IB Physics work, energy and power questions become much easier once you stop treating them as formula-selection exercises. The reliable method is to define the system, identify the initial and final energy stores, account for energy transferred by forces, and then check signs, units, and efficiency.
Topic A.3 Work, energy and power is common to SL and HL in the current course. It can appear in Paper 1A multiple-choice questions, Paper 1B data-based questions, and Paper 2 short-answer or extended-response questions. The contexts change, but the underlying structures are predictable.
What you need to know for IB Physics A.3
The current IB Physics course expects students to understand conservation of energy, work as energy transfer, mechanical energy, power, efficiency, Sankey diagrams, and energy density. The core equations are summarized below.
| Quantity | Relationship | Essential condition or meaning |
|---|---|---|
| Work by a constant force | W = Fs cos θ | θ is the angle between force and displacement |
| Translational kinetic energy | Eₖ = ½mv² | Speed is squared, so direction does not affect kinetic energy |
| Change in gravitational potential energy | ΔEₚ = mgΔh | Valid near Earth's surface where g is approximately constant |
| Elastic potential energy | Eₕ = ½k(Δx)² | Δx is extension or compression from equilibrium |
| Average power | P = ΔW/Δt = ΔE/Δt | Power is the rate of work or energy transfer |
| Mechanical power | P = Fv | Use the force component parallel to velocity |
| Efficiency | η = useful output / total input | The numerator and denominator must both be energies or both be powers |
Mechanical energy is the sum of kinetic, gravitational potential, and elastic potential energy included in the chosen system. It is conserved only when frictional and other resistive effects do not transfer energy out of those mechanical stores.
This distinction matters. Total energy is conserved, but mechanical energy may decrease as energy is transferred to thermal energy or the surroundings.
The exam method for work, energy and power questions
Use the following sequence before substituting numbers.
- Define the object or system. Decide whether gravitational or elastic potential energy belongs inside your system.
- Mark the initial and final states. Record speeds, heights, spring extensions, and relevant times at each state.
- List the energy stores. Include only terms that exist in that situation.
- Add external transfers or losses. Work by an external force adds or removes energy; friction commonly transfers mechanical energy into thermal energy.
- Write one complete symbolic equation. Substitute values only after the physical model is clear.
- Check signs, units, and scale. A negative kinetic energy or efficiency above 100% usually reveals a modelling or arithmetic error.
A useful general structure is:
initial mechanical energy + external work = final mechanical energy + dissipated energy
This is not an additional formula to memorize. It is an accounting model that helps you place each quantity on the correct side.
Predictable IB question types
Work done at an angle
If a force of 50 N pulls a box 8.0 m at 30° above the horizontal, the work done by that force is:
W = Fs cos θ = 50 × 8.0 × cos 30° = 3.46 × 10² J
The recurring trap is to use Fs without resolving the force. Only the component parallel to the displacement transfers energy through mechanical work. A perpendicular force does zero work, even if its magnitude is large.
Be careful when the angle is shown relative to the vertical rather than the direction of motion. The angle in the equation must be the angle between the force and displacement vectors.
Work-energy questions with several forces
The work-energy theorem is:
Wnet = ΔEₖ = ½mv² - ½mu²
Here, Wnet means the work done by the resultant force, not automatically the work done by one named force. Suppose a 4.0 kg block is pulled 5.0 m by a horizontal force of 18 N while friction is 6.0 N. If it starts from rest:
Wnet = (18 - 6.0) × 5.0 = 60 J
60 = ½(4.0)v², giving v = 5.5 m s⁻¹ to two significant figures.
A frequent error is to calculate 18 × 5.0 and call it the net work. That is the work done by the pulling force alone; friction does negative work.
Conservation of mechanical energy
For an object falling without significant air resistance:
mgh + ½mu² = ½mv²
If it begins from rest, mass cancels and v = √(2gh). This is often quicker than using constant-acceleration equations because energy connects the initial and final states without requiring time.
Do not cancel mass if another term is not proportional to mass. For example, elastic energy ½kx² prevents automatic cancellation unless the rest of the equation permits it.
When resistance is present, write an explicit loss term:
initial mechanical energy = final mechanical energy + energy dissipated
Never state that energy has been “lost.” It has been transferred to less useful stores, usually thermal energy in the object and surroundings.
Springs and elastic energy
For an ideal spring, stored elastic energy is:
Eₕ = ½k(Δx)²
If a spring launches a mass horizontally without losses, then:
½k(Δx)² = ½mv²
The extension must be measured from the spring's equilibrium length and converted to metres. Students commonly forget the factor of one-half or fail to square the extension.
