Free fall is motion under the influence of gravity alone, with no other forces (like air resistance) acting. Near Earth's surface, all objects in free fall accelerate at approximately 9.8 m/s² regardless of their mass.

Gravity for Beginners: What It Is, How It Works, and Why It Matters

Science › Physics | Tutorial with worked examples | Grades 6–10

Learning Objectives By the end of this tutorial you will be able to: define gravity and explain what causes it; state Newton’s law of universal gravitation; distinguish between mass and weight; calculate weight from mass; describe free fall and the acceleration due to gravity; compare gravity across different planets.

Gravity is the invisible force that keeps your feet on the ground, holds the Moon in orbit, and governs the motion of every planet, star, and galaxy in the universe. It is one of the four fundamental forces of nature and the one most directly felt in everyday life. Understanding gravity is the gateway to understanding how the entire cosmos works.

1. What Is Gravity?

Gravity is a force of attraction between any two objects that have mass. Every object in the universe pulls on every other object. The strength of that pull depends on two things: the masses of the objects and the distance between them.

Greater mass → stronger gravitational pull
Greater distance → weaker gravitational pull

Because Earth has an enormous mass, its gravitational pull is strong enough to keep the atmosphere, oceans, and everything on its surface from floating away into space.

2. Newton’s Law of Universal Gravitation

Isaac Newton formulated the mathematical law of gravity in 1687. It states that the gravitational force between two objects is:

F = G × (m₁ × m₂) ÷ r²

F = gravitational force (Newtons)
G = gravitational constant (6.674 × 10&sup-;¹¹ N·m²/kg²)
m₁ and m₂ = masses of the two objects (kg)
r = distance between their centres (metres)
r² = this “inverse square” relationship means doubling the distance reduces gravity to one-quarter

At the student level, the key insight is the inverse-square relationship: moving twice as far away makes gravity four times weaker; three times farther makes it nine times weaker.

Worked Example 2-A: Inverse-Square Relationship Suppose gravity between two objects at distance d is 100 N. What is the force if the distance doubles to 2d? New force = original force ÷ (distance ratio)² New force = 100 N ÷ (2)² New force = 100 N ÷ 4 New force = 25 N Doubling the distance reduces gravity to one-quarter.

3. Mass vs. Weight

One of the most common sources of confusion in physics is mixing up mass and weight. They are related but fundamentally different:

Mass is the amount of matter in an object. It is measured in kilograms (kg). It is the same everywhere in the universe.
Weight is the gravitational force acting on that mass. It is measured in Newtons (N). It changes depending on the gravitational field strength.

Weight = mass × gravitational acceleration W = mg

On Earth, g ≈ 9.8 m/s². On the Moon, g ≈ 1.62 m/s². On Mars, g ≈ 3.72 m/s².

Worked Example 3-A: Weight on Earth A student has a mass of 50 kg. What is their weight on Earth? W = m × g W = 50 kg × 9.8 m/s² W = 490 N Their weight on Earth is 490 Newtons.

Worked Example 3-B: Weight on the Moon The same student (mass = 50 kg) travels to the Moon (g = 1.62 m/s²). What is their weight on the Moon? W = m × g W = 50 kg × 1.62 m/s² W = 81 N Their weight on the Moon is only 81 N — about one-sixth of Earth weight. But their MASS is still 50 kg everywhere.

4. Free Fall and the Acceleration Due to Gravity

Free fall means an object is moving under gravity alone, with no air resistance. Near Earth’s surface, all freely falling objects accelerate at the same rate regardless of their mass:

g ≈ 9.8 m/s² (downward)

This is called the acceleration due to gravity. Every second an object is in free fall, its speed increases by 9.8 m/s. After 1 second: 9.8 m/s. After 2 seconds: 19.6 m/s. After 3 seconds: 29.4 m/s.

