The nice thing about student centred learning is that you get to talk with the students instead of to them. Today I was talking with Nikita, he was doing the gravitational potential activity and was wondering if he could simulate the gravitational field inside the Earth in Algodoo. First he tried with a solid circle but it showed the field increasing as a small ball passed through a hole. The reason for this is that the attraction force acts from the centre of mass. To model the Earth you have to treat it like a load of particles, this can easily be done by liquefying the circle. The force now gets less as expected.