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Newton's second law of motion is F = ma, or force is equal to mass times acceleration. Learn how to use the formula to calculate acceleration.

Learn about Newton's third law of motion, which states that for every action there is an equal and opposite reaction. Look at multiple examples that illustrate this law, including pushing a block on ice, pushing against a desk, walking on sand, how rockets work, and how an astronaut could save themselves from drifting in space.

This section introduces you to the realm of physics, and discusses applications of physics in other disciplines of study. It also describes the methods by which science is done, and how scientists communicate their results to each other.

Physics is a branch of science. The word science comes from a Latin word that means having knowledge, and refers the knowledge of how the physical world operates, based on objective evidence determined through observation and experimentation. A key requirement of any scientific explanation of a natural phenomenon is that it must be testable; one must be able to devise and conduct an experimental investigation that either supports or refutes the explanation. It is important to note that some questions fall outside the realm of science precisely because they deal with phenomena that are not scientifically testable. This need for objective evidence helps define the investigative process scientists follow, which will be described later in this chapter.