This counterintuitive fact — that a heavy cannonball and a light feather fall at the same rate (in a vacuum) — was demonstrated by Galileo in the late 1500s and confirmed by Apollo 15 astronauts on the Moon in 1971.

Worked Example 4-A: Free-Fall Velocity A ball is dropped from rest. How fast is it moving after 3 seconds? (Ignore air resistance.) v = g × t v = 9.8 m/s² × 3 s v = 29.4 m/s The ball is falling at 29.4 m/s after 3 seconds.

Worked Example 4-B: Free-Fall Distance How far does an object fall from rest in 4 seconds? d = ½ × g × t² d = ½ × 9.8 m/s² × (4 s)² d = ½ × 9.8 × 16 d = 78.4 metres The object falls 78.4 metres in 4 seconds.

5. Gravity and Orbits

If gravity is always pulling the Moon toward Earth, why doesn’t the Moon crash into us? The answer is orbital mechanics. The Moon is also moving sideways at about 1 km/s. As it falls toward Earth, it simultaneously moves sideways enough to “miss” Earth. The result is a continuous falling motion that curves around Earth — an orbit.

This principle applies to all satellites, including the International Space Station and GPS satellites. They are in a state of continuous free fall; they just have enough sideways speed that Earth curves away beneath them as fast as they fall.

Worked Example 5: Why Astronauts Feel Weightless Astronauts on the ISS orbit at about 400 km altitude. At that height g ≈ 8.7 m/s² (only slightly less than at Earth’s surface). They are NOT outside gravity — they are in free fall around Earth. Because both the astronaut AND the station are falling at the same rate, the astronaut feels no contact force from the floor. This “weightlessness” is actually continuous free fall, not absence of gravity.

6. Gravity Across the Solar System

The surface gravity of a planet depends on its mass and radius. Here is how your weight would change on different worlds if your mass is 60 kg:

World	Surface g (m/s²)	Your weight (60 kg mass)
Mercury	3.7	222 N
Venus	8.87	532 N
Earth	9.8	588 N
Moon	1.62	97 N
Mars	3.72	223 N
Jupiter	24.8	1,488 N
Saturn	10.4	624 N

Worked Example 6: Your Weight on Mars A person has a mass of 75 kg. What is their weight on Mars? (g on Mars = 3.72 m/s²) W = m × g W = 75 kg × 3.72 m/s² W = 279 N They weigh 279 N on Mars compared to 735 N on Earth. Their mass remains 75 kg on both planets.

7. Practice Problems

A box has a mass of 20 kg. What is its weight on Earth? (g = 9.8 m/s²)
An object is dropped from rest. How fast is it moving after 5 seconds of free fall?
How far does a ball fall from rest in 2 seconds? (Ignore air resistance.)
If gravity between two objects at distance d is 200 N, what is the force at distance 3d?
An astronaut has a mass of 80 kg. What is their weight on the Moon? (g = 1.62 m/s²)

Answers: 1) 196 N 2) 49 m/s 3) 19.6 m 4) 200 ÷ 9 ≈ 22.2 N 5) 129.6 N

Frequently Asked Questions

What is gravity? Gravity is a fundamental force of attraction between any two objects that have mass. The greater the mass and the closer the objects, the stronger the gravitational pull between them.

What is the difference between mass and weight? Mass is the amount of matter in an object and does not change regardless of location. Weight is the gravitational force on that mass (W = mg) and changes with location.

What is free fall? Free fall is motion under gravity alone, with no other forces acting. Near Earth’s surface, all objects in free fall accelerate at approximately 9.8 m/s² regardless of mass.

Why does the Moon orbit Earth instead of falling into it? The Moon is constantly falling toward Earth due to gravity, but it also has a sideways velocity. The combination means the Moon continually misses Earth — that is what an orbit is.

Would a feather and a hammer fall at the same rate in a vacuum? Yes. Without air resistance, all objects fall at the same rate. This was demonstrated on the Moon by Apollo 15 astronaut David Scott in 1971.