A force-extension graph may test the same idea without giving the equation directly. The elastic energy is the area under the force-extension graph, so a straight-line Hooke's-law graph forms a triangle with area ½FΔx.
Power questions
Use P = ΔE/Δt when a total energy transfer and time are given. Use P = Fv when a force acts along the direction of motion at a known instantaneous speed.
For a lift raising a 600 kg load vertically at a constant speed of 2.0 m s⁻¹, the useful mechanical power is:
P = Fv = mgv = 600 × 9.81 × 2.0 = 1.18 × 10⁴ W
Constant speed means the resultant force is zero, not that the motor force is zero. The motor must balance the weight and may need additional force to overcome resistance.
For forces at an angle, the general scalar form is P = Fv cos θ. Using Fv without checking direction is the power equivalent of forgetting cos θ in a work calculation.
Efficiency and Sankey diagrams
Efficiency compares useful output with total input:
η = useful energy output / total energy input
or
η = useful power output / total power input
If a motor receives 2.5 kW and supplies 1.8 kW of useful mechanical power, its efficiency is 1.8/2.5 = 0.72, or 72%. Do not multiply by 100 unless the answer is requested as a percentage.
In a Sankey diagram, arrow width represents energy or power. The useful and wasted outputs must add to the input, so the diagram also provides a conservation check.
Graph and data-based questions
Paper 1B can place familiar energy ideas inside an unfamiliar data set. Expect to calculate gradients, interpret areas, compare a model with data, or explain why measured mechanical energy changes.
Common graph relationships include:
- area under a force-displacement graph equals work done
- area under a power-time graph equals energy transferred
- gradient of an energy-time graph equals power
- kinetic energy is proportional to
v², notv - elastic potential energy is proportional to
(Δx)²
Always inspect axis multipliers. If an energy axis is labelled E / 10³ J, a plotted value of 4.2 represents 4.2 × 10³ J. Include units in intermediate graph calculations because they help distinguish a gradient from an area.
Common pitfalls that cost marks
| Pitfall | Better approach |
|---|---|
| Assuming every force does positive work | Decide whether each force is parallel, perpendicular, or opposite to displacement |
| Using work by one force as net work | Sum the work done by all relevant forces |
| Conserving mechanical energy despite friction | Include dissipated energy explicitly |
| Confusing energy with power | Energy is measured in joules; power is measured in watts or joules per second |
| Mixing input energy with output power | Compare energy with energy or power with power |
| Substituting before modelling | Write a symbolic energy equation first |
| Reporting excessive digits | Round consistently with the data and include units |
| Treating constant speed as zero force | State that the resultant force is zero |
Explanations should use causal physics. Instead of writing “the energy decreases because of friction,” state that friction does negative work on the mechanical system and transfers energy to thermal stores.
The fastest practice loop
Re-reading notes can clarify definitions, but it does not train the decisions required in an exam. A faster route to mastery is to attempt a question without help, inspect a complete worked solution, identify the first incorrect decision, and then repeat a similar question.
Use this loop:
- Attempt one question under a short time limit.
- Commit to a diagram and symbolic equation before checking anything.
- Watch the question worked through step by step.
- Record whether the error involved the system, equation, sign, algebra, unit, or interpretation.
- Reattempt the question from a blank page the next day.
RevisionDojo's A.3 Questionbank supports targeted exam-style practice, while the A.3 worked physics videos let you compare your reasoning with a worked method. This attempt-then-watch sequence is more useful than passively watching several solutions because it exposes the precise step at which your model failed.
Use the A.3 topic hub to move between questions, lessons, and revision materials. The work, energy and power notes are useful for repairing conceptual gaps, while the A.3 flashcards help consolidate definitions and conditions. For mixed-topic preparation, return to the broader IB Physics resource hub.
Conclusion
Successful answers to IB Physics work, energy and power questions begin with an energy model, not a memorized formula. Define the system, compare initial and final states, include external work or dissipation, and check that signs and units match the physical situation.
The question formats recur often enough that deliberate practice produces rapid improvement. Attempting questions and then studying their worked video solutions is the most direct way to recognize those patterns. RevisionDojo's A.3 Questionbank and worked physics videos are the most relevant tools for building that exam-ready method.
Sources and referenced URLs
- Official IB Physics subject brief, first assessment 2025
- Official IB Physics curriculum updates
- Official IB Physics in the Diploma Programme overview
- OpenStax explanation of the work-energy theorem
- RevisionDojo A.3 Work, Energy and Power topic hub
- RevisionDojo A.3 Work, Energy and Power Questionbank
- RevisionDojo A.3 worked physics videos
- RevisionDojo A.3 Work, Energy and Power notes
- RevisionDojo A.3 Work, Energy and Power flashcards
- RevisionDojo IB Physics resources
