Gravitation: Universal Law, Free Fall & Orbits: quick revision, 15 bites

Every Mass Pulls on Every Other Mass

You attract the Earth — and it attracts you.

Gravitation is a **universal attractive force** between any two objects with mass. It is always attractive, never repulsive, and acts along the line joining their centres.

Takeaway. Gravity is universal: every pair of masses in the universe attracts each other with this same law.

The Universal Gravitational Constant G

One tiny number rules all of gravity.

Takeaway. G is fixed everywhere; its tiny value is why gravity feels weak between everyday objects.

Mass vs Weight: Not the Same Thing

Your mass is constant; your weight is not.

Takeaway. Mass is intrinsic; weight is the force gravity exerts on that mass at a given place.

Weight: The Force of Gravity on a Body

Weight is just mass times g.

Takeaway. Weight = mass × local g; change location, and weight changes while mass does not.

Free Fall: Gravity's Solo Act

Drop it — gravity does the rest.

**Free fall** is motion under gravity alone, with no air resistance or other forces. All objects in free fall accelerate at the same rate g, regardless of mass.

Takeaway. In free fall, mass drops out — a feather and a hammer fall together in vacuum.

Heavier Objects Do NOT Fall Faster

Bigger isn't faster in free fall.

Takeaway. Mass does not affect free-fall acceleration; air resistance is the real culprit for everyday differences.

Feather vs Hammer on the Moon

No air, no excuses — they land together.

Takeaway. Remove air resistance, and free fall becomes truly mass-independent.

Acceleration Due to Gravity (g)

g is gravity's fingerprint on motion.

Takeaway. g is a property of the planet, not the falling object.

Why Satellites Don't Fall Down

They're falling — just missing Earth.

A satellite is in **continuous free fall** toward Earth, but its high horizontal speed makes it curve around Earth instead of hitting it. This balance of gravity and tangential velocity produces an orbit.

Takeaway. Orbiting is free fall with enough sideways speed to keep missing the ground.

Orbital Velocity: Speed to Stay in Orbit

The speed that keeps you circling.

Takeaway. Orbital speed depends only on the central body's mass and the orbital radius.

Escape Velocity: Breaking Free

The speed to leave the planet for good.

Takeaway. Escape velocity is the minimum speed to leave a planet's gravity permanently.

Inverse-Square: Double Distance, Quarter Force

Twice as far, one-fourth the pull.

Remember the **inverse-square law**: if distance doubles, gravitational force becomes 1/4; if distance triples, it becomes 1/9. Force ∝ 1/r².

Takeaway. Distance squared in the denominator — double r, quarter F.

Exam Tip: Watch the Distance Definition

r is centre-to-centre, not surface-to-surface.

Takeaway. r is always centre-to-centre; add the planet's radius when given height.

Gravity Is the Weakest Fundamental Force

Weakest force, biggest cosmic job.

Gravity is by far the **weakest** of the four fundamental forces, yet it dominates on cosmic scales because it is always attractive and has infinite range — it never cancels out.

Takeaway. Weak locally, dominant cosmically — gravity's always-attractive nature wins at scale.

Real Applications of Gravitation

From tides to GPS — gravity at work.

Takeaway. Gravitation underpins tides, satellite tech, space travel, and everyday weighing.