Equal masses M of 200 grams are hung on the ends of a light string passing over a light, frictionless pulley. When an additional mass of 100g is hung on one end, the resulting acceleration can be measured by timing the motion of either mass over a distance S between two points. The acceleration of gravity g can then be calculated: g = a (2M + m)/m, where a is the acceleration of the system: a = 2S /t^2.