# Energy

- calculation of work done for constant forces, including force not along the line of motion
- calculation of exchanges between gravitational potential energy and kinetic energy
- principle of conservation of energy

The first thing to be aware of is that the recommended language for discussing energy has been changing recently. Currently the KS3 and GCSE specifications have caught up, but the students will probably not yet be familiar with it. The focus is very much on calculations, which fits very well with the quantitative approach needed at A level. The biggest change is that light, sound and current are best described as pathways or processes that explain the shifting of energy between stores. Equations can and should be used to describe the energy stored elastically, in motion and so on. For more information, the IoP's Supporting Physics Teaching resource (listed as 11-14 but current students will not have been taught in this way) is a good place to start.

When discussing work done, it is worth reminding them that energy has been shifted to another store - usually kinetic and/or thermal (work done against friction). Some will need to be reminded that lifting an object means overcoming the weight, while others will appreciate the link between *Work done = force x distance* and *GPE = (mg)h*. You may also want to revisit their use of trigonometry when forces acting at an angle to the motion are considered.

Falling objects, or objects thrown upwards to a maximum height, offer many opportunities to link kinetic and gravitational energy store calculations. Any still relying on triangles to rearrange equations will soon struggle. Doing any classroom measurements, for example with lightgates, will show how important air resistance is with most examples. Ignoring it means that the numbers do not seem to follow the principle of conservation of energy.

This principle is one that students must be able to state clearly and explain. Challenging them to justify the common phrase of 'losing energy' will help them to appreciate that rather than energy being lost or destroyed, we are often **losing track** of it - an important distinction. (Page 5 of this SEP resource explains it well.) Occasionally students will need to be reminded of the difference between energy conservation from a physics point of view, and the 'everyday' use which revolves around turning off lights.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

## Links and Resources

### Episode 214: Work Done by a Force

This introductory lesson suggests questions you might use to gauge the current understanding of your class. For many the concepts will be familiar, but the practical work allows them to extend their skills while demonstrating the principles involved. The investigation guidance - with associated prompts - uses basic equipment to show a block pulled up a slope by a hanging/falling weight. This means they will need to practise resolving forces along and into a slope, a familiar exam question.

### Work Done

The diagrams on this page at School Physics are particularly useful and help students to understand the effect of a force acting at an angle to the motion. They should quickly recognize the result of simple cases where the force is parallel or perpendicular to the motion, as well as the link to trigonometry for other angles.

There are not many worked examples here but they are fairly easy to generate. Alternatively you may find that challenging them to set problems for fellow students, with worked answers, helps them to gain a deeper appreciation of both the physics and the likely marking approaches taken by exam boards. Using official mark schemes and examiners' reports early on means they are more likely to show full and clear working from the beginning.

A second page at SchoolPhysics includes an explanation of the use of force-displacement graphs to calculate the work done on an object.

### Calculating Work Done

This is one of several linked pages on the Physics Classroom website which guide students through the calculations they will need for energy and work done. It's worth being careful as the umbrella term 'mechanical energy' is one we don't use much in the UK, but the more familiar language of potential and kinetic energy is explained also; you'll just want to be prepared for the question.

This page gives the opportunity for students to attempt problems and check their answers. You might find it a useful way to monitor their understanding, although clearly this is limited to numerical solutions. One approach is to ask students to add a follow-up question to each example that demands a written answer, then try them out with each other.

### Episode 216: Energy Changes

In this resource students consider what happens to the initial kinetic energy of a moving object. Links to braking vehicles are considered and there are practical guidelines for a useful investigation; you may see this as a chance to consolidate scientific vocabulary from your specification glossary.

As a quick reference to consider the combination of equations in specific circumstances, your students may find this list from SchoolPhysics useful.

### Energy Skate Park

This simulation from PhET allows students to change heights and slopes on a skate ramp, and then measure how the speed of a skater changes at various points on the journey. Values of both gravitational and kinetic energy are calculated from the 'measurements' taken.

It is a vivid example and the main strength lies in the small wait between making a prediction and getting an answer. Students who are familiar with the PEOE structure - Predict, Explain, Observe, Evaluate - will be able to quickly make and test hypotheses, which can be quantitative rather than simply qualitative.

### The Law of Conservation of Energy

Inevitably, some of these methods from the Practical Physics website will be familiar in your setting; some may have been used at GCSE with your pupils. The pendulum instructions in particular include useful tips for your students. Asking them to discuss the differences between previous approaches and the quantitative method here often leads to a better recognition of the demands of A level practical work. Depending on your resources, you may wish to have students create a spreadsheet that calculates potential energy changes when measured data is added.

You may wish to mention that you will be returning to the pendulum later on as an example of an oscillating system, but for now no time measurements are needed. This in itself can be pointed out to students as evidence that the measurements are about energy, not power.

You may find the accompanying guidance interesting and useful; students' responses to the historical perspective will be varied but it can be a good change of pace to the calculations.

### Episode 217: Conservation of Energy

In this resource it is the sequence of practical work that is important. Students are guided in reducing causes of energy 'loss', each of which results in a closer match between the calculated values. This explicit process is one they will wish to revisit for other practicals, where they can consider ways to eliminate sources of error to improve data.

At the end of the document there are problems which could be set for homework, along with model answers and explanation.

Subject(s) | Science |
---|---|

Tags | n.a |

Age | 16-19 |

Last updated | 12 July 2016 |

Log in to rate this resource | |

URL | https://www.stem.org.uk/lxpax |