Force and Motion – Historical Development of Ideas of Motion

Vocabulary flashcards covering major contributors, theories, and key terms from ancient philosophy to modern physics on motion.

Natural Motion

Aristotle’s idea that motion is inherent to an object’s nature—heavy objects move downward, light elements like fire move upward.

Violent Motion

Aristotle’s category for motion imposed by an external force, contrary to an object’s natural tendency (e.g., pushing a cart).

Aristotle’s View on Falling Objects

Heavier bodies fall faster than lighter ones—a conclusion later proven incorrect by Galileo.

Impetus Theory

Medieval concept that an impressed force keeps a projectile moving without continuous push; a precursor to momentum.

John Philoponus

6th-century Byzantine thinker who first criticized Aristotle and introduced the impetus concept, arguing objects of different weights fall at nearly the same rate.

Jean Buridan

14th-century French scholar who refined impetus theory, proposing motion persists unless opposed by external forces like air resistance.

Heliocentric Model

Copernicus’s proposal that the Sun, not Earth, is at the center of the solar system, challenging Aristotelian cosmology.

Nicolaus Copernicus

Renaissance astronomer who revived heliocentrism, prompting new questions about Earth’s motion.

Kepler’s First Law (Law of Ellipses)

Planets move in elliptical orbits with the Sun at one focus, replacing the belief in perfect circles.

Kepler’s Second Law (Law of Equal Areas)

A planet sweeps out equal areas in equal times, meaning it travels faster when closer to the Sun.

Kepler’s Third Law (Harmonic Law)

The square of a planet’s orbital period is proportional to the cube of its average distance from the Sun (T² ∝ r³).

Galileo’s Concept of Inertia

Motion continues without a continuous cause; objects resist changes in their state of motion.

Inclined-Plane Experiment

Galileo’s test showing objects accelerate uniformly under gravity, enabling precise study of acceleration.

Isochronism of the Pendulum

Galileo’s finding that pendulums of equal length have the same period, regardless of amplitude.

Newton’s First Law (Law of Inertia)

An object remains at rest or in uniform straight-line motion unless acted on by a net external force.

Newton’s Second Law (Law of Acceleration)

Acceleration of an object is proportional to net force and inversely proportional to mass (F = ma).

Newton’s Third Law (Law of Interaction)

For every action force, there is an equal and opposite reaction force; forces always occur in pairs.

Universal Gravitation

Newton’s idea that every mass attracts every other mass with a force proportional to their masses and inversely proportional to the square of their separation.

Philosophiæ Naturalis Principia Mathematica

Newton’s 1687 work that unified celestial and terrestrial mechanics through mathematical laws.

Special Relativity

Einstein’s 1905 theory stating that the laws of physics are the same for all inertial observers and that space and time are relative, not absolute.

Spacetime

Einstein’s concept combining three-dimensional space and one-dimensional time into a four-dimensional continuum.

Mass–Energy Equivalence (E = mc²)

Einstein’s relation showing mass at rest is a form of energy, with c representing the speed of light.

Length Contraction

Relativistic effect where an object moving close to light speed appears shorter along the direction of motion to a stationary observer.

Relativistic Momentum

Momentum expression modified by special relativity, increasing drastically as an object’s speed approaches the speed of light.