Why students “understand” Physics in class and then freeze in the exam

“I understood everything in class.” We hear it from students every week, usually a few days after a Physics test that went badly. And they are telling the truth. They followed the lesson, nodded at every step of the worked example, and could explain it back to the teacher.

Then the exam paper arrived, the first question was about a ball, a ramp and a height, and nothing happened. The formulas were all there in their head. They just could not tell which one the question wanted.

That moment has a cause, and it is not a lack of intelligence or a lack of revision. It is a skill that most Physics lessons never practise.

Knowing a formula is not the same as choosing one

Solving a Physics problem takes three separate skills, and they are easy to confuse because they look like one:

1. Knowing the formula. Being able to write down v² = u² + 2as, or E = ½mv², from memory.
2. Knowing when it is true. Every formula has conditions. The equations of motion only hold when acceleration is constant. Potential energy as mgh only holds close to the Earth’s surface, where g barely changes.
3. Choosing it. Reading a situation described in words, working out which principle governs it and picking the one formula that connects what you know to what you want.

Classroom learning builds the first skill thoroughly and the second one partly. The exam tests the third.

Why class feels like understanding

In a lesson, the choice has already been made for the student.

A lesson on the equations of motion is followed by an exercise on the equations of motion. The chapter heading at the top of the page tells the student which tools to use before they have read a word of the question. The teacher’s worked example begins by writing down the right equation. Homework is set from the same chapter. Everything the student does that week is correct, and none of it required them to decide anything.

This is not bad teaching. Practising one idea at a time is how ideas are first learned. But it creates a convincing illusion: following a solution feels exactly like being able to produce one. The student has recognised the method every time it was shown to them and has never once had to retrieve it from a blank page.

The exam removes the label. Questions arrive mixed, from every chapter, in an order nobody announces. Across the six boards we teach (IB, Cambridge International, the US High School Diploma with AP, CBSE, CISCE and NIOS), Physics papers reward exactly this step. Several of them, IB, A-level and AP among them, even print the formulas in a data booklet or on an equation sheet. When the formulas are handed out, the marks cannot be for remembering them. They are for choosing.

The freeze, slowed down

Here is a question almost every Physics student meets in some form:

A ball is thrown vertically upwards at 20 m/s. How high does it rise? Take g = 10 m/s².

A student who has learned the equations of motion knows four of them. The freeze happens because the question seems to give only one number, and none of the equations can be solved with one number.

The missing step is reading the situation before reaching for a formula. At the top of its flight the ball stops for an instant, so its final velocity is zero. The question never says so. It is a hidden given, carried by the word “how high”. With that, the list of what is known looks like this:

• u = 20 m/s (thrown upwards)
• v = 0 (the top of the flight)
• a = −10 m/s² (gravity, acting against the motion)
• s = ? (what the question wants)
• t is not given and not asked for

Now the choice almost makes itself. The equation to use is the one that contains u, v, a and s, and leaves out t: v² = u² + 2as. So 0 = 400 − 20s, and the ball rises 20 m.

None of that needed new Physics. It needed a procedure for choosing.

The question where the “right” formula is wrong

The second kind of freeze is quieter, because the student does not stop. They start confidently and use a formula that does not apply.

A block slides from rest down a smooth curved ramp from a height of 5 m. How fast is it moving at the bottom?

Many students reach for the equations of motion, because the question is about speed and distance. But on a curved ramp the acceleration changes all the way down, and those equations need it to be constant. Whatever they write next, the method is wrong.

The word “smooth” is the clue. It means no friction, so no energy is lost, and the question belongs to conservation of energy rather than to kinematics. The potential energy at the top becomes kinetic energy at the bottom: mgh = ½mv², the mass cancels, and v = √(2gh) = √100 = 10 m/s.

A student who knows each formula together with the condition that makes it true is protected from this. A student who learned the formulas as a list is not.

Signs a student has this gap

The pattern is consistent enough that parents can usually spot it:

• They say “I understood it in class” after a poor test, and mean it.
• They score well on chapter tests and noticeably worse on mixed papers and end-of-year exams.
• Their most common question is “which formula do I use?”
• They start writing equations before they have drawn a diagram or said in words what is happening.
• Their exam scripts show several formulas tried and abandoned at the top of an answer.

How we teach the choosing

Every formula comes with its conditions. When a student learns an equation, they write down three things: the equation itself, what each symbol means with its unit, and when it is true. The equations of motion: constant acceleration only. Conservation of momentum: only when no external force acts. Learned this way, a formula carries its own warning label.

Physics first, mathematics second. Before any equation is written, we ask the student to say in one sentence what is happening and to sketch it. Then a second question: which big idea is this about? Forces, energy, momentum, motion, fields, circuits. Most school Physics questions are governed by a small number of principles, and naming the principle narrows a page of formulas to two or three.

Knowns, unknowns and hidden givens. Students list what they know and what they want, as in the ball question above, and we train them to translate the words examiners use. “From rest” means u = 0. “Highest point” means the vertical velocity is zero. “Smooth” means no friction. “Light string” means its mass can be ignored. “The two trolleys stick together” means they share one final velocity. A surprising share of frozen students were missing a number the question had given them in words.

Mixed practice, without chapter labels. Once a topic has been learned, we stop setting questions from it in a block. Problems come from the whole course, in no particular order, so that the student has to decide what each one is about. Research on interleaved practice, mostly from Mathematics, has found that mixing problem types makes practice feel harder but improves students’ ability to pick the right method later. That matches what we see.

“Why this one?” Before a student solves a problem, they tell us which formula they are going to use and why, and which one they considered and rejected. It takes ten seconds and it turns an unconscious guess into a decision they can check.

Past papers under exam conditions. Eventually the choosing has to happen alone, against the clock, with the same paper and data booklet the student will have on the day. That is where the skill is confirmed rather than assumed.

What parents can do at home

You do not need to know any Physics to help.

Ask your child to explain a problem to you before solving it, without using any numbers. “What is happening here? What is the question really asking for?” If they can describe the situation, the choice of formula usually follows. If they cannot, that is exactly where the gap is.

It also helps to look past chapter-test scores. A good mark on a test about one topic shows the topic was learned. A good mark on a mixed paper shows it can be used.

The freeze is a missing step, not a missing brain

Students who freeze in Physics exams usually know more than their marks suggest. What they lack is a reliable way to get from a paragraph of words to the right formula, because in class that step was always taken for them.

Make that step visible, give it a procedure and practise it on mixed questions, and “I understood it in class” starts to show up in the exam too.

At TutorWyz, we teach Mathematics, Physics, Chemistry, Biology and English to students on every continent. If your child understands Physics in lessons but freezes in tests, the first assessment is free, and it is usually the quickest way to see which of the three skills is missing.