The BPhO Year 7/8 and Junior Physics Challenges are, in BPhO's own words, "a Quiz, not an exam" — but the marks that do slip away tend to come from the same handful of misconceptions, again and again. Below are eleven of the most common, drawn straight from our own BPhO challenge toolkit flashcard deck, each paired with the fact that fixes it. This is the reverse side of BPhO facts every KS3 child should know — read that one for the facts themselves; this one is about where they go wrong.

Forces

The mistake: believing a moving object must have a bigger force pushing it forward than holding it back. It feels intuitive — surely something needs a "winning" force to keep going? But an object moving at a steady speed has balanced forces, not an imbalance in its favour. A skydiver falling at constant speed has air resistance exactly equal to their weight. Motion at a steady speed is what balanced forces look like, not evidence of an unbalanced one.

The mistake: dividing time by distance, or multiplying the two, without noticing the unit stops making sense. Speed is distance divided by time — full stop. The fastest check is the unit itself: if an answer doesn't come out in something sensible like metres per second, the calculation was inverted somewhere. Comparing the result to a known speed (a walking pace is about 1.5 m/s, a car about 30 m/s) catches the same error a different way.

The mistake: multiplying force by area instead of dividing, when calculating pressure. Pressure is force divided by area — spreading a force over a bigger area reduces the pressure, it doesn't increase it, which is precisely why snowshoes work. A quick sanity check: a huge area under a modest force should give a small pressure, not a large one.

Gravity

The mistake: treating weight and mass as the same thing, both measured in kilograms. Mass is how much matter something is made of, in kilograms, and never changes. Weight is the force of gravity on that mass, in newtons, and does change — with location. A 60 kg astronaut is still 60 kg on the Moon, but weighs roughly a sixth as much there as on Earth, because the Moon's gravity is weaker.

The mistake: writing the weight as, say, "3 N" — copying the mass number across and just changing the unit. Weight isn't the mass number wearing a different label; it's mass multiplied by gravitational field strength (about 10 N/kg on Earth). A 3 kg object weighs about 30 N, not 3 N — the calculation has to actually happen.

Energy

The mistake: saying an object "has no energy" because it isn't moving. A ball held still at the top of a slide isn't moving, but it's full of energy in its gravitational potential store, purely by virtue of having been lifted. "Not moving" only rules out the kinetic store — it says nothing about the others.

The mistake: mixing up kilo- (×1000) with centi- (÷100) or milli- (÷1000). These prefixes scale in opposite directions, and a challenge question that mixes km, cm and mm in the same sentence is deliberately testing whether that distinction is solid. The fix is mechanical: convert everything to one unit before calculating anything, rather than trying to juggle the conversions mid-calculation.

Electricity

A quieter trap worth flagging, even though it's less of a classic misconception than a mix-up: which meter goes where. An ammeter reads current and belongs in series, in the main loop; a voltmeter reads potential difference and belongs in parallel, connected across a component. Getting the wiring the wrong way round is an easy slip under time pressure, even for a child who knows the definitions perfectly well.

The mistake: thinking bulbs in parallel must be dimmer because they "share" the battery. It's series bulbs that share the supply voltage between them and burn dimmer as a result. In parallel, each branch has its own path back to the cell and gets the full supply voltage — which is also why a parallel circuit keeps working when one branch breaks, unlike a series loop, where a single break stops everything.

Sound

The mistake: thinking sound, like light, can cross empty space. Sound is a vibration passed from particle to particle, so it needs a medium — solid, liquid or gas — to travel through. Pump the air out of a sealed jar with a ringing bell inside, and the sound vanishes even though the hammer can still be seen striking, because light needs no medium at all. That's the entire reason space is silent.

The mistake: forgetting that an echo has made a round trip, and ending up with double the true distance. If a clap returns after 2 seconds, the sound has travelled there and back in that time — not just one way. At roughly 340 m/s, that's a total round-trip distance of about 680 m, meaning the wall itself is about 340 m away, not 680 m. Skipping the halving step is the single most common echo-question error.

Matter

The mistake: believing heavy things sink and light things float, instead of comparing each with water. A huge log floats; a tiny nail sinks — so "heavy" and "light" clearly aren't what decides it. What matters is density: wood, at around 0.7 g/cm³, is less dense than water and floats; iron, at around 7.9 g/cm³, is denser and sinks. Size and absolute weight are distractions from the real comparison.

Why these particular mistakes are worth knowing in advance

None of these are exotic — they're the everyday slips that show up in ordinary KS3 physics lessons, and a challenge paper with a strict time limit simply doesn't leave room to reason your way back to the correct idea mid-question the way a classroom discussion might. Recognising a misconception by name, in advance, is often enough to stop it happening: once a child has consciously noticed "oh, that's the mass-and-weight mix-up," they tend not to fall for it again.

For the full set of facts these misconceptions sit alongside, see BPhO facts every KS3 child should know, and for how spaced practice helps these facts actually stick rather than fade by the time the quiz window opens, see preparing for BPhO with spaced repetition.

FAQ

Are these mistakes specific to the BPhO, or general KS3 physics misconceptions?

General. Every one of these turns up in ordinary KS3 physics lessons long before a child ever sits a BPhO challenge — the quiz format just makes them visible, because there's no time to reason your way back to the right idea mid-question.

How were these misconceptions identified?

They're logged directly on our own BPhO challenge toolkit flashcard deck at aitutors.me, alongside the fact each one is confused with — written and checked in-house, not drawn from any BPhO paper or mark scheme.

Is it better to drill these mistakes directly, or just learn the facts?

Learn the facts first. A misconception usually isn't a knowledge gap so much as the wrong fact sitting where the right one should be — once the correct idea is genuinely familiar, most of these mistakes stop happening on their own.

Duke Harewood built aitutors.me's KS3 physics tutor, Professor Newton, around a predict-observe-explain method that catches exactly these kinds of misconceptions before they harden. Updated 26 August 2026.