Gravity

The force pulling on you right now — what it really is, and why everything falls the same way.

The most familiar force, explained badly in school

You feel gravity right now. Sit, stand, lie down — something is constantly pulling you toward the floor. That “something” is gravity, and it’s the most underestimated force in physics.

Here’s the rule in one sentence:

Every object with mass pulls on every other object with mass.

You. The Earth. The Moon. Your coffee cup. A faraway galaxy. They’re all pulling on each other right now. It’s just that most of those pulls are too weak to notice.

Why we only notice Earth’s pull

The strength of gravity depends on mass. Earth has a lot of mass (about $6 \times 10^{24}$ kg), so its pull dominates. Your coffee cup has gravity too — it’s pulling on you — but its mass is tiny, so the force is laughably small.

Newton wrote it as:

$F = G \frac{m_1 m_2}{r^2}$

Don’t panic. Read what it says:

• Bigger masses → stronger pull. (Makes sense.)
• Greater distance → weaker pull. (Also makes sense.)
• And the distance is squared: double the distance, the pull becomes four times weaker.

Weight vs. mass (the confusion ends here)

Mass

How much stuff you’re made of. Measured in kilograms. Never changes, no matter where you go.

Weight

How hard gravity pulls on you. Measured in newtons. Changes depending on where you are.

You have the same mass on Earth, the Moon, and Mars. But your weight is different in each place because gravity is different.

$\text{Weight} = m \cdot g$

On Earth, $g \approx 9.8$ m/s². On the Moon, $g \approx 1.6$ m/s². A 70 kg person:

• On Earth: weight = 70 × 9.8 = 686 N
• On the Moon: weight = 70 × 1.6 = 112 N

Same person, same mass. About one-sixth the weight. That’s why astronauts bounce around.

The famous experiment: everything falls the same

Drop a hammer and a feather at the same time. Hammer hits first, right? Yes — but only because of air resistance.

In 1971, astronaut David Scott did this exact experiment on the Moon (no air). Hammer and feather hit the ground at the exact same time.

Why? Newton’s 2nd Law says $a = F/m$. The gravitational force on a heavier object is bigger ($F = mg$), but the object is also more massive. The two effects cancel perfectly:

$a = \frac{mg}{m} = g$

Mass cancels out. Every object accelerates at the same $g$ — about 9.8 m/s² near Earth’s surface — regardless of how heavy it is.

Galileo figured this out 400 years ago by rolling balls down ramps. Newton put the math behind it. It still surprises every student. If it surprises you, you’re in good company.

How fast does something fall?

If you drop something from rest:

$v = g \cdot t$

• After 1 second: 9.8 m/s (about 35 km/h)
• After 2 seconds: 19.6 m/s (about 70 km/h)
• After 3 seconds: 29.4 m/s (about 106 km/h)

Notice it just keeps getting faster. There’s no “top speed” from gravity alone.

So why don’t skydivers accelerate forever?

Air resistance. As you fall faster, the air pushes back harder. At some point, the air’s push equals gravity’s pull. Net force = zero. No more acceleration. You’ve hit terminal velocity (around 200 km/h for a person belly-down).

At that point you’re falling fast, but at a constant speed. Skydiving is mostly just hanging at terminal velocity.

Why the Moon doesn’t fall on us (it does)

Here’s a brain-bender: the Moon is constantly falling toward Earth. It just keeps missing.

The Moon is moving sideways really fast. Gravity pulls it toward Earth, but by the time it has fallen a little, it has also moved sideways enough to “miss” Earth. So it falls forever, in a loop. That loop is called an orbit.

Same story for satellites. Same story for the Earth around the Sun. Orbits are just falling and missing.

What gravity is, deep down

Newton’s equations describe gravity perfectly for everyday use. But Einstein took it further: in his picture, mass bends space itself, and objects “fall” because they’re following the curves.

Imagine a bowling ball on a stretched bedsheet. It dents the sheet. Roll a marble nearby, and the marble curves toward the bowling ball — not because the bowling ball is “pulling” it, but because the sheet itself is curved.

That’s general relativity in one sentence. For 99% of engineering, Newton is good enough. For GPS satellites and black holes, you need Einstein.

Next: Pressure and Fluids

Why ships float, straws work, and your ears pop on airplanes.