Is work, energy and power included in both IB Physics SL and HL?

Yes, A.3 Work, energy and power is studied by both SL and HL students in the current IB Physics course. Its core content includes conservation of energy, work done by forces, mechanical energy, power, efficiency, and Sankey diagrams. HL students are not given a separate A.3 equation set, although HL examination questions may connect these ideas with additional higher-level material elsewhere in the course. Questions can appear in multiple-choice, data-based, short-response, or extended-response contexts. Both levels therefore need conceptual understanding as well as accurate equation use.

When should I use conservation of energy instead of force equations?

Use conservation of energy when the question links positions, speeds, heights, spring extensions, or dissipative losses without requiring the time taken. It is especially efficient when acceleration varies or when the forces are difficult to resolve directly. Force equations are often preferable when the question asks for acceleration, tension, normal reaction, or motion over time. Some problems require both approaches, such as using energy to find speed and then using Newton's second law to find a force. Before calculating, decide which requested quantity is most directly connected to the available information.

Why can an object moving at constant speed still involve work and power?

Constant speed means that kinetic energy is unchanged, so the net work on the object is zero. It does not mean that every force does zero work. A driving force may do positive work while friction or drag does an equal amount of negative work. Energy is then continuously supplied and transferred to thermal energy even though the object's kinetic energy remains constant. The driving system therefore develops power because it transfers energy each second.

Is work done by gravity positive or negative?

The sign depends on the direction of displacement relative to the gravitational force. Gravity does positive work when an object moves downward because force and displacement have components in the same direction. It does negative work when the object moves upward because gravity opposes the displacement. For horizontal displacement in a uniform vertical gravitational field, gravity does zero work. The corresponding potential-energy change has the opposite sign to the work done by gravity, so $Wg=-\Delta Ep$.

How do I calculate work from a force-displacement graph?

The work done is the signed area between the force-displacement graph and the displacement axis. Divide an irregular piecewise-linear graph into rectangles, triangles, or trapezia and calculate each area separately. Areas above the axis are positive, while areas below it are negative. Add the signed areas to obtain the total work over the interval. Do not use the gradient, because the gradient represents the rate at which force changes with displacement rather than work.

What is the best way to practise IB work, energy and power questions?

First revise the meaning and conditions of each equation rather than memorizing symbols alone. Practise separate groups of questions on angled forces, energy conservation, friction, springs, power, efficiency, and graphs. Then complete mixed problems without being told which principle to use, because equation selection is a major part of IB assessment. Mark every response for method, units, signs, and explanation quality rather than checking only the final number. Finally, use RevisionDojo's Questionbank and worked video solutions to compare your reasoning with a complete exam-focused method.

IB Physics Work, Energy & Power Explained

RevisionDojo Team

•8/2/2026•11 min read

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IB Physics Work, Energy & Power Explained

RevisionDojo Team

•8/2/2026•11 min read

Work, energy and power in IB Physics centre on a small set of closely connected ideas: work transfers energy, energy is conserved, and power measures the rate of energy transfer. Most exam questions test whether you can identify the correct system, track energy changes, account for resistive forces, and present a logically structured calculation.

In the current course, first assessed in 2025, these ideas appear in A.3 Work, energy and power for both SL and HL. They can be examined through Paper 1A multiple-choice questions, Paper 1B data analysis, and Paper 2 short or extended responses, so understanding the relationships matters more than memorizing isolated equations.

What the IB expects you to understand

The official course requires students to understand that work done by a force is equivalent to an energy transfer. Students must also use conservation of energy, represent transfers with Sankey diagrams, calculate mechanical energy, and work with power and efficiency.

