The Zimbabwe Secondary School Science Project (ZIM-SCI) developed student study guides, corresponding teaching guides, and science kits for a low-cost science course which could be taught during the third year of secondary school without the aid of qualified teachers and conventional laboratories. This ZIM-SCI study guide is a five-part unit providing activities and experiments which build on concepts about forces learned in ZIM-SCI units H1 and H2. The material included expands students' knowledge to cover requirements of the O-Level General Science Syllabus. Parts 1 and 2 focus on two groups of forces found in the natural environment--forces acting at a distance and forces of reaction. Students are also taught about various man-made devices that apply or overcome these natural forces in a useful way. The remaining three parts focus on measurement and calculation of the size of these forces (part 3), nature of pressure and applications of pressure to useful work (part 4), and principles that govern all simple machines (part 5). Actual O-level examination questions (with answers) as well as information on principles behind the activities and experiments are provided. (JN)