AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy stores and systems: a moving object hitting an obstacle
When a moving object strikes an obstacle, its kinetic energy store decreases as it slows or stops.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When a moving object strikes an obstacle, its kinetic energy store decreases as it slows or stops. Energy is transferred mechanically by the contact force, doing work on the obstacle and on the object. Some energy raises the thermal energy store of both bodies because friction and deformation generate heating; some may transfer by sound to the surroundings. The total energy is conserved, so the decrease in kinetic energy equals the work done plus the energy dissipated.
For example: A car braking to a stop transfers energy from its kinetic store to thermal stores in the brakes and road, with a little sound.
What earns the marks
- Identify the moving object's kinetic energy store as the initial store that decreases.
- State that the contact force between object and obstacle does mechanical work, transferring energy.
- Recognise that friction and deformation transfer energy to thermal energy stores of the object, obstacle and surroundings.
- Note that some energy may be transferred by sound to the surroundings.
- Apply conservation of energy: total energy before impact equals total energy after, accounting for all stores.
- Use Ek = ½mv² to calculate kinetic energy where mass and speed are given.
Common mistakes and how to put them right
Saying energy is 'used up' or 'lost'.
Put it right by stating energy is dissipated to thermal stores and remains conserved.
Treating kinetic energy as destroyed on impact.
Put it right by explaining it is transferred by forces doing work.
Ignoring sound or thermal transfers and claiming all kinetic energy becomes one store.
Put it right by listing all pathways and stores involved.
Exam tips
- Name the stores and the transfer pathway explicitly rather than writing only 'energy is transferred'.
- Link each energy transfer to a mechanism such as friction, deformation or a contact force.
- Check numerical answers by confirming the decrease in kinetic energy matches the sum of other energy transfers.
Try the lesson
The start of the Energy stores and systems: a moving object hitting an obstacle lesson, made from checked teaching notes for this specification point.
Have a go first
A moving car hits a brick wall and stops. Which energy store decreases as the car slows down?
Listen
When a moving object strikes an obstacle, its kinetic energy store decreases as it slows or stops. Energy is transferred mechanically by the contact force, doing work on the obstacle and on the object. Some energy raises the thermal energy store of both bodies because friction and deformation generate heating; some may transfer by sound to the surroundings. The total energy is conserved, so the decrease in kinetic energy equals the work done plus the energy dissipated. For example, a car braking to a stop transfers energy from its kinetic store to thermal stores in the brakes and road, with a little sound. A strong answer earns marks for these points. Identify the moving object's kinetic energy store as the initial store that decreases. State that the contact force between object and obstacle does mechanical work, transferring energy. Recognise that friction and deformation transfer energy to thermal energy stores of the object, obstacle and surroundings. Note that some energy may be transferred by sound to the surroundings. A common slip: Saying energy is 'used up' or 'lost'. Instead, stating energy is dissipated to thermal stores and remains conserved. Exam tip: Name the stores and the transfer pathway explicitly rather than writing only 'energy is transferred'.
See it
Key words
Say what each word means before you turn the card.
Conservation of energy
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Mechanical work and contact forces
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Dissipation to thermal stores
Try answering aloud or in your head before turning the card.
See every definition
- Conservation of energy
- Total energy before impact equals total energy after, accounting for all stores.
- Mechanical work and contact forces
- The contact force between object and obstacle does mechanical work, transferring energy.
- Dissipation to thermal stores
- Friction and deformation transfer energy to thermal energy stores of the object, obstacle and surroundings.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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