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 an object will tend to keep moving at a constant speed and in one constant 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 forces 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 to do otherwise.

Inertia helps to explain why many objects act in specific ways. For example, understanding the role of external forces in the principle of inertia helps a person understand why a ball that is thrown up in the air doesn't keep flying forever. It also 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.

As a basic example, one could consider a driver in a car. When the car is moving at a certain speed, all objects in that car — including the driver — are moving at that speed. When the car stops, the objects in the car continue to move forward unless an outside force, like a seat belt, act upon them.

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. Understanding the basic concept of inertia, that an object will keep moving until influenced otherwise, helps human beings understand the way many objects in the universe behave. It is also important to product development, helping explain why many products are designed the way they are. 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.