Does total mechanical energy always stay constant?

No, and that “no” is one of the most testable ideas in IB Physics. Total mechanical energy stays constant only when the system has no significant non-conservative forces doing work, such as friction and air resistance. In many exam questions, the phrase “smooth track” or “negligible air resistance” is your hint to use conservation of mechanical energy. If those phrases are missing, you should pause and consider whether energy is being transferred into thermal or sound stores. The good news is that the method is still the same: you write an energy balance, but include energy transferred out of the mechanical account. Practicing both versions in the A.3 work, energy and power questionbank helps you stop guessing.

Can total mechanical energy increase?

Yes, if something external adds energy to the system in a mechanical way. In IB Physics, that usually means an external force does positive work, like a motor pulling an object up, a person pushing a cart, or a spring being wound by an external agent. The key idea is system boundaries: if you include the motor as part of the system, the “increase” may become an internal transfer instead. Examiners like questions where students forget to define the system and then misinterpret what changed. A clean way to avoid that is to write, in words, what’s inside your system before you write equations. RevisionDojo’s work, energy and power notes model this habit with consistent language.

How do I know which potential energy equation to use?

Start by asking what force is storing energy, which is exactly how IB Physics wants you to think. If the energy comes from height in a uniform gravitational field near Earth, use (mgh). If the energy comes from stretching or compressing a spring-like object that follows Hooke’s law, use (\\tfrac12 kx^2). Some questions include both at once, like a spring launcher on a raised platform, and then total mechanical energy includes multiple potential terms. The most common mistake is choosing formulas too early, before listing the energy stores present. Train yourself to write “KE + GPE + EPE” first, and only then substitute expressions. If you want a fast refresher on units (which catches a lot of mistakes), What is the unit for energy? is a quick sanity check.

IB Physics: How to Find Total Mechanical Energy

IB Physics

RevisionDojo Team

•11/15/2025•6 min read

When an exam question says “find the total mechanical energy,” it often feels like it’s asking for a single neat number in a messy world. You picture a roller coaster, a bouncing mass, or a swinging pendulum. Then your brain adds the unhelpful voice: What if I pick the wrong energy? What if I forget friction?

That tension is exactly why this skill matters in IB Physics. Total mechanical energy is a simple idea, but it’s also a reliable way to stay calm when motion problems try to rush you.

Total mechanical energy in IB Physics (definition)

Total mechanical energy is the energy tied to motion and position in a system. In IB Physics, you’ll almost always treat it as:

Total mechanical energy, (E_{mech} = E_k + E_p)

Where:

Kinetic energy: (E_k = \tfrac12 mv^2)
Gravitational potential energy (near Earth): (E_p = mgh)
Elastic potential energy (springs): (E_p = \tfrac12 kx^2)

If you want the official syllabus-aligned framing, RevisionDojo’s A.3 Work, energy and power notes and A.3.1 conservation of energy notes keep the language exam-friendly.

A quick checklist to find total mechanical energy

Use this mini routine (it’s fast enough for timed IB Physics questions):

Choose the system (object + Earth? object + spring?)
List energy stores present (KE, GPE, EPE)
Write (E_{mech} = E_k + E_p)
Substitute formulas only after you’ve listed the stores
Decide if mechanical energy is conserved (friction/air resistance?)

For targeted practice after you learn the routine, use the A.3 questionbank or the broader Mechanics questionbank.

Stick figures arguing about friction stealing energy

Worked setup patterns you’ll reuse

Most IB Physics problems are just variations of two templates.

Gravity + motion

A moving object at height (h):

(E_{mech} = \tfrac12 mv^2 + mgh)

At the top of a track: (mgh) dominates. At the bottom: (\tfrac12 mv^2) dominates. The total can stay constant only if losses are negligible.

Spring + motion

A mass attached to a spring stretched by (x):

(E_{mech} = \tfrac12 mv^2 + \tfrac12 kx^2)

This shows up constantly alongside SHM. If you’re practicing that crossover, the C.1 Simple harmonic motion questionbank helps you mix energy and oscillations in exam style.

Conservation of mechanical energy (and when it fails)

In idealized IB Physics setups (no friction, no air resistance), mechanical energy is conserved:

(E_{mech,initial} = E_{mech,final})

In real setups, mechanical energy often decreases because energy transfers into thermal and sound stores. Energy is still conserved overall, but mechanical energy isn’t.

If you want the bigger-picture reasoning, Why is energy conservation considered universal? is a helpful read before you do mixed questions.

Roller coaster friction exists error comic

How this connects to IAs and exam strategy

Total mechanical energy is a great IA backbone because it’s measurable (speed, height, extension) and naturally invites evaluation (losses, uncertainties, modeling assumptions). For exams, it’s even more valuable because it reduces a long story problem into a single balance statement.

On RevisionDojo, students usually combine:

Study Notes for clean definitions (start at 2.3 Work, energy, and power)
Flashcards for formulas and units
Questionbank for exam-style repetition with solutions
AI Chat to diagnose “why did I choose the wrong system?” moments
Mock Exams, Predicted Papers, and Grading tools to practice finishing under time pressure
Tutors when you want feedback on method, not just answers

Closing: turn energy into a repeatable method

The simplest advantage in IB Physics is having a method you trust when you’re tired. Total mechanical energy gives you that: name the stores, write (E_{mech} = E_k + E_p), decide whether it’s conserved, and move.

When you’re ready to make this automatic, RevisionDojo is built for it: use the Study Notes for clarity, the Flashcards for retention, the Questionbank for repetition, and the AI Chat plus Grading tools to sharpen your method until it matches markschemes. Then test it under pressure with Mock Exams and Predicted Papers -- and walk into your next IB Physics exam knowing energy questions are one of your safest points.

IB Physics: How to Find Total Mechanical Energy

IB Physics: How to Find Total Mechanical Energy

Total mechanical energy in IB Physics (definition)