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Equations of motion

physical science Maturity 9-11 Next article →

Equations of motion are mathematical functions used to describe how a physical system behaves as it moves through time. These equations act as a map for movement, using variables to track an object's behavior. Most commonly, these variables include spatial coordinates, which tell us where an object is in space, and time. In more complex systems, scientists might also use momentum components to understand the motion. In classical mechanics, these functions are defined within a flat Euclidean space. However, in the study of relativity, these functions must account for curved spaces.

Kinematics
Kinematics

There are two primary ways to categorize these descriptions: kinematics and dynamics. Kinematics is a simpler approach that focuses only on variables derived from position and time. It describes how things move without worrying about why they move. Dynamics is a much broader field because it considers the causes of motion. It takes into account forces, energy, and the momentum of particles. When scientists solve differential equations to find the motion of a system, they are working within dynamics.

Least action principle
Least action principle

When an object experiences constant acceleration, physicists use a specific set of kinematic equations known as the SUVAT equations. The name comes from the five key variables involved in the process. These include displacement (s), initial velocity (u), final velocity (v), acceleration (a), and time (t). To solve a problem using these equations, one often starts with a differential equation, such as Newton's second law, which is F = ma. By applying the definitions of these physical quantities, you can set up an equation to solve for the unknown movement.

Suvat eom any direction constant acceleration
Suvat eom any direction constant acceleration

The history of these ideas spans thousands of years of human thought. In ancient times, astronomers used algorithms to predict eclipses and solar events, but they did not use formal equations of motion. During the thirteenth century, scholars at universities in Oxford and Paris began developing the foundations of physics. At Merton College in Oxford, thinkers like Thomas Bradwardine and Nicholas Oresme explored how velocity and force related to distance. Later, in 1545, the Spanish theologian Domingo de Soto correctly identified that certain types of motion were proportional to time. He even noted that acceleration would be negative when an object moves upward.

Velocity vs time graph for average acceleration that shows dependence on time
Velocity vs time graph for average acceleration that shows dependence on time

Galileo Galilei significantly advanced the field through geometric deduction and observation. He was the first to prove that the path of a projectile is a parabola. Galileo also studied the pendulum, observing that its period of oscillation is independent of its mass. He discovered that the period varies with the square root of the pendulum's length. While Galileo understood momentum as the product of velocity and weight, the concept of mass was developed later by Huygens and Newton. Newton eventually expanded these ideas to apply to all bodies in the universe, moving beyond just Earth's gravity.

Geodesic deviation on a sphere
Geodesic deviation on a sphere

In modern science, these equations have evolved to describe many different phenomena. In the field of electrodynamics, the Lorentz force equation describes how charged particles move through electric and magnetic fields.

Lorentz force particle
Lorentz force particle

As our understanding of the universe grew, special and general relativity required modifications to these classical equations. These updates account for the finite speed of light and the way gravity curves spacetime. Even in the tiny world of quantum mechanics, equations of motion exist. In that field, they are differential equations of the wavefunction, which describes how a quantum state behaves over space and time.

Mathematically, an equation of motion is often expressed as a second-order ordinary differential equation. It is a function of the object's position, its velocity, and its acceleration. Velocity is the first time derivative of position, while acceleration is the second time derivative. For a particle moving in a straight line with constant acceleration, the math becomes much simpler. In these cases, the velocity increases linearly over time. This means the average velocity multiplied by the time interval can help determine the total distance traveled.

Kinematics
Kinematics
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File:Velocity vs time graph for average acceleration that shows dependence on time.jpg
Velocity vs time graph for average...
File:Kinematics.svg
Kinematics.svg
File:Suvat eom any direction constant acceleration.svg
Suvat eom any direction constant acceleration.svg
File:Least action principle.svg
Least action principle.svg
File:Lorentz force particle.svg
Lorentz force particle.svg
File:Geodesic deviation on a sphere.svg
Geodesic deviation on a sphere.svg
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