IB Physics Friction: How It's Taught, Examined and Revised

Friction is the resistive force opposing relative motion between two surfaces in contact, covered under Topic A.2 (Forces) in the current IB Physics guide, first examined 2025. It's examined through calculation questions using $f=\mu N$, free-body diagrams, and experimental work on the coefficient of friction — at both SL and HL, with HL adding inclined-plane and multi-body problems.

Static friction acts on an object at rest and adjusts up to a maximum value ($f_{s,max}={\mu }_{s}N$) to prevent motion starting. Kinetic (dynamic) friction acts once the object is already sliding, using $f_k={\mu }_{k}N$, and is almost always smaller than the maximum static value for the same surfaces.

The core relationship is $f=\mu N$, where $f$ is the friction force, $\mu$ is the coefficient of friction (static or kinetic, unitless), and $N$ is the normal reaction force. This appears on the IB Physics data booklet, so you don't need to memorise it — you need to know when to use ${\mu }_s$ versus ${\mu }_k$.

Yes — friction sits in Topic A.2 (Forces and momentum), which is common to both SL and HL. The core concept and $f=\mu N$ apply equally, but HL problems more often combine friction with connected bodies, inclined planes at variable angles, or energy-loss calculations across multiple stages of motion.

Resolve gravity into components parallel ($mg\sin \theta$) and perpendicular ($mg\cos \theta$) to the slope, find the normal force from the perpendicular component, then calculate friction as $f=\mu N=\mu mg\cos \theta$. Compare the parallel weight component to friction to find net force and acceleration.

The biggest one is confusing static and kinetic friction — using ${\mu }_k$ before an object has actually started moving. The second is forgetting friction acts opposite to the direction of relative motion (or intended motion), which flips the sign in equations of motion and changes the final answer entirely.

Friction makes a solid IA topic because it's easy to measure but has real depth — you might investigate how μ varies with surface roughness, mass, or contact area using an inclined plane or a pulley system. Examiners look for a clear research question, systematic uncertainty analysis, and enough data points (5+ trials, ideally more) to justify a trend line.

Almost always it's the free-body diagram, not the formula. Examiner reports on mechanics questions consistently note that students calculate $f=\mu N$ correctly but then misapply it — using the wrong normal force, forgetting a second force acting on an incline, or mixing up direction when summing forces for Newton's second law.

Both. Paper 1 (multiple choice, no calculator at SL for some sections) tests conceptual understanding and quick calculations of $\mu$ or friction force. Paper 2 (structured, calculator allowed) is where friction gets combined with kinematics, energy, or connected-body problems worth several marks each.

It varies by paper, but a Paper 2 structured question involving friction typically carries 4-8 marks across several sub-parts — free-body diagram, calculation of normal force, calculation of friction, then acceleration or energy loss. Standalone Paper 1 questions are usually worth 1 mark each.

Friction does negative work on a moving object, converting kinetic energy into heat — calculated as $W_f=f\times d$, where $d$ is the distance moved against the friction force. This connects directly to Topic A.4 (Energy), where friction is the go-to example of a non-conservative force causing mechanical energy loss.

Not conceptually — the formula is simple and sits on the data booklet. What makes friction feel harder is that it rarely appears alone; it's usually layered onto inclined planes, connected objects, or energy questions, so the difficulty comes from combining topics, not from friction itself.