What is Inertia ?

If that stone were a piece of polished marble, it would slide considerably farther than a rough paving stone. It is apparent that the force of friction is greater on the rough paving stone than on the polished marble.

Despite its general acceptance, Aristotle's concept of motion was disputed on several occasions by notable philosophers over nearly two millennia. For example, Lucretius (following, presumably, Epicurus) stated that the "default state" of matter was motion, not stasis.

Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

However, while the frictional force between the marble and the ice is less than that between the rough stone and the ice, it is still not zero.

In the 6th century, John Philoponus criticized the inconsistency between Aristotle's discussion of projectiles, where the medium keeps projectiles going, and his discussion of the void, where the medium would hinder a body's motion. Philoponus proposed that motion was not maintained by the action of a surrounding medium, but by some property imparted to the object when it was set in motion.

While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.

This property of massive bodies to resist changes in their state of motion is called inertia, and this leads to the concept of inertial reference frames.

In the 14th century, Jean Buridan rejected the notion that a motion-generating property, which he named impetus, dissipated spontaneously. Buridan's position was that a moving object would be arrested by the resistance of the air and the weight of the body which would oppose its impetus.

An inertial reference frame can be described as a 3-dimensional coordinate system that is neither accelerating nor rotating; however, it may be in uniform linear motion with respect to some other inertial reference frame.

Newton never explicitly described inertial reference frames, but they are a natural consequence of his First Law of Motion.

Buridan also maintained that impetus increased with speed; thus, his initial idea of impetus was similar in many ways to the modern concept of momentum. Despite the obvious similarities to more modern ideas of inertia, Buridan saw his theory as only a modification to Aristotle's basic philosophy, maintaining many other peripatetic views, including the belief that there was still a fundamental difference between an object in motion and an object at rest.

When we say that a body is in motion, one might ask, in motion compared to what?

Buridan's thought was followed up by his pupil Albert of Saxony (1316–1390) and the Oxford Calculators, who performed various experiments that further undermined the classical, Aristotelian view. Their work in turn was elaborated by Nicole Oresme who pioneered the practice of demonstrating laws of motion in the form of graphs.

Could you catch a baseball going 100 mph in your bare hand? You could if you were riding on a train going 100 mph, and someone on that train gently tossed you the ball.

The train and the track both exist in their own inertial reference frames, and the speed of the ball depends on the inertial reference frame from which it is viewed.

If you were standing on the platform, and a passenger on that train tossed the ball out the window to you, it would not be wise to attempt to catch it in your bare hand.

The principle of inertia which originated with Aristotle for "motions in a void" states that an object tends to resist a change in motion. According to Newton, an object will stay at rest or stay in motion (i.e. "maintain its velocity") unless acted on by a net external force, whether it results from gravity, friction, contact, or some other force.

Before a rocket is even launched, it is at rest on the surface of Earth.

It would stay at rest indefinitely without any external force acting upon it.

Newton's first law also applies when the rocket is gliding through space with no external forces on it, it will travel in a straight line at a constant speed forever.

The Aristotelian division of motion into mundane and celestial became increasingly problematic in the face of the conclusions of Nicolaus Copernicus in the 16th century, who argued that the earth (and everything on it) was in fact never "at rest", but was actually in constant motion around the sun

Now that we know how an object behaves when there is no outside force acting upon it, what happens when there is an outside force, such as the engines firing up in order to launch the rocket into space?

Galileo writes that "all external impediments removed, a heavy body on a spherical surface concentric with the earth will maintain itself in that state in which it has been; if placed in movement towards the west (for example), it will maintain itself in that movement

This notion which is termed "circular inertia" or "horizontal circular inertia" by historians of science, is a precursor to, but distinct from, Newton's notion of rectilinear inertia.

For Galileo, a motion is "horizontal" if it does not carry the moving body towards or away from the centre of the earth, and for him, "a ship, for instance, having once received some impetus through the tranquil sea, would move continually around our globe without ever stopping.