Inertia is the idea that an object keeps moving unless acted upon by an outside force. Much of the initial work written about inertia was done by Isaac Newton in the 17th century and became known as his first law of physics. It is one of the basic principles of physics and has a number of real-life applications, as it helps to explain much of why objects behave the way they do.

The idea of inertia is quite simple. Inertia is the principle that is that an object will tend to keep moving at a certain speed and direction unless something else acts to change it. That external force could be almost anything. Friction is a common external force that can change the speed and direction of a moving object. Many other objects can also affect motion, however, from gravity of the earth's pull to another person. The key idea is that an object will keep moving (or stay still) unless something else influences it otherwise.

Inertia helps to justify why many objects act in certain ways. For example, inertia is a reason why a ball that is thrown up in the air doesn't keep flying forever. It helps to explain why brakes use friction to help slow down a bike, or why an apple falling from a tree is stopped when it hits the ground. Without inertia, the world would be quite different. The principle of inertia touches a surprising number of aspects of daily life that few ever consider.

While the principle of inertia sounds relatively simple, its application in everyday life can be much more complex. Many aspects of inertia are studied and considered by physicists, engineers and product designers. Principles of inertia are used in many different fields where motion, speed and directional control are important factors.

The inertia principle is a basic and fundamental concept in physics. The basic concept of inertia that an object will keep moving until influenced otherwise is a key reason why many everyday objects behave in certain ways. It also helps justify why many products are designed the way they do.