Class 9 Science

1.1 Why science uses models A model keeps the few quantities that decide the answer to one question and leaves out the rest, on purpose. 1.2 The language of science Words such as work have exact meanings in science, and an equation states how quantities are related. 1.3 Why units must be standard A measurement is a number and a unit. One airliner ran out of fuel because pounds were used in place of kilograms. 1.4 Laws, theories and principles A law describes a pattern, a theory explains it, and a principle applies across many situations. 1.5 Prediction and testing A prediction is an expected result worked out from evidence. A claim is testable when it names something measurable. 1.6 The art of estimation Rough values of each quantity give an answer of the right size, good enough to check a calculation.