A pair of trolleys for investigating momentum, velocity, acceleration and energy conservation. Both trolleys are identical and each consists of a rectangular wooden body carried upon three low friction wheels to ensure smooth straight operation. One trolley is supplied with one strong spring plunger. The trolley is provided with three dowel pegs so that the trolleys can be stacked upon one another. This enables the effective mass to be increased. In front of each trolley, two holes are drilled and fitted with corks. If long pins are pushed through the corks of one of the trolleys, they can be made to stick together after collision.