Density Simulation: Float or Sink

This simulation is a float-or-sink lab for the whole class on one screen. Drag a wood block, an ice cube, an apple, an egg, an empty bottle, a steel ball or a stone into a glass tank and watch it bob up to the surface or drop to the bottom. Every object has a real mass and volume, so its density decides what happens, and floating things settle at exactly the depth Archimedes' principle predicts. Change the liquid to salt water, cooking oil, honey or even mercury, and the same things behave differently.

How to use it

• Drop things in: drag anything from the shelf into the tank, or tap it to drop it in. Drag it out of the tank to take it back. Several students can drag at once on a touch screen.
• Choose the liquid: fresh water, salt water, cooking oil, honey or mercury along the top. The number on each button is its density in kg/m³.
• Read the numbers: tap a thing in the tank. The panel shows its mass, volume, density, the liquid's density, how much of it is under the surface, its weight and the buoyant force. The strip underneath places every thing on a density scale: left of the liquid's line floats, right of it sinks.
• Forces: switch on Forces to draw the weight (red) and the buoyant force (green) as arrows, plus the floor pushing up (violet) on things that sink.
• My block: drop in the pink block, then set its mass and volume with the sliders. Can you make it hover in the middle of the water?
• Egg trick: one button sets up the classic demonstration. Then add salt a spoon at a time with the button or the slider.
• Keyboard and projector: 1 to 5 choose the liquid, the arrow keys move the chosen thing, Delete takes it out. Full screen makes the text bigger for the back row.

Density: how heavy for its size

Density tells you how much mass is packed into each bit of volume:

ρ = m / V

A 5 cm wooden cube has a volume of 5 × 5 × 5 = 125 cm³ and a mass of 62.5 g, so its density is 62.5 ÷ 125 = 0.5 g/cm³, or 500 kg/m³. Water is about 1 g/cm³ (998 kg/m³ at 20 °C). A big thing is not always dense: the empty bottle has the biggest volume on the shelf but is mostly air, and the small steel ball is the densest of all.

Why things float: Archimedes' principle

A liquid pushes up on anything in it. The push, called the buoyant force, equals the weight of the liquid the thing pushes aside:

F_B = ρ_liquid × g × V_under

A thing sinks into the liquid until the buoyant force matches its own weight. If it is less dense than the liquid, that happens before it is fully under, and it floats. The fraction under the surface is simply:

V_under / V = ρ_object / ρ_liquid

So the wood cube (500 kg/m³) floats half under in water, and ice (917 kg/m³) floats with about 92% hidden, which is why most of an iceberg is under the sea. Switch on Forces and watch: while a floating thing bobs, the two arrows change length; once it is still, they are exactly equal. A thing denser than the liquid can never push aside enough liquid, so it sinks, and the floor holds up the rest of its weight.

Same thing, different liquid

Density is a contest between the object and the liquid. In cooking oil (about 920 kg/m³), ice only just floats, with 99.7% under. In honey (1 420 kg/m³) the egg floats. In mercury (13 546 kg/m³) even the steel ball and the stone float, riding high on the surface. Only something denser than mercury, such as gold at 19 300 kg/m³, would sink in it.

The egg trick

A fresh egg is a little denser than water, so it sinks. Stirring salt into the water makes the water denser without making it much bigger. Press Egg trick, then add salt:

The egg starts to float at about 10.3% salt, about 115 g of salt in a litre of water: around six or seven heaped tablespoons. You can do the same in class with a tall glass, an egg and a bag of salt.

Try this

1. Before dropping anything, ask the class to sort the shelf into "floats" and "sinks". Then test each one.
2. Drop the ice cube into fresh water and read the percentage under the surface. Switch to salt water and to oil: does it rise or sink deeper?
3. Use My block to make a block that floats exactly half under. What has to be true about its mass and volume?
4. Press Egg trick. How many spoons of salt until the egg leaves the floor?
5. Put the steel ball and the stone on mercury. Why do they float there?

Teaching ideas

The simulation fits lessons on materials and their properties, density and buoyancy, and pressure in liquids (Grades 4–9; NGSS 5-PS1-3 on identifying materials by their properties, and MS-PS2 on forces; GCSE density and upthrust).

1. Predict (5 minutes): "Will a big apple sink because it is heavy?" Vote, then drop it in. Then ask why the tiny steel ball sinks.
2. Measure (10 minutes): groups copy mass and volume from the panel for each thing, compute ρ = m/V and order the shelf by density. Compare with float or sink.
3. Explain (5 minutes): switch on Forces and pause the talk while a floating thing settles. Weight down, buoyant force up: when are they equal?
4. Stretch (5 minutes): honey and mercury. Can a liquid make steel float?
5. Review (10 minutes): make a review game with the button above and play it as a class, in teams, with your phone as the remote.

Questions teachers ask

Are the numbers real?

Yes. Each thing has a typical real mass and volume, and the liquids use measured densities at 20 °C, including a published table for salt water. Real woods, apples and eggs vary a little, so your own egg might need a bit more or less salt.

Why does a floating thing bob before it settles?

When it is dropped in, it goes too deep, the buoyant force is then bigger than its weight and pushes it back up, and so on. The liquid's drag and the waves it makes slowly calm it. Honey calms things much faster than water because it is so thick. The motion runs a little slower than real time so the class can follow it.

What does "hovers" mean?

If a thing has the same density as the liquid, the buoyant force on it matches its weight anywhere under the surface, so it stays where you leave it, neither rising nor sinking. Fish and submarines do this by adjusting how much air they carry. Try it with My block at 400 g and 400 cm³.

Why is weight in newtons?

Weight is a force: mass × g, with g = 9.8 m/s². A 100 g apple weighs about 1 N. The buoyant force is also a force, so the two can be compared directly.

Do I need to install anything or sign up?

No. It runs in any modern browser on a computer, an interactive whiteboard, a tablet or a phone, and it's free.