The maths hiding inside GCSE Physics
"He's fine with the ideas, he just can't do the questions." I hear a version of that sentence most months, usually from a parent who's puzzled by it, and it's nearly always describing the same thing. The physics went in. The maths underneath it didn't. And because the report says physics, that's where everyone looks.
In this guide
How much of it is maths
At least 30 per cent. That's not an estimate, it's a floor set by Ofqual: exam boards must give a minimum of 30 per cent of GCSE Physics marks to mathematical skills.
Compare that to the other two sciences. Chemistry sits at a minimum of 20 per cent, biology at 10. So a student who's coasting in biology and struggling in physics may not be worse at physics as a subject. They're meeting three times as much maths, and it's finding them out.
Nearly a third of the paper, before we've said anything about physics at all. Worth knowing when you're deciding what to spend the revision hours on.
Standard form and prefixes
Physics deals in the very large and the very small, so numbers arrive in standard form constantly, and they arrive wearing prefixes: kilo, mega, giga, milli, micro, nano.
Two things go wrong here, and both are arithmetic rather than physics.
The first is conversion. A question quotes a distance in kilometres and wants an answer in metres, or gives a current in milliamps when the equation expects amps. Nothing conceptual is being tested; the question is simply checking whether your child noticed. Plenty don't, under time pressure, and the answer comes out a thousand times too big with no other error in it.
The second is the calculator. Entering standard form correctly requires knowing which button does it, and a student who types out all the zeros instead will eventually mistype one. That's a mark gone for a reason that has nothing to do with physics and everything to do with an unfamiliar keypad.
The algebra underneath the equations
Your child knows that force equals mass times acceleration. The question gives force and acceleration and asks for mass.
If rearranging is automatic, this is a non-event. If it isn't, the student stares at the page, tries something, and either gets it or doesn't. That's the whole difference, and it repeats across every calculation question on the paper.
What makes it frustrating to watch is that the physics was fine. They picked the right equation. They knew what the question was about. The mark went because the algebra wasn't secure enough to survive being put under time pressure in an unfamiliar subject. I've written separately about which equations have to be memorised, but knowing the equation is only ever half of it.
Worth trying at home: write down any physics equation from your child's book and ask them to rearrange it for each of the other letters, without a calculator and without a question attached. Thirty seconds. If that's hard in the calm of a kitchen, it will not happen in an exam hall, and you've just found the actual problem.
Graphs, gradients and what a slope means
Physics asks students to draw graphs, read values off them, and calculate a gradient. Then it asks the question that separates the grades: what does that gradient represent?
Drawing the line of best fit is a skill. Choosing a sensible scale so the graph fills the paper is a skill, and a surprising number of marks hinge on it. Calculating rise over run is straight GCSE Maths. But interpreting the slope as a speed, or a resistance, or a spring constant is where the two subjects meet, and it's the bit that can't be bluffed.
Students who are comfortable with gradients in maths tend to find this straightforward once someone points out that it's the same operation. Students who found gradients shaky in maths meet them again here, in a harder context, with units attached.
Significant figures, the last-step tax
This one is small, avoidable, and costs marks every single year.
A question asks for an answer to two significant figures. Your child does the physics correctly, does the arithmetic correctly, and writes down everything the calculator displayed. Mark lost, at the very last step, after all the hard work was already done.
Rounding too early causes the opposite problem: an answer that's slightly wrong because a value was cut short halfway through. The rule is to carry the full number through the working and round only at the end, and it takes about ten seconds to teach and a fortnight of noticing to embed.
Which problem does your child actually have?
This is the useful question, and it's answerable at home in twenty minutes.
Take a marked physics paper and look only at the questions that lost marks. For each one, ask which step failed. Did they not know what was going on? That's physics. Did they know what was going on, pick the right equation, and then come unstuck on the numbers? That's maths.
Sort ten lost marks that way and the pattern is usually obvious within a few minutes. If most of them landed in the second pile, then more physics revision is the wrong prescription, and your child will keep doing it faithfully while the marks stay where they are.
There's a reason I find this the most cheering diagnosis I can give a family. Not understanding physics is a broad problem that takes months. Not being able to rearrange an equation is narrow, specific, and usually sorted in a handful of sessions. It's also the same work that lifts the maths grade, so it pays twice. My guide on how to revise for GCSE Maths covers the drilling side of it.
Common questions
How much maths is there in GCSE Physics?
At least 30 per cent of the marks, because that's the minimum Ofqual requires boards to allocate to mathematical skills in GCSE Physics. The comparable floors are 20 per cent for chemistry and 10 per cent for biology, which is why physics feels so different from the other two sciences to a student whose algebra is shaky.
Can students use a calculator in GCSE Physics?
Yes, a calculator is allowed throughout, which surprises parents who assume the non-calculator rule from maths carries across. It doesn't rescue a student who can't rearrange an equation or doesn't notice a unit mismatch, since a calculator answers the sum it's given rather than the sum the question asked for.
Does weak maths hold back a GCSE Physics grade?
Frequently, and it's often mistaken for weakness at physics. A student who understands the concept but can't rearrange, convert units or handle standard form loses marks across the whole paper rather than in one topic. The useful news is that this is a narrower and more fixable problem than not understanding physics.