MYP Physics: Density & Floating FAQ

In MYP Physics, density and floating come down to one relationship: density equals mass divided by volume, usually in kg/m³ or g/cm³. An object floats when its average density is lower than the fluid around it. You need to calculate density accurately, compare it to water's 1.00 g/cm³, and explain floating using upthrust and displaced fluid.

Density is mass divided by volume: ρ = m ÷ V. Measure mass in grams or kilograms on a balance, find volume by measuring dimensions (regular solids) or water displacement (irregular solids), then divide. Always state units — g/cm³ for small objects, kg/m³ for larger ones — because MYP markschemes penalise missing or wrong units.

An object floats when its overall density is less than the density of the fluid it's in, and sinks when it's denser. Wood floats in water because it's less dense than 1.00 g/cm³; a steel nail sinks because steel is roughly eight times denser than water.

Mass is the amount of matter in an object, measured in kilograms and constant everywhere. Weight is the force of gravity on that mass, measured in newtons and changes on the Moon or in space. Density is mass per unit volume, in kg/m³, and tells you how tightly packed that matter is.

Archimedes' principle states that the upthrust on an object in a fluid equals the weight of fluid it displaces. MYP Physics doesn't require the formal formula, but you do need to explain floating and sinking qualitatively using displaced fluid and upthrust — it usually appears in Criterion A inquiry or Criterion D application tasks.

Start by identifying what's being asked — calculation, explanation, or comparison — then show your working with units at every step. For floating questions, always compare the object's density directly to the fluid's density rather than just saying 'heavy' or 'light', since examiners look for that precise comparison in Criterion C and D responses.

The three mistakes I see most: mixing up mass and weight (using grams where newtons are needed), forgetting to convert units before dividing (cm³ vs m³), and stating an answer without units at all — which loses marks even when the number is correct. Always double-check units before you finalise a density value.

Density and floating usually show up across Criterion A (Knowing and Understanding) for calculations and definitions, and Criterion C (Processing and Evaluating) or Criterion D (Reflecting) if you run a displacement experiment. Expect short-answer calculation questions plus at least one extended response asking you to explain floating using density and upthrust.

Density is usually expressed in g/cm³ for small lab objects and kg/m³ for larger or real-world objects like ships and air. To convert, multiply g/cm³ by 1000 to get kg/m³ — so water at 1.00 g/cm³ is exactly 1000 kg/m³. Always check which unit the question expects before you calculate.

Yes — density calculations and floating/sinking scenarios are common in the on-screen MYP eAssessment Sciences paper, often as data-based questions with a graph or diagram. According to the IB's MYP: From Principles into Practice guide, the Sciences eAssessment tests conceptual understanding across all four criteria, so expect both calculation and explanation items.

A steel ship floats because its overall shape spreads that steel mass across a huge volume of mostly air, so its average density — total mass divided by total volume — comes out lower than water's 1.00 g/cm³. It's the hollow hull's volume, not the material alone, that determines whether something floats.

The best home experiment is simple: fill a tub with water and test which household objects float — a coin, an orange (peeled and unpeeled), a piece of foil scrunched into a ball versus flattened. Ask your child to predict first, then explain each result using density, not just 'heavy' or 'light'.

Focused revision notes covering density and floating with worked calculations, paired with topical worksheets in the same style as MYP eAssessment questions, help far more than re-reading a textbook chapter. Short, targeted practice on calculation-and-explanation questions builds the confidence needed for both classwork and the on-screen exam.