IB Physics Mechanics: A Complete Topic Guide

Mechanics is the foundation of IB Physics. It appears in Topic 2 (and for HL students, it extends into circular motion, gravitation, and more). Whether you're sitting SL or HL, a strong understanding of mechanics is what holds the rest of the course together — and it's one of the most reliably tested areas on all three papers.

This guide covers every core mechanics topic, how each one is examined, and the conceptual gaps that cost students marks.

1. Kinematics — Describing Motion

Kinematics deals with how objects move, without asking why. The IB exam tests kinematics through numerical problems, velocity-time graph interpretation, and projectile motion.

The four SUVAT equations

For uniform acceleration, you'll use these four equations (they're given in the data booklet):

• v = u + at
• s = ut + ½at²
• v² = u² + 2as
• s = ½(u + v)t

The key skill is identifying which three quantities you know and choosing the equation that contains your unknown. List your knowns first — this alone prevents most errors.

Velocity-time graphs

The IB loves v-t graph questions. Remember: the gradient is acceleration, and the area under the curve is displacement (not distance if the object reverses direction). On Paper 2, you may be asked to sketch or annotate a v-t graph — this is conceptual, not purely numerical.

Projectile motion

Projectile motion is treated as two independent motions: constant horizontal velocity and constant vertical acceleration (g = 9.81 m s⁻²). The most common error is mixing horizontal and vertical equations. Keep them strictly separate, and identify t (time of flight) as the link between the two components.

2. Newton's Laws — Forces and Motion

Newton's three laws underpin almost everything in mechanics. The IB expects you to apply them, not just state them.

Free body diagrams (FBDs)

Drawing a correct FBD before solving any force problem is non-negotiable. An FBD shows all forces acting on a single object as arrows from the object's centre. Common forces: weight (mg downward), normal force (perpendicular to surface), tension, friction, and applied force.

Newton's second law: F = ma

This is the workhorse of mechanics. For problems involving multiple forces, resolve into components. On inclined planes, choose axes parallel and perpendicular to the surface — this simplifies the algebra considerably.

Newton's third law

Every force has an equal and opposite reaction force acting on a different object. A common exam question pairs this with FBDs — you need to identify which forces form third-law pairs and which act on the same object (which don't).

3. Work, Energy, and Power

The energy approach is often faster than force-based methods for problems involving motion over a distance. Choose it when you don't need to find forces at intermediate points.

Work-energy theorem

The net work done on an object equals its change in kinetic energy: W_net = ΔKE. This is powerful for problems where you know the initial and final speeds but not the individual forces throughout the motion.

Conservation of energy

Where no non-conservative forces act (or their work is zero), total mechanical energy is conserved: KE + PE = constant. For problems with friction, you'll need to account for energy dissipated as heat: W_friction = ΔKE + ΔPE.

Power

P = Fv is particularly useful for problems about engines, vehicles, and motors — where you're given speed and need to find power or vice versa.

4. Momentum and Impulse

Momentum (p = mv) is conserved in all collisions if no external net force acts. The IB tests both elastic collisions (KE conserved) and inelastic collisions (KE not conserved). For perfectly inelastic collisions, the objects stick together after impact.

Impulse-momentum theorem

Impulse J = FΔt = Δp. This links force and time to change in momentum — it explains why car airbags and crumple zones work. On Paper 1, expect to calculate impulse from the area under a force-time graph.

Two-dimensional collisions (HL)

HL students need to apply momentum conservation in both x and y directions separately for 2D collisions. Set up two equations, one per direction, and solve simultaneously.

5. Circular Motion and Gravitation (HL extension)

For HL students, Topic 6 extends mechanics to circular motion and gravitational fields. The key insight: centripetal force is not a separate force — it's the name we give to the net force directed toward the centre of the circular path.

Common sources of centripetal force: tension (for a mass on a string), normal force (for a car on a banked curve or a roller-coaster loop), gravity (for satellite orbits), and static friction (for a car rounding a bend).

For gravitational fields, understand both gravitational field strength (g = GM/r²) and gravitational potential (V = -GM/r) — these are often tested through satellite problems and energy comparisons.

How Mechanics Is Examined Across the Papers

• Paper 1 (MCQ): Kinematics, Newton's laws, energy/momentum concepts, graph interpretation. Often tests conceptual understanding through distractors.
• Paper 2 (Structured): Multi-step calculation problems, FBDs, energy analysis. Show all working — even a wrong answer can earn method marks.
• Paper 3 (HL): May include data analysis involving mechanics, or questions on the option topic if mechanics-related.

If you want personalised support with IB Physics mechanics — including worked examples, past paper walkthroughs, and targeted problem-solving practice — working with an IB Physics HL tutor online or IB Physics SL tutor online can make a significant difference to your understanding and confidence.