Example 1
A 2.0 kg object is raised vertically by 3.0 m. Take g = 10 N/kg. Find its gain in gravitational potential energy.
- Gain = mgh = 2.0 × 10 × 3.0.
- The gain is 60 J. This is the minimum input work for an ideal lift starting and ending at rest.
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Energy calculations: work, kinetic energy and efficiency
Choose a system before writing an energy equation. A falling object transfers gravitational potential energy into kinetic energy and energy dissipated to the surroundings. Only an ideal model lets you equate the first two exactly.
Choose the method
Work done by a constant force in its direction is W = Fs. Gravitational potential energy change is mgh; kinetic energy is ½mv².
Power is energy transferred/time. Efficiency is useful output/total input × 100%, using energy on both sides or power on both sides.
Use kilograms, metres and seconds to obtain joules and watts. State whether air resistance or friction is neglected, and use the value of g supplied in the question.
Worked examples
Example 2
A motor takes in 500 J and does 350 J of useful lifting work in 5.0 s. Find its efficiency and useful output power.
- Efficiency = 350/500 × 100% = 70%.
- Useful output power = 350/5.0 = 70 W.
- The remaining 150 J is transferred to other forms, such as thermal energy and sound; energy is conserved.
Try it yourself
A 0.50 kg ball moves at 4.0 m/s. Find its kinetic energy. What happens if its speed doubles?
Show the worked answer
- Energy = ½ × 0.50 × 4.0² = 4.0 J.
- Doubling speed multiplies v² by four, so the new energy is 16 J.
Common mistakes
- Do not square the mass in ½mv².
- Efficiency cannot exceed 100% for the defined total input. A larger result usually means you reversed the ratio or mixed energy and power.
Sources and review
SEAB 2026 syllabus: 2026 Combined Science physics component: energy; also useful for Pure Physics revision.
Original Rae practice, prepared with AI assistance. Selected numerical results and their displayed working are automatically checked at publication; this does not verify every explanation. Curriculum references checked on 5 September 2026. No teacher review or SEAB endorsement is claimed.
What the syllabus expects
- Recall and apply the kinetic energy formula (Ek = ½ mv2) and, for positions close to the Earth's surface, the gravitational potential energy formula (Ep = m g h), in unfamiliar situations or problem solving.
- State the principle of conservation of energy and apply it in unfamiliar situations or problem solving.
- Recall and use power = energy transfer / time taken in unfamiliar situations or problem solving.
- Compute the efficiency of an energy transfer with efficiency = useful energy output / total energy input.
- Discuss generating electricity from non-renewable resources like fossil fuel and nuclear fuel and renewable ones like biofuel, wind, tides, hydropower, geothermal reservoirs and solar, weighing transfer efficiency, cost, reliability and environmental impact.
How it's examined
Questions on this topic most often ask you to calculate, estimate, find, state. About 6% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Continue revising
- DC circuits: series and parallel resistance with worked answers
- Chemical calculations: moles, reacting masses and concentration
- Acids, bases and salts: neutralisation and choosing a preparation method
More O-Level Pure Physics topics
Physical Quantities, Units and Measurement · Kinematics · Dynamics · Turning Effects of Forces · Pressure · Kinetic Particle Model of Matter · all of O-Level Pure Physics