Crumple zones

What the Activity is for

Working reduces the energy in the kinetic store.

Here you can investigate the trade-off between the pair of compensated quantities: force and distance, the combined action of which alters the energy in the kinetic store.

You might also make the two-step connection between:

• The pair of compensated quantities, force and velocity, the combined action of which alters the power in the mechanical working pathway.
• The accumulation of energy in the kinetic store as a result of the power in this pathway acting for a duration.

As is often the case in physics, true understanding is achieved by making connections between many ways of looking at the same process: here the artefact from the lived-in world, an imagined-world model, and a physical-world laboratory experiment. It might be a good opportunity to explore how physics works by investigating the extent to which the different systems seem to exhibit the same behaviour. Clearly, cycle helmets are not designed just by trial and error, nor are the standards to which they have to comply arrived at by accident.

What to Prepare

• a collection of items which rely on crumple zones, for example, a cycle helmet
• a copy of a computer modelling program, displaying on a large screen
• a sensor-driven set-up to measure energy in the kinetic store, force and distance, during a collision.
• The interactive diagram, or one like it

What Happens During this Activity

We'd suggest starting with the physical artefacts (although in some cases you might justifiably choose to substitute images where these more clearly indicate the focus). You'll want to clearly identify their function as a sacrificial zone, what is being protected, the kinetic store which needs to be emptied, and the importance of the distance that can be crushed in determining the force that is acting on the object to be protected.

In your measurements you'll almost certainly find that the retarding force on the protected object is not constant, and so will want to refine the model. We hope your set-up will allow you to make some changes to the sacrificial material or structure and to control the initial energy in the kinetic store, so as to make these comparisons fruitful.

Finally, the model will allow you to explore many more situations, limited only by your technical skill and imagination in coding up how the retarding force varies. The core of the model will be statements like this:

new value = old value + change

Δ →p = →F ×  Δ t

momentum_new = momentum_old +  Δ momentum

change in energy = power × duration

energy_new = energy_old +  Δ energy

Do keep in mind that it'll be only too easy to code in variations of force that fire the object back off in the direction from which it came, or fail to bring it to a halt. Models need to be constrained by what they're designed to mimic, so there should be no surprise there.

You might also use one or more of the interactives from the SPT: Electricity and energy topic Physics Narrative or the SPT: Force and motion topic Physics Narrative to open up this exploration.