Two-penny physics: Teaching 2D linear momentum conservation

A low-cost setup is used to quantitatively introduce the momentum conservation law in two dimensions. The apparatus consists of two small identical coins (e.g., 1 euro cent), a small cardboard box, and two A4 sheets of paper; it is technology-free, low-cost, works with energy supplied by the gravitational field, and takes up little space. For these reasons, it can be assembled on a desk, allowing the entire class to directly engage with it, and is a suitable device for schools with a low budget. Educational results of a test stage with 15 secondary school students are also discussed.