Momentum
- definition
- principle of conservation of momentum
- calculations for one-dimensional problems
For many students, calculations of momentum and impulse will have been the most challenging encountered during GCSE. By A level it is hoped that they have a much clearer understanding of force, energy and motion but for many previous difficulties may cause anxiety. It is worth ensuring students can correctly place momentum in the context of other physical properties; units in particular can cause some difficulties. The resources listed offer one route that might be useful to consider.
It starts with some review material and progresses to quantitative experiments that students can use to check predicted changes in momentum. A distinction between perfectly elastic momentum changes - where no external forces act and there is no shift from kinetic energy to other stores - and inelastic collisions, which might involve chemical explosions or permanent shape changes.
The use of simulations is discussed and there are links to practice questions that might be useful. Discussion of causes and effects - often in the form of a flow chart - can help students understand the ideas and then describe them verbally. Thorough practice and a methodical approach to problems set in class are, for most students, the best way to achieve familiarity with the quantitative problems they will encounter in exams.
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.
Investigating RTAs *suitable for home teaching*
Several of the examples in this Catalyst article use the ideas of momentum and how this relates to road safety. It would be a good reminder of the basics or introduction to the topic.
These are straightforward calculations and A level students should be encouraged to suggest how the real data might vary from the simplified versions described here. This is a valuable opportunity to discuss how we always eliminate some complications simply to make solving a problem realistic.
Episode 221: Elastic Collisions
As well as giving practical ideas about hands-on activities, this guidance from the IoP includes suggested difficulties and ways to address them successfully. The specific examples about why a collision/interaction might not be elastic, in terms of energy changes, are particularly useful for students. Some of the practicals may seem a little alarming to less-experienced colleagues; for example most schools no longer keep an air rifle for firing into a pendulum of known mass! There are equivalents, however, using less intimidating foam dart guns as suggested on Dr Dav's teaching blog.
The worked examples and problems will be helpful when setting consolidation questions for students.
Isolated Systems
This link is to a specific activity, but the whole sequence from the Physics Classroom for momentum is worth reading through. This section provides a useful response, with calculations, to answer claims from students that air resistance and/or friction mean that momentum predictions are never accurate (including resistive forces makes only 2% difference in the example given).
Momentum Conservation
The clear definition here will be a useful reference for students. The classic explanation of a boat being pushed away from the shore as an occupant gets out is one that gives rise to straightforward numbers and an easy situation for students to visualize. There is no mention of the complications raised by an inelastic collision/interaction, which students will need to be able to explain.
Collision Lab
What makes this PhET simulation particularly useful is that momentum and kinetic energy can be shown at any point for the moving objects. Students often start with a fairly confused idea of momentum as being somewhere between force and KE - this is demonstrated by their assumption that a thrown object somehow has a forwards force acting on it in mid-flight.
Students can start by colliding objects of varying masses in a straight line and then extend this to more complex possibilities. It is a useful follow-up to a demonstration with an air track, which can be made quantitative by the use of slow-motion video capture or similar.