A pair of trolleys fitted with ball bearing wheels to minimize friction. A spring loaded exploder plunger with 2 levels of spring compression is activated by a trigger on top of the trolley. A tough laminated surface mounted on each trolley allows the placing of additional masses for increasing the effective weight of the trolley. Velcro at the back end of each trolley enables the study of inelastic collisions. Dimensions: 350 x 80 x 44mm.