This trial version of an Australian Science Education Project unit concerns the nature and measurement of force. The teachers' guide, an overprinted copy of the student manual, lists objectives for each section of the unit, discusses the role of the teacher, provides hints in the use of the materials, suggests suitable references, and lists apparatus required. The basic core of the unit is designed so that students can visualize unseen forces by observing their effects, measure them (includes activities teaching the requirements for a measuring system), accept the newton as the appropriate unit, and obtain the skills of graphing and scale construction and reading. Options provide background on the development of measurement systems, introduce magnetic and gravitational forces, demonstrate the magnitude of surface tension forces, use an account of moon exploration to distinguish between mass and weight, provide a narrative account of the development of the theories of gravitation, provide experience with frictional forces, and demonstrate that measuring instruments can be overloaded. At the end of the core and each option there is a self-administered test, the answers to which are appended together with answers to questions in the activities in each section of the unit. The unit has been written in the expectation that students will proceed at their own pace. There is a student record book in addition to the teachers' and pupils' manuals. (AL)