This lab works on a phone. On a bigger screen you can see the drop, the graph and the readouts at the same time.

Gravity Basics

Mass is how much matter something has. Weight is how hard gravity pulls on it. Drop two balls on ten different worlds and compare them. Then move a ball away from a planet and watch gravity get weaker with distance.

Mass vs weight g changes by world Free fall Gravity and distance

Free

Drop lab

Pick a world, set the height and the two masses, then press Drop

g = 9.81 m/s² Air: off Both balls are 22 cm across · not to scale

Speed 1×

World

Drop height 60 m

Ball A mass 10.0 kg

Ball B mass 0.5 kg

Speed vs time

Ball A (10 kg) weighs

Readout

Live readout

WorldEarth g (pull on each kg)9.81 m/s² 1 kg weighs9.81 N Airoff Time0.00 s Fall time with no air3.50 s

Quantity	Ball A	Ball B
Mass	10.0 kg	0.5 kg
Weight	98.1 N	4.91 N
Speed	0.00 m/s	0.00 m/s
Fallen	0.0 m	0.0 m
Landed after	–	–
Landing speed	–	–

What you're learning

Mass (in kilograms, kg) is how much matter an object has. It is the same on every world.

Weight (in newtons, N) is the force of gravity on that mass: W = m·g. On Earth 1 kg weighs 9.81 N. On the Moon it weighs only 1.62 N.

g is how fast a falling object speeds up, in m/s² (metres per second, every second). With no air, every object's speed grows by g each second, whatever its mass. A heavier ball gets a bigger pull, but it is also harder to speed up. The two cancel, so the balls land together.

In the picture the arrows show acceleration, in m/s²: gravity's arrow is the same on both balls, and the air's arrow is longer on the lighter ball, because the same push changes a small mass more.

Air pushes up on a falling ball (drag), and pushes harder the faster it goes. The same push slows a light ball more than a heavy ball of the same size. When the push equals the ball's weight, it stops speeding up: that is its top speed.

Good to know

In the US, weight is often given in pounds: 1 pound is about 4.45 N, so 1 kg weighs about 2.2 pounds on Earth.

Venus's air is about 50 times as dense as Earth's. It pushes up on the balls even before they move, a little like water holds you up (this push is called buoyancy), and it slows them a lot.

Jupiter, Saturn, Uranus, Neptune and the Sun have no solid ground. Their g is measured at the equator, at the height where the gas pressure is about the same as Earth's air pressure at sea level (for the Sun, at its visible surface).

Gravity and distance

Drag ball A away from the world, or use the slider

r = 2.00 R g = 2.45 m/s² Distances to scale · R = the world's radius

World

Distance from centre 2.00 R

At this distance

Readout

WorldEarth Distance from centre12,742 km Height above surface6,371 km g at the surface9.81 m/s² g here2.45 m/s² Compared with surface1/4 Ball A (10 kg) weighs24.5 N

Newton's law of gravity

Two masses pull on each other with a force F = G·m₁·m₂ / r². G is a tiny number, 6.674 × 10⁻¹¹ N·m²/kg², and r is the distance between their centres.

Because r is squared, twice as far means 1/4 the pull (2² = 4). Three times as far means 1/9.

So at a distance r from a world's centre, g = g₀·(R/r)², where g₀ is g at the surface and R is the world's radius.

Why astronauts float

The International Space Station (ISS) orbits about 400 km above Earth. Gravity there is still about 8.7 m/s², almost 90% of its pull on the ground.

Astronauts float because they and the station are falling together, all the time. They also move sideways at about 7.7 km per second, so they keep falling around Earth instead of into it. That is free fall, not zero gravity.

Practice

Answer a quick gravity question. Score counts answers right on the first try.

Challenge

Press New challenge.

Score: 0 Streak: 0 Attempted: 0

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