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Work done is measured in joules (J). Friction does work, transferring energy to thermal energy stores. Free GCSE Physics notes and 9 FAQs.
Work Done and Energy Changes is part of Energy stores and transfers in GCSE Edexcel Higher Physics.
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GCSE Edexcel Higher
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Work done is measured in joules (J). Friction does work, transferring energy to thermal energy stores. Free GCSE Physics notes and 9 FAQs.
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Use this lesson to stay inside Energy stores and transfers while connecting the detail back to the wider Topic 8: Energy - Forces doing work topic.
Read the core explanations from Work Done and Energy Changes before testing yourself from memory.
When you push a shopping trolley across a car park, you're doing work. When a crane lifts steel beams to the top of a building, it's doing work. But when you hold a heavy box without moving it, no work is done at all.
This might seem strange at first. You feel tired holding that box, so surely you're doing work? The key insight is that work has a very specific scientific meaning. Work is only done when a force causes movement in the direction of that force.
Think of it this way: work transfers energy from one place to another. When you push the trolley, energy transfers from your muscles to the trolley's kinetic energy store. When the crane lifts beams, energy transfers from the motor to the beam's gravitational potential energy store.
The formula for work done is beautifully simple: W = Fs. Here, W is work done in joules, F is the force in newtons, and s is the distance moved in the direction of the force in metres. Notice that both force and distance must be in the same direction for work to be done.
Every time work is done, energy changes happen within a system. Picture a system as everything we're interested in studying - it could be a single object, multiple objects, or even entire regions of space. The beauty of energy is that it's always conserved within any system.
When an object accelerates due to a constant force, work is done on the object. This work transfers energy into the object's kinetic energy store. The faster the object moves, the more kinetic energy it has. This is why cars need more energy to reach 60 mph than 30 mph.
Heating also changes energy within systems, but differently from mechanical work. When you rub your hands together, friction does work and transfers energy to the thermal energy stores of your hands. The temperature rises because the average kinetic energy of the particles increases.
Consider a ball thrown upwards. Initially, it has kinetic energy. As it rises, work is done against gravity, transferring energy from the kinetic store to the gravitational potential energy store. At the highest point, all kinetic energy has become potential energy. As it falls, the reverse happens.
Electric current can do work too, and this is how most of our modern world operates. When current flows through a circuit, it can transfer energy to different components. Think about a motor lifting a weight - electrical energy transfers to gravitational potential energy.
The work done when current flows depends on three factors: the voltage across the component, the current flowing through it, and the time for which it flows. This gives us the formula W = VIt, where W is work done in joules, V is voltage in volts, I is current in amperes, and t is time in seconds.
This electrical work often appears as different energy transfers. In a resistor, electrical energy transfers to thermal energy - this is how electric heaters work. In a motor, electrical energy transfers to kinetic energy. In a battery being charged, electrical energy transfers to chemical energy.
Worked example: Calculating Electrical Work Done
A 12 V motor draws 2 A of current for 5 seconds. Calculate the work done. [3 marks]
Answer: 120 J
The principle of energy conservation is one of the most powerful ideas in physics. Energy cannot be created or destroyed - it can only be transferred from one store to another or from one object to another. This means we can track energy through any process.
Consider the following example of energy transfer: a cyclist pedalling up a hill. Chemical energy in the cyclist's muscles is converted into the bike's kinetic energy. As the bike climbs, some of this kinetic energy is transformed into gravitational potential energy, while some energy is dissipated as thermal energy due to friction and air resistance.
The total energy before equals the total energy after - it's just redistributed among different stores. This is why we can calculate energy changes by comparing the initial and final states of a system, regardless of the complex processes in between.
Energy calculations become powerful tools for understanding real systems. If we know the energy input and can measure some outputs, we can calculate efficiency. If we know the initial energy stores and the work done, we can predict the final energy distribution.
Real exam questions often ask you to show how energy redistributes when a system changes. The key is to identify all the energy stores before and after the change, then use calculations to show the energy transfers quantitatively.
Start by listing the initial energy stores and their values. Then identify what work is done or what forces act during the change. Finally, calculate the final energy stores. The total energy before must equal the total energy after - this gives you a powerful check on your calculations.
Worked example: Energy Redistribution Analysis
A 2 kg ball is dropped from 5 m height. Calculate the kinetic energy just before impact. (g = 9.8 N/kg) [4 marks]
Answer: 98 J
This approach works for any energy change scenario. Whether it's electrical work charging a battery, mechanical work stretching a spring, or thermal energy heating water, the same principle applies: track the energy stores before and after, account for any work done, and check that energy is conserved.
Work done and energy questions are mark-winners if you follow the right approach. Always start by identifying what type of work is being done - mechanical work (W = Fs) or electrical work (W = VIt). Then set up your energy stores clearly before attempting any calculations.
For calculation questions, examiners award method marks for correct formulas and substitution, even if your final answer is wrong. Always show your formula first, then substitute values with units, then calculate. This structured approach maximises your marks even if you make arithmetic errors.
Energy conservation questions often ask you to 'show' that energy is conserved. This means you must calculate the total energy before and after the change, then demonstrate they're equal. Don't just state that energy is conserved - prove it with numbers.
Watch out for common traps in exam questions. Questions about holding objects stationary test whether you understand that work requires movement. Questions mixing different types of work test whether you can choose the right formula. Energy efficiency questions often require you to account for 'wasted' energy that transfers to thermal stores.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Work done is measured in joules (J). In W = Fs, force is in newtons and distance in metres, and in W = VIt, voltage is in volts, current in amperes and time in seconds.
Electrical work done = voltage × current × time, W = VIt. For a 12 V motor drawing 2 A for 5 seconds, W = 12 × 2 × 5 = 120 J.
Use energy conservation: the gravitational potential energy at the start becomes kinetic energy at impact. A 2 kg ball dropped from 5 m has GPE = mgh = 2 × 9.8 × 5 = 98 J, so its kinetic energy just before impact is 98 J.
Friction does work, transferring energy to thermal energy stores. When hands are rubbed together, friction does work and the temperature rises because the average kinetic energy of the particles increases.
Holding a heavy box without moving it does no work, even though it feels tiring. Work is only done when a force causes movement in the direction of that force.
When a ball is thrown upwards, work is done against gravity, transferring energy from the kinetic store to the gravitational potential energy store. At the highest point the kinetic energy has become potential energy, and as it falls the reverse happens.
In an electric heater, electrical energy transfers to thermal energy as current flows through a resistor. In a motor electrical energy transfers to kinetic energy, and in a charging battery it transfers to chemical energy.
When cycling up a hill, chemical energy in the cyclist's muscles becomes kinetic energy of the bike, and some is transformed into gravitational potential energy. Some energy is dissipated as thermal energy due to friction and air resistance.
To show energy is conserved, the total energy before and after a change is calculated and shown to be equal. For a 2 kg ball dropped from 5 m, GPE = 2 × 9.8 × 5 = 98 J at the top and KE = 98 J just before impact.