A boy (25kg) and a girl (20kg) are playing on a see-saw which is 4m long. If the boy sits 1m from the centre on the left side and the girl 2m from the centre on the other, which direction will the see-saw will rotate around its centre?

Student should calculate the moments about the pivot point (from diagram recognise this is the centre of the see-saw) for each child. Using the equation, moment = mass x acceleration x distance from pivot, with acceleration due to gravity = 9.81m/s^2 The moment for the boy is 25 x 9.81 x 1 = 245.25N anti-clockwise and, the moment for the girl is 20 x 9.81 x 2 = 392.4N clockwise. Therefore, the see-saw will rotate clockwise.