Gravity and Motion – Study Notes

General Science July 28, 2025 5 min read

Key Concepts

Gravity is a force that attracts two bodies toward each other.
Every object with mass exerts gravity, but the force becomes noticeable only with massive objects (e.g., planets, stars).
Analogy: Imagine gravity as an invisible elastic band pulling objects together.

Formula: F = G * (m₁ * m₂) / r²
- F = gravitational force
- G = gravitational constant
- m₁, m₂ = masses of objects
- r = distance between centers
Real-world example: If you drop a ball, gravity pulls it toward the ground.

Gravity affects the motion of planets, moons, and satellites.
Analogy: Think of planets as marbles rolling around a stretched rubber sheet (space-time), where the Sun makes a deep dent, pulling planets toward it.
Real-world example: The Moon’s orbit around Earth is a result of gravity providing the centripetal force.

Objects in free fall accelerate at the same rate, regardless of mass (ignoring air resistance).
Astronauts in orbit experience “weightlessness” because they are in continuous free fall around Earth.
Analogy: Imagine jumping off a trampoline and feeling the brief sensation of floating.

Apple Falling: Just as an apple falls from a tree due to gravity, satellites “fall” toward Earth but move fast enough to keep missing it, remaining in orbit.
Roller Coaster: The feeling of being pulled down on a roller coaster is gravity in action.
Tides: The Moon’s gravity pulls on Earth’s oceans, causing tides.

Gravity only acts downward.
- Gravity acts between any two masses, not just toward Earth’s center.
Heavier objects fall faster.
- In the absence of air resistance, all objects fall at the same rate.
There is no gravity in space.
- Gravity exists everywhere; astronauts orbit Earth because of gravity.
Gravity is the same everywhere.
- Gravity varies depending on location (e.g., altitude, latitude, planetary body).

The ISS orbits Earth at ~400 km altitude.
It stays in orbit because its forward velocity balances the pull of gravity, creating continuous free fall.
Astronauts experience microgravity, affecting muscle and bone health.

In 2020, LIGO and Virgo detected gravitational waves from colliding black holes and neutron stars.
These ripples in space-time confirmed Einstein’s predictions and opened new ways to study cosmic events.

NASA’s spacecraft use gravity assists (slingshot maneuvers) to gain speed by passing near planets.
Example: The Juno mission to Jupiter used Earth’s gravity to increase velocity.

Bone Density: Prolonged exposure to microgravity (space) causes bone loss and muscle atrophy. Astronauts must exercise to counteract these effects.
Blood Circulation: Gravity helps blood flow from head to toe. In microgravity, fluids shift toward the head, causing “moon face” and vision changes.
Balance and Coordination: Gravity is essential for maintaining balance; lack of gravity can affect the inner ear and spatial orientation.

Source: “Spaceflight and Bone Loss: Mechanisms and Countermeasures” (Frontiers in Physiology, 2021)
- Study highlights how microgravity leads to bone density reduction and explores exercise and nutrition as countermeasures.
News: In 2020, NASA published findings on how gravity changes affect the cardiovascular system, emphasizing the need for tailored health protocols for astronauts.

Gravity: The force that attracts objects with mass toward each other.
Free Fall: Motion under the influence of gravity alone.
Microgravity: Condition of very weak gravity, as experienced in orbit.
Orbit: The path an object follows as it moves around another object due to gravity.
Gravitational Waves: Ripples in space-time caused by massive accelerating objects.
Weightlessness: Sensation of having no weight, experienced in free fall.
Centripetal Force: Force that keeps an object moving in a curved path.
Gravity Assist: Using a planet’s gravity to change the speed and direction of a spacecraft.

Concept	Real-World Example	Analogy
Gravity	Apple falling from a tree	Elastic band pulling objects
Orbit	Moon around Earth	Marbles on a rubber sheet
Free Fall	Skydiver before parachute opens	Jumping off trampoline
Microgravity	Astronauts in ISS	Floating in water
Gravity Assist	Juno mission	Slingshot maneuver

Gravity is the weakest of the four fundamental forces but dominates at large scales.
The human brain has more connections than there are stars in the Milky Way, highlighting the complexity of systems influenced by gravity.
Gravity affects everything from the motion of galaxies to the flow of blood in your body.

Frontiers in Physiology. (2021). Spaceflight and Bone Loss: Mechanisms and Countermeasures. Link
NASA News Release, 2020: “Gravity and Cardiovascular Health in Space.”