IB Physics Circular Motion: Exam Guide & Common Mistakes

Circular motion appears across Paper 1 (multiple choice on direction of force and velocity), Paper 2 (calculation questions on centripetal force, banked curves, vertical circles) and occasionally Paper 3 (data analysis with a rotating apparatus). According to the IB Physics guide (first exams 2025), it sits under Topic 6: Circular motion and gravitation at SL, extended by Topic 10 at HL.

The essentials are $F=\frac{m{v}^2}{r}$ (centripetal force), $a=\frac{v^2}{r}$ (centripetal acceleration), and $v=\frac{2\pi r}{T}$ (linear speed from period). HL adds angular versions: $\omega =\frac{2\pi }{T}$, $v=\omega r$, and $a={\omega }^{2}r$. All are on the data booklet, so memorising them isn't the issue — knowing when to use which is.

A 0.20 kg ball on a 0.50 m string moves in a horizontal circle at 4.0 m/s. Find the tension. Using $F=\frac{m{v}^2}{r}=\frac{0.20\times 4.0^2}{0.50}=6.4$ N. That's the net centripetal force — for a purely horizontal circle with no other forces, tension supplies it directly.

Because the net force needed for circular motion changes direction relative to gravity as the object moves around the loop. At the top of a vertical circle, gravity and the normal/tension force both point toward the centre; at the bottom, they point in opposite directions. Students who use one equation for the whole circle get it wrong.

It's moderately difficult — the maths itself is simple (mostly rearranging $F=\frac{m{v}^2}{r}$), but students struggle with the conceptual side: identifying which real force (tension, friction, normal force, gravity) provides the centripetal force in a given scenario. That conceptual gap, not the algebra, is what separates a 5 from a 7 on these questions.

HL circular motion is harder because it links to angular quantities (angular velocity, angular acceleration) and rotational dynamics — moment of inertia and angular momentum in Topic 10 — none of which SL students touch. SL circular motion stays confined to Topic 6: linear speed, centripetal force and basic gravitational orbits.

Start by drawing free-body diagrams for every classic scenario — horizontal circle, vertical loop, banked curve, conical pendulum — before touching a calculation. Then work through past paper questions grouped by scenario type rather than randomly, since the IB reuses the same handful of setups every session with different numbers.

Centripetal force is real — it's the net force pointing toward the centre of the circle, provided by tension, gravity, friction or a normal force. Centrifugal force isn't a real force at all; it's the apparent outward push you feel in a rotating frame of reference, and the IB syllabus does not require you to use it.

Resolve the normal force into horizontal and vertical components. The vertical component balances gravity; the horizontal component provides the centripetal force. For a frictionless banked track at angle $\theta$, this gives $\tan \theta =\frac{v^2}{rg}$ — a useful shortcut worth memorising, since it appears almost every exam session.

Yes — it links directly to gravitation (orbital motion uses the same centripetal force equation with gravitational force as the provider), and at HL it connects to simple harmonic motion, since uniform circular motion is the geometric basis for describing oscillations with $\omega$ and phase.

Circular motion is a relatively compact topic in the DP Physics guide — typically 3-5 teaching hours at SL — compared to topics like Thermal Physics or Fields, so schools often move through it quickly. It still appears reliably on Paper 1 and Paper 2 every session, so a short teaching allocation doesn't mean it's low-stakes for the final grade.