Video narration
You're on a bus as it starts suddenly; a loose ball seems to roll backward. The bus moves forward, but the ball resists changing its motion. That tendency is inertia, not a force. The first law says motion stays the same unless a net outside force acts. Push a shopping trolley: a net force changes its motion. The second law, F equals m a, links net force, mass, and acceleration. For a two-kilogram trolley with a four-newton net force, acceleration is four divided by two: two metres per second squared. Here's the flip: a net force changes motion, and in every interaction, each object pushes the other equally in the opposite direction. A swimmer pushes water backward; water pushes the swimmer forward. A rocket moves forward by expelling exhaust backward. Exam check: inertia resists change; net force causes acceleration; action and reaction are equal, opposite pushes on different objects.