The central equations are provided in the IB Physics data booklet:

Quantity	Equation	Essential condition or interpretation
Work	$W=Fs\cos\theta$	$\theta$ is the angle between force and displacement
Kinetic energy	$E_k=\frac{1}{2}mv^2=\frac{p^2}{2m}$
Gravitational potential energy change	$\Delta E_p=mg\Delta h$	Valid in an approximately uniform gravitational field
Elastic potential energy	$E_H=\frac{1}{2}k(\Delta x)^2$
Power	$P=\frac{\Delta W}{\Delta t}=Fv$	For $Fv$ , use the force component parallel to velocity
Efficiency	$\eta=\frac{E_{\text{useful out}}}{E_{\text{total in}}}=\frac{P_{\text{useful out}}}{P_{\text{total in}}}$

The formulas are available, but the booklet does not decide which equation applies. Examiners award marks for recognizing the physical process, substituting consistently, and interpreting the result.

Work as an energy transfer

Work is done when a force has a component in the direction of displacement. For a constant force,

$W=Fs\cos\theta.$

The angle must be measured between the force vector and displacement vector. This produces three common cases:

At $0^\circ$ , work is positive and $W=Fs$ .
At $90^\circ$ , no work is done by that force.
At $180^\circ$ , work is negative and .

For example, the normal force on a box moving horizontally does no work because it is perpendicular to the displacement. Friction usually does negative work because it acts opposite the motion, transferring mechanical energy into thermal energy.

Variable forces and force-displacement graphs

When force varies with position, multiplying one force value by the total displacement is generally invalid. The area under a force-displacement graph gives the work done:

$W=\int F\,ds.$

IB questions often use triangles, rectangles, or trapezia so that integration is unnecessary. Keep areas below the displacement axis negative, because they represent negative work.

Energy and the work-energy connection

Mechanical energy is the sum of kinetic, gravitational potential, and elastic potential energy:

$E_{\text{mech}}=E_k+E_p+E_H.$

If no frictional or resistive forces transfer energy out of the mechanical system, total mechanical energy is conserved. A useful exam statement is

$E_{k,i}+E_{p,i}+E_{H,i}=E_{k,f}+E_{p,f}+E_{H,f}.$

The net work done on an object equals its change in kinetic energy:

$W_{\text{net}}=\Delta E_k.$

Do not confuse net work with the work done by one selected force. If gravity, friction, and an applied force all act, their individual works add to give net work.

When friction is present

Total energy remains conserved, but mechanical energy may not. A reliable model is

$E_{\text{mechanical, initial}}+E_{\text{input}}=E_{\text{mechanical, final}}+E_{\text{dissipated}}.$

Calling energy “lost” is imprecise unless you clarify that it has left the chosen mechanical system. In most situations, friction transfers energy to the thermal energy of the object and surroundings.

Worked IB-style example

A $2.0\ \text{kg}$ block starts from rest and slides $5.0\ \text{m}$ down a slope, losing $1.5\ \text{m}$ of vertical height. A constant frictional force of $2.0\ \text{N}$ opposes the motion. Find its final speed, taking $g=9.8\ \text{m s}^{-2}$ .

First identify the transfers:

decrease in gravitational potential energy
increase in kinetic energy
energy dissipated by friction

The gravitational energy decrease is

$mg\Delta h=(2.0)(9.8)(1.5)=29.4\ \text{J}.$

Friction dissipates

$E_{\text{dissipated}}=Fs=(2.0)(5.0)=10.0\ \text{J}.$

Therefore,

$29.4=\frac{1}{2}(2.0)v^2+10.0.$

This gives $v^2=19.4$ , so

$v=4.4\ \text{m s}^{-1}.$

This solution earns marks because it shows the energy model before substitution. A frequent mistake is to use the distance along the slope in $mg\Delta h$ ; gravitational potential energy depends on vertical height change, not path length.

Power and efficiency

Power is the rate at which work is done or energy is transferred. Average power is

$P=\frac{\Delta E}{\Delta t}.$

One watt is one joule per second. Two machines can perform the same work while having different powers if they take different times.

Mechanical power can also be written as

$P=Fv$

when force and velocity are parallel. More generally, $P=Fv\cos\theta$ , so only the force component parallel to the velocity transfers energy at that rate. At constant engine power, increasing speed means the available driving force decreases because $F=P/v$ .

Efficiency compares useful output with total input:

$\eta=\frac{\text{useful output}}{\text{total input}}.$

The numerator and denominator must use matching quantities. Compare energy with energy or power with power, not output energy with input power.

