MYP Physics: Mass, Weight & Gravity — Student FAQ

Start by defining terms precisely: mass (kg) is the amount of matter in an object and stays constant; weight (N) is the gravitational force on that mass and changes with location. Always show the formula weight = mass × gravitational field strength (W = mg), substitute units correctly, and state your final answer with correct units and significant figures.

Mass measures how much matter an object has and is measured in kilograms (kg) — it doesn't change wherever you go. Weight is the force gravity exerts on that mass, measured in newtons (N), and it depends on the local gravitational field strength. Take a rock to the Moon and its mass is identical; its weight drops to roughly a sixth.

Gravitational field strength (g) tells you how much force, in newtons, acts on each kilogram of mass at a given location — on Earth's surface it's about 9.8 N/kg. It changes because it depends on the mass of the planet or moon and the distance from its centre; more massive bodies, or being closer to the centre, both increase g.

Mass is a fixed property of matter — it counts particles and their arrangement, which doesn't depend on where you are. Weight is a force, and forces depend on the gravitational field acting at that specific location. Move the object, keep the same field strength, and weight stays the same too — it's location, not the object, that changes.

Weight equals mass multiplied by gravitational field strength: W = mg. Weight is measured in newtons (N), mass in kilograms (kg), and gravitational field strength in newtons per kilogram (N/kg). On Earth's surface, g is usually taken as 9.8 N/kg unless a question tells you otherwise.

Use W = mg with the gravitational field strength for that planet instead of Earth's 9.8 N/kg. Mass stays exactly the same number regardless of planet; only g changes, so weight changes proportionally. This is the classic MYP assessment-style question — examiners love a table of planets and asking you to calculate weight for each.

Identify what you're given and what you need. If you have mass and need weight, multiply by g (W = mg). If you have weight and need mass, divide by g (m = W ÷ g). Check your units match the formula before substituting — a common error is mixing grams with kilograms.

In MYP and IB-style mark schemes, the unit is part of the answer — writing 588 instead of 588 N usually loses a mark even if the number is correct. Another frequent cause is skipping the formula line: examiners award method marks for showing W = mg before substitution, not just for the final figure.

Common command terms include "state" (give a short answer with no explanation), "calculate" (show working and reach a numerical answer with units), "explain" (give reasons or a mechanism, not just a description), and "compare" (identify similarities and differences, often used for mass vs weight on different planets).

Mass, weight and gravity are typically assessed under MYP Sciences Criterion B (Inquiring and designing) and Criterion C (Processing and evaluating data) if it's linked to a practical investigation, or through short-answer tests assessing Criterion D (Knowing and understanding) style factual and calculation questions. Expect a mix of definitions, formula application, and short explanation questions.

The single biggest mistake is treating mass and weight as interchangeable — saying a person "weighs 70 kilograms" instead of stating their mass is 70 kg and weight is roughly 686 N. The second most common error is forgetting that gravitational field strength, not gravity itself, is what changes between planets in these calculations.

No — MYP treats mass, weight and gravity conceptually and numerically with simple formulas like W = mg, while DP Physics (both SL and HL) builds on this with Newton's law of gravitation, gravitational potential and orbital mechanics. MYP 4-5 is genuinely the foundation year; getting it solid here makes DP mechanics far less intimidating later.

This MYP topic is the direct groundwork for DP Physics's gravitational fields option and core mechanics — the same W = mg relationship reappears, just alongside Newton's universal law of gravitation and gravitational potential energy. A student who's shaky on mass vs weight in MYP tends to struggle when DP introduces variable g near massive objects.

Look for resources with clear worked calculations rather than just definitions — students need repeated practice substituting into W = mg with different planets and units to build real fluency. Topical worksheets that isolate this one skill, paired with short revision notes summarising the mass-versus-weight distinction, tend to work better than a general textbook chapter alone.