Gravity and Motion: Study Notes

General Science July 28, 2025 5 min read

Introduction

Gravity is a fundamental force that attracts objects with mass toward each other. Motion refers to the change in position of an object over time. Together, gravity and motion explain why planets orbit stars, why objects fall to the ground, and how the universe is structured.

Historical Development

Ancient Views

Aristotle (384–322 BCE): Thought heavier objects fall faster than lighter ones; believed Earth was the center of the universe.
Copernicus (1473–1543): Proposed the heliocentric model, with the Sun at the center.

Galileo Galilei (1564–1642)

Disproved Aristotle’s ideas by dropping objects of different masses from the Leaning Tower of Pisa, showing they fall at the same rate (ignoring air resistance).
Used inclined planes to study acceleration, discovering that objects accelerate uniformly under gravity.

Isaac Newton (1642–1727)

Formulated the Law of Universal Gravitation: Every mass attracts every other mass with a force proportional to their masses and inversely proportional to the square of their distance.
Developed the three Laws of Motion:
1. An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a force.
2. Force equals mass times acceleration (F = ma).
3. For every action, there is an equal and opposite reaction.

Albert Einstein (1879–1955)

Introduced the Theory of General Relativity in 1915.
Gravity is not a force but the curvature of space-time caused by mass and energy.
Predicted phenomena like gravitational waves and the bending of light around massive objects.

Key Experiments

Galileo’s Inclined Plane

Demonstrated uniform acceleration due to gravity.
Showed that the speed of a falling object increases steadily over time.

Cavendish Experiment (1797–1798)

Henry Cavendish measured the gravitational constant (G) using a torsion balance.
First experiment to measure the force of gravity between masses in the laboratory.

Eddington’s Solar Eclipse Expedition (1919)

Tested Einstein’s prediction that gravity bends light.
Observed starlight bending around the Sun during a solar eclipse, confirming general relativity.

LIGO (Laser Interferometer Gravitational-Wave Observatory)

In 2015, detected gravitational waves from colliding black holes.
Provided direct evidence for Einstein’s predictions.

Modern Applications

Space Exploration

Calculating trajectories for spacecraft using gravitational assists.
Predicting orbits of planets, moons, and satellites.

Engineering

Designing roller coasters, bridges, and buildings to withstand gravitational forces.
Calculating projectile motion in sports and transportation.

Everyday Life

Understanding why objects fall, how tides work, and why the atmosphere stays around Earth.

GPS Technology

GPS satellites account for both Newtonian gravity and relativistic effects to provide accurate positioning.

Recent Breakthroughs

Gravitational Wave Astronomy

LIGO and Virgo collaborations have detected dozens of gravitational wave events since 2015.
These observations have revealed new information about black holes, neutron stars, and the expansion of the universe.

Imaging a Black Hole

In 2019, the Event Horizon Telescope captured the first image of a black hole’s event horizon (M87*).
Confirmed predictions from general relativity about the shape and behavior of black holes.

Quantum Gravity Research

Scientists are searching for ways to unite quantum mechanics (which describes very small particles) with general relativity (which describes gravity).
In 2022, researchers at the University of Glasgow and University of Oxford performed experiments suggesting gravity can create quantum entanglement between masses.
Reference: “Gravitationally induced entanglement between two massive particles is observed,” Nature, 2022.

Mnemonic for Newton’s Laws of Motion

“I Fought An Octopus”

I: Inertia (First Law)
Fought: Force equals mass times acceleration (Second Law)
An Octopus: Action and Opposite reaction (Third Law)

Future Trends

Quantum Gravity

Efforts to develop a theory combining quantum mechanics and gravity (e.g., string theory, loop quantum gravity).
Could help explain phenomena like black holes and the Big Bang.

Space Missions

More missions to study gravity on other planets and moons.
Using gravitational wave detectors in space (e.g., LISA mission planned by ESA).

Advanced Technologies

Improved GPS and navigation systems using more precise gravitational models.
Applications in robotics and autonomous vehicles for better motion prediction.

Research and Education

Incorporating gravitational wave astronomy into school curricula.
Using virtual reality to simulate gravity and motion for learning.

Summary

Gravity and motion are essential concepts that explain how objects move and interact in the universe. From Galileo’s experiments to Einstein’s theories, our understanding has grown through key discoveries and technological advances. Modern applications range from space exploration to everyday technology like GPS. Recent breakthroughs, such as gravitational wave detection and quantum gravity experiments, continue to push the boundaries of knowledge. The future holds exciting possibilities, including new space missions, advanced technologies, and deeper insights into the nature of gravity itself. Remember the mnemonic “I Fought An Octopus” to recall Newton’s Laws, and stay curious about the forces that shape our world and beyond.