moving fluid, commonly expressed as \(p + \frac{1}{2}\rho v^2 + \rho gh = \text{constant}\) along a streamline.

Fluid mechanics is the branch of applied mechanics concerned with the statics (fluids at rest) and dynamics (fluids in motion) of liquids and gases. It applies fundamental principles of mass, momentum, and energy conservation to analyze and predict fluid behavior. Its applications are vast, ranging from aerodynamics and hydraulics to meteorology and oceanography.

A partial differential equation (PDE) is an equation that imposes relations between the various partial derivatives of a multivariable function. The function is often called the unknown, and a PDE describes a relationship between this unknown function and its derivatives. Unlike ordinary differential equations (ODEs), which involve functions of a single variable, PDEs are fundamental to modeling multidimensional systems.

The Euler characteristic is a topological invariant, a number that describes a topological space's structure or shape regardless of how it is bent. For polyhedra, it is defined by the formula \(\chi = V - E + F\), where V, E, and F are the number of vertices, edges, and faces, respectively. For a sphere, \(\chi = 2\), while for a torus, \(\chi = 0\).

A second-order linear elliptic partial differential equation that describes systems in a steady-state or equilibrium condition. It is written as \(nabla^2 u = 0\) or \(Delta u = 0\), where \(nabla^2\) (or \(Delta\)) is the Laplace operator. Solutions, called harmonic functions, are the smoothest possible functions and represent potentials in fields like electrostatics, gravitation, and fluid flow.

Generalized Hooke's Law is the constitutive equation for linear elastic materials, stating that the stress tensor is linearly proportional to the strain tensor. The relationship is expressed as \(\sigma = C : \varepsilon\), where \(\sigma\) is the stress tensor, \(\varepsilon\) is the strain tensor, and \(C\) is the fourth-order stiffness tensor containing the material's elastic constants.

This law states that every particle attracts every other particle in the universe with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. The formula is \(F = G \frac{m_1 m_2}{r^2}\), where \(G\) is the gravitational constant. It unified terrestrial and celestial mechanics.

This is a historically notable problem in mathematics. Its negative resolution by Leonhard Euler in 1736 laid the foundations of graph theory and prefigured the idea of topology. The problem asked if the seven bridges of the city of Königsberg could all be traversed in a single trip without doubling back, with the trip ending on the same landmass it began.

A second-order linear hyperbolic partial differential equation that governs the propagation of various types of waves. In its simplest form, it is written as \(\frac{\partial^2 u}{\partial t^2} = c^2 \nabla^2 u\), where \(u(\vec{x},t)\) is the amplitude of the wave, \(c\) is the wave speed, and \(\nabla^2\) is the Laplace operator. It models phenomena like vibrating strings, sound waves, and light waves.

A fundamental theorem in topology and geometry stating that for any convex polyhedron, the number of vertices (V), edges (E), and faces (F) are related by the formula \(V - E + F = 2\). This value, 2, is the Euler characteristic of a sphere, revealing a deep topological property independent of the polyhedron's specific shape.

In continuum mechanics, the principle of mass conservation states that the mass of a closed system must remain constant over time. For a fluid, this is expressed by the continuity equation. In its Eulerian differential form, it is written as \(\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf{u}) = 0\), where \(\rho\) is the density and \(\mathbf{u}\) is the velocity field.

Bézout's theorem is a fundamental statement in intersection theory. It asserts that the number of intersection points of two plane algebraic curves of degrees \(m\) and \(n\) is exactly \(mn\), provided that one works in a projective plane over an algebraically closed field, counts points with multiplicity, and includes points at infinity where parallel asymptotes meet.

These are two ways to describe motion in continuum mechanics:

• the Lagrangian specification follows individual material particles, tracking their properties over time, like watching a specific car in traffic
• the Eulerian specification focuses on fixed points in space, observing the properties (velocity, density) of whatever particles pass through those points, like a traffic camera observing a fixed intersection.