Take a selection of balls and marbles along to a nearby playground slide and let students investigate factors that may influence how balls accelerate down an inclined plane. Students can make hypotheses in small groups, plan investigations to test multiple possible explanations and draw conclusions about the importance of different variables. The experiments illustrate the principle of equivalence between inertial and gravitational mass, as well as of the importance of mass distribution. Students can develop an intuitive feeling for these concepts even without dealing with the full mathematical treatment, which involves torque, angular momentum and moment of inertia, typically treated in high-school or introductory university physics courses. The paper discusses this investigation as part of a setup of teacher professional development events on a playground.