MYP Physics: Friction & Drag — Frequently Asked Questions

Quick distinction:

• Friction: solid-on-solid contact (a box sliding across a table)
• Drag: solid-through-fluid contact (a parachutist falling, a cyclist against wind)

I tell my Year 10 classes to picture friction as a handshake between two surfaces, and drag as swimming through treacle — the harder you push, the more it pushes back.

There are two flavours worth knowing for MYP assessment:

1. Static friction — stops an object starting to move; this is always slightly higher than kinetic friction
2. Kinetic (sliding) friction — acts once the object is already moving

Common mistake: students assume friction only depends on speed. It doesn't, in the simple model used at MYP level — it depends on the normal force and the roughness of the surfaces (the coefficient of friction), not on how fast something is moving.

Four factors examiners expect you to name:

• Speed (drag rises sharply as speed increases)
• Cross-sectional area (bigger surface = more resistance)
• Fluid density (water creates far more drag than air)
• Shape/streamlining (teardrop shapes reduce turbulence)

Quick tip: if an exam question mentions a skydiver reaching terminal velocity, it wants you to explain that drag has increased until it exactly balances weight — resultant force becomes zero, so acceleration stops.

Worked example: A skydiver of mass 80 kg experiences weight = mass × gravitational field strength = 80 × 9.8 = 784 N. As they fall, drag increases with speed until it also reaches 784 N. At that point, resultant force = 784 − 784 = 0 N, so by Newton's first law they stop accelerating and continue at constant terminal velocity — typically around 55 m/s for a human in free fall before a parachute opens.

Worked example: A 5 kg box rests on a table where the coefficient of friction is 0.3. Normal force $N=mg=5\times 9.8=49$ N. Friction force $F=\mu N=0.3\times 49=14.7$ N. This is the minimum horizontal push needed to start the box sliding.

Common mistake: students multiply mass directly by $\mu$ and forget to convert to a force using $g$ first — that's an easy method mark lost in an assessed piece.

If your class does go further, the idea that drag is roughly proportional to speed squared (doubling speed roughly quadruples drag) is a useful qualitative fact to quote in a Criterion D investigation, even without the full equation.

5-step checklist for a strong response:

1. Identify friction vs drag correctly for the scenario
2. State the direction opposes relative motion
3. Name the factors affecting its size (surface roughness/normal force, or speed/area/density)
4. Link to resultant force and acceleration using Newton's second law
5. Use correct units throughout (Newtons for force, no units for $\mu$)

According to the IB's MYP: From Principles into Practice guide, Criterion A tasks assess scientific knowledge applied to unfamiliar situations — so expect friction/drag questions set in real contexts (car brakes, parachutes, swimming) rather than pure textbook recall.

A well-designed friction investigation for Criterion C should vary one thing at a time — surface material, for example — while keeping mass and normal force constant, then plot force needed to start motion against surface type.

Three things to check before your next assessment:

1. Have you correctly identified whether the surfaces are solid-solid (friction) or solid-fluid (drag)?
2. Have you given direction as well as size where the question asks for a full description?
3. Have you shown working for any calculation, not just a final number — Criterion A rewards method?

Parents often ask why a topic that sounds simple shows up as a weak area on reports. Usually it's not the physics that's hard — it's translating a real-world scenario (a car braking, a feather falling) into the correct force name and formula under exam time pressure.

On revisionprep.com, students can work through Topical Worksheets and Revision Notes specifically organised by MYP unit, which helps target forces and motion without wading through unrelated topics.