So your skating down the street and you see that hot chick/guy.
You make eye contact and then suddenly you hit the most microscopical rock and your board stops dead in it's tracks. Unfortunately your body keeps moving forward, causing you to hit the ground hurting your pride in front of that hottie,
Here is why your board stopped but you didn't.
You keep moving because of a property of matter called Inertia.
This property was described in Sir Isaac Newton’s first law of motion.
The first law of motion states:
All non-moving objects will remain non-moving and all moving objects will remain moving at the same speed in the same direction,
until acted upon by an external force. External Forces could be a push, a pull, gravity, friction, etc.
So your riding on your board and suddenly it stops because something acts upon it with a force.
However, you keep moving because nothing has acted on YOU with a net force,
Only your skateboard has had the force acting upon it.
The same thing happens with seatbelts in cars. If the car stops, and you are not wearing a seatbelt.
You’ll keep moving until acted upon by a force. And a painful result not to mention.
The force would more than likely be the dashboard or windshield .
Inertia is the resistance an object has to a change in its state of motion. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Sir Isaac Newton defined inertia in Definition 3 of his Philosophies Naturalis Principia Mathematica, which states:
In common usage, however, people may also use the term inertia to refer to an object's amount of resistance to change in velocity (which is quantified by its mass), and sometimes its momentum, depending on context (e.g. this object has a lot of inertia).
The term inertia is more properly understood as a shorthand for the principle of inertia as described by Newton in Newton's First Law of Motion which, expressed simply, says:
On the surface of the Earth the nature of inertia is often masked by the effects of friction which brings moving objects to rest relatively quickly unless they are coasting on wheels, well lubricated or perhaps falling or going downhill, being accelerated by gravity.
This is what misled classical theorists such as Aristotle who believed objects moved only so long as force was being applied to them.
Prior to the Renaissance in the 15th century, the generally accepted theory of motion in Western philosophy was that proposed by Aristotle (around 335 BC to 322 BC), which stated that in the absence of an external motive power, all objects (on earth) would naturally come to rest in a state of no movement, and that moving objects only continue to move so long as there is a power inducing them to do so.
Aristotle explained the continued motion of projectiles, which are separated from their projector, by the action of the surrounding medium which continues to move the projectile in some way. As a consequence, Aristotle concluded that such violent motion in a void was impossible for there would be nothing there to keep the body in motion against the resistance of its own gravity.
Then in a statement regarded by Newton as expressing his Principia's first law of motion, Aristotle continued by asserting that a body in (non-violent) motion in a void would continue moving forever if externally unimpeded:
No one could say why a thing once set in motion should stop anywhere;
for why should it stop here rather than here? So that a thing will either be at rest or must be moved ad infinitum, unless something more powerful gets in its way.
Despite its remarkable success and general acceptance, Aristotle's concept of motion was disputed on several occasions by notable philosophers over the nearly 2 millennia of its reign.
For example, Lucretius (following, presumably, Epicurus) clearly stated that the 'default state' of matter was motion, not stasis. In the 6th century, John Philoponus criticized Aristotle's view,
noting 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 the surrounding medium but by some property implanted in the object when it was set in motion. This was not the modern concept of inertia, for there was still the need for a power to keep a body in motion.
This view was strongly opposed by Averroës and many scholastic philosophers who supported Aristotle. However this view did not go unchallenged in the Islamic world, where Philoponus did have several supporters.
The law of inertia states that it is the tendency of an object to resist a change in motion.
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. Galileo, in his further development of