Inertia is the ability of an object to resist changes in its motion, in other words, to resist acceleration.

Newton's First Law of Motion is often called the Principle of Inertia.

An everyday example of inertia and its effect on us is the effect of it during driving and braking. When a car is in motion, so is the body of the driver. The brakes of the car act on the tires, forcing the car to stop moving. At the same time, if no force is exerted on the driver, then the driver will continue moving at the same speed as before, resulting in an accident. The body is resisting changes in its motion according to the Principle of Inertia.

Image of two scenarios in a car accident. The scenario on the left shows a driver wearing the seatbelt, which is stopping the body to crash into the car´s windshield. On the right, the driver has not the seatbelt on and the body is breaking through the windshield.

Figure: A seat-belt is forcing the body to stop as the car brakes in the first image. As shown in the second image, the body would continue to move with its initial speed unless a force is exerted to it by the seat-belt according to the Principle of Inertia.