A Sankey diagram represents these transfers using arrow widths. The total width entering equals the combined widths leaving, while the useful and dissipated outputs must add to the input.

How examiners phrase these questions

Different command terms require different responses:

Calculate or determine: show the equation, substitution, result, and unit.
Show that: demonstrate enough intermediate reasoning to reach the supplied value without circularly assuming it.
Explain: connect a physical principle to the observed outcome, usually in more than one linked statement.
State: give a concise fact without an extended derivation.
Sketch: show the correct trend, intercepts, and significant features rather than decorative detail.

A question may describe an object moving at constant speed and ask about work. Constant speed means no change in kinetic energy and therefore zero net work, but individual forces may still do non-zero work. For example, an engine can do positive work at exactly the same rate that drag does negative work.

Paper 1B can present force-displacement, energy-time, or power-time data. Remember that a gradient on an energy-time graph represents power, while the area under a power-time graph represents energy transferred.

Common mistakes that lose marks

Using $W=Fs$ without checking the angle.
Treating work as a vector because force and displacement are vectors. Work is a scalar.
Assuming mechanical energy is conserved when friction or drag is present.
Writing $mgs$ instead of $mg\Delta h$ for gravitational potential energy.
Forgetting to square speed or spring extension.
Using total force rather than the parallel force component in $P=Fv$ .

A quick dimensional check catches many errors: work and energy use joules, $\text{J}=\text{N m}$ , while power uses watts, $\text{W}=\text{J s}^{-1}$ .

An efficient revision method

Begin by learning to identify the system and energy stores before selecting equations. Then practise one-question sets on angled forces, conservation with friction, springs, efficiency, and power graphs.

Use the A.3 Work, energy and power Questionbank to apply each idea under exam-style wording. After attempting a question independently, compare your setup with the worked A.3 video solutions, focusing on how the solution converts the description into an energy equation.

If the underlying theory remains unclear, review the A.3 study notes and step-by-step lessons. Jojo AI can then give feedback on your reasoning, but you should still write every equation and unit yourself before checking the explanation.

Conclusion

Work, energy and power questions become manageable when you treat them as energy-accounting problems. Identify the system, list initial and final energy stores, include dissipative transfers, and use power only when the rate or time matters.

RevisionDojo can support this process with targeted Physics notes, the Questionbank, and worked videos. The most useful next step is to attempt several A.3 questions without notes, then use the per-question solutions to compare methods rather than merely checking final answers.

Sources and referenced URLs

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IB Physics Work, Energy & Power Explained | RevisionDojo

What the IB expects you to understand

The central equations are provided in the IB Physics data booklet:

Quantity	Equation	Essential condition or interpretation
Work	$W=Fs\cos\theta$	$\theta$ is the angle between force and displacement
Kinetic energy	$E_k=\frac{1}{2}mv^2=\frac{p^2}{2m}$
Gravitational potential energy change	$\Delta E_p=mg\Delta h$	Valid in an approximately uniform gravitational field
Elastic potential energy	$E_H=\frac{1}{2}k(\Delta x)^2$
Power	$P=\frac{\Delta W}{\Delta t}=Fv$	For $Fv$ , use the force component parallel to velocity
Efficiency	$\eta=\frac{E_{\text{useful out}}}{E_{\text{total in}}}=\frac{P_{\text{useful out}}}{P_{\text{total in}}}$

The formulas are available, but the booklet does not decide which equation applies. Examiners award marks for recognizing the physical process, substituting consistently, and interpreting the result.

Work as an energy transfer

Work is done when a force has a component in the direction of displacement. For a constant force,

$W=Fs\cos\theta.$

The angle must be measured between the force vector and displacement vector. This produces three common cases:

At $0^\circ$ , work is positive and $W=Fs$ .
At $90^\circ$ , no work is done by that force.
At $180^\circ$ , work is negative and .

Variable forces and force-displacement graphs

When force varies with position, multiplying one force value by the total displacement is generally invalid. The area under a force-displacement graph gives the work done:

$W=\int F\,ds.$

IB questions often use triangles, rectangles, or trapezia so that integration is unnecessary. Keep areas below the displacement axis negative, because they represent negative work.

