MYP Physics: Kinetic & Potential Energy FAQ

In fifteen years of marking this topic, the single most common error is students substituting velocity into the $E_p=mgh$ equation, or forgetting to convert grams to kilograms and centimetres to metres before they calculate anything. Quick tip: underline every value and its unit before you touch a formula — half the marks lost here are unit errors, not conceptual ones.

Think of a rollercoaster at the top of a drop: it's almost all potential energy and barely any kinetic energy. At the bottom of the drop, that's reversed. The total mechanical energy — ignoring friction and air resistance — stays constant throughout.

Check which value your school uses for $g$ — some MYP schools round to 10 m/s² for simplicity, others use 9.8 m/s² to match DP conventions. Always state which one you're using in your working; examiners mark the method, not just the final number.

This is why $E_p=mgh$ is linear in height rather than squared like kinetic energy's dependence on velocity — lifting an object twice as high takes exactly twice the work, assuming mass and gravitational field strength stay constant.

Worked example: A 0.5 kg ball travels at 6 m/s.

1. Square the velocity: $6^2=36$
2. Multiply by mass: $0.5\times 36=18$
3. Halve it: $18÷2=9$ J

Answer: $E_k=9$ J. The most common slip is halving before squaring — always square the velocity first.

Worked example: A 1.5 kg book sits on a shelf 1.2 m high, using $g=9.8$ m/s².

1. Multiply mass and $g$: $1.5\times 9.8=14.7$
2. Multiply by height: $14.7\times 1.2=17.64$ J

Answer: $E_p\approx 17.6$ J. Always measure height from the reference point stated in the question, not from the floor by default.

Worked example: A 2 kg ball is dropped from 5 m, using $g=10$ m/s².

1. Find starting $E_p$: $2\times 10\times 5=100$ J
2. At the ground, height is 0, so $E_p=0$ and all 100 J has converted to $E_k$.
3. Solve for velocity: $100=\frac{1}{2}(2)v^2\Rightarrow v^2=100\Rightarrow v=10$ m/s

This conservation-of-energy method works for pendulums, rollercoasters and falling objects alike, as long as friction and air resistance are ignored.

Checklist before you submit any energy calculation:

1. Are mass, height and speed all in kg, m and m/s?
2. Have you squared velocity before halving, not after?
3. Have you stated which value of $g$ you used?
4. Does your final answer have the correct unit — joules (J)?
5. If energy is 'lost', have you accounted for it as heat or sound rather than ignoring it?

Criterion A is marked out of 8, across two strands covering explanation of scientific knowledge and application to solve problems — energy calculations usually sit in the second strand. Command terms like 'calculate' and 'determine' signal that examiners want a clear numerical method with units, not just a final number.

According to the IB, MYP eAssessment on-screen exams for Sciences combine multiple-choice, short-answer and extended-response questions, often built around real-world scenarios like transport or sport — expect an energy calculation embedded in a longer stimulus rather than presented in isolation.

Mixing these up — describing when the question asks you to explain — is one of the most common ways students lose marks even with correct science.

Three habits that separate a 6 from a 7:

1. Show every substitution step, not just the final answer.
2. State assumptions explicitly (e.g. 'ignoring air resistance').
3. Connect the number back to the scenario — explain what a high or low value of energy actually means physically.

Practising past-paper-style extended-response questions on RevisionPrep's Topical Worksheets is one of the fastest ways to build this habit before a summative assessment.

On revisionprep.com, the MYP Physics Revision Notes summarise the formulas and worked examples in one place, while the Topical Worksheets provide graded practice questions organised by criterion — useful for building the calculation-plus-explanation habit examiners look for.

Most students who struggle here aren't weak at maths — they're rushing past the units and skipping the 'state your assumptions' step examiners look for. A few minutes of your child explaining their working out loud each week catches this faster than any amount of extra worksheet time.

According to the IB, the current DP Physics guide (first exams 2025) builds directly on these MYP foundations before introducing gravitational potential energy near massive bodies and elastic potential energy in Topic 2 (Mechanics).