State Newton's first law of motion. Explain, with two daily-life examples, how it shows the property of inertia.

1. 1 · State the law. A body remains in its state of rest or of uniform motion in a straight line unless a net external force acts on it.
2. 2 · Define inertia. Inertia is the tendency of a body to resist a change in its state of rest or motion. Mass is its measure.
3. 3 · Example of inertia of rest. When a bus starts suddenly, a standing passenger falls backward: the feet move with the bus but the upper body stays at rest.
4. 4 · Example of inertia of motion. When a moving bus stops suddenly, the passenger falls forward because the body keeps moving.
5. 5 · Link to safety. Seat belts supply the force that stops the body safely in a sudden stop.
6. 6 · Close in one line. So the first law is also called the law of inertia.

Scoring tip: Name the type of inertia (of rest, of motion or of direction) in each example. The mark is for the reason, not the story.