Energy and the work-energy connection

Mechanical energy is the sum of kinetic, gravitational potential, and elastic potential energy:

$E_{\text{mech}}=E_k+E_p+E_H.$

If no frictional or resistive forces transfer energy out of the mechanical system, total mechanical energy is conserved. A useful exam statement is

$E_{k,i}+E_{p,i}+E_{H,i}=E_{k,f}+E_{p,f}+E_{H,f}.$

The net work done on an object equals its change in kinetic energy:

$W_{\text{net}}=\Delta E_k.$

Do not confuse net work with the work done by one selected force. If gravity, friction, and an applied force all act, their individual works add to give net work.

When friction is present

Total energy remains conserved, but mechanical energy may not. A reliable model is

$E_{\text{mechanical, initial}}+E_{\text{input}}=E_{\text{mechanical, final}}+E_{\text{dissipated}}.$

Worked IB-style example

First identify the transfers:

decrease in gravitational potential energy
increase in kinetic energy
energy dissipated by friction

The gravitational energy decrease is

$mg\Delta h=(2.0)(9.8)(1.5)=29.4\ \text{J}.$

Friction dissipates

$E_{\text{dissipated}}=Fs=(2.0)(5.0)=10.0\ \text{J}.$

Therefore,

$29.4=\frac{1}{2}(2.0)v^2+10.0.$

This gives $v^2=19.4$ , so

$v=4.4\ \text{m s}^{-1}.$

Power and efficiency

Power is the rate at which work is done or energy is transferred. Average power is

$P=\frac{\Delta E}{\Delta t}.$

One watt is one joule per second. Two machines can perform the same work while having different powers if they take different times.

Mechanical power can also be written as

$P=Fv$

Efficiency compares useful output with total input:

$\eta=\frac{\text{useful output}}{\text{total input}}.$

The numerator and denominator must use matching quantities. Compare energy with energy or power with power, not output energy with input power.

A Sankey diagram represents these transfers using arrow widths. The total width entering equals the combined widths leaving, while the useful and dissipated outputs must add to the input.

How examiners phrase these questions

Different command terms require different responses:

Calculate or determine: show the equation, substitution, result, and unit.
Show that: demonstrate enough intermediate reasoning to reach the supplied value without circularly assuming it.
Explain: connect a physical principle to the observed outcome, usually in more than one linked statement.
State: give a concise fact without an extended derivation.
Sketch: show the correct trend, intercepts, and significant features rather than decorative detail.

Common mistakes that lose marks

Using $W=Fs$ without checking the angle.
Treating work as a vector because force and displacement are vectors. Work is a scalar.
Assuming mechanical energy is conserved when friction or drag is present.
Writing $mgs$ instead of $mg\Delta h$ for gravitational potential energy.
Forgetting to square speed or spring extension.
Using total force rather than the parallel force component in $P=Fv$ .

A quick dimensional check catches many errors: work and energy use joules, $\text{J}=\text{N m}$ , while power uses watts, $\text{W}=\text{J s}^{-1}$ .

An efficient revision method

Conclusion

Sources and referenced URLs

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What the IB expects you to understand

Work as an energy transfer

Variable forces and force-displacement graphs

Energy and the work-energy connection

When friction is present

Worked IB-style example

Power and efficiency

How examiners phrase these questions

Common mistakes that lose marks

An efficient revision method

Conclusion

Sources and referenced URLs

What the IB expects you to understand

Work as an energy transfer

Variable forces and force-displacement graphs

Energy and the work-energy connection

When friction is present

Worked IB-style example

Power and efficiency

How examiners phrase these questions

Common mistakes that lose marks

An efficient revision method

Conclusion

Sources and referenced URLs

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Frequently asked questions

Is work, energy and power included in both IB Physics SL and HL?

When should I use conservation of energy instead of force equations?

Why can an object moving at constant speed still involve work and power?

Is work done by gravity positive or negative?

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How do I calculate work from a force-displacement graph?

What is the best way to practise IB work, energy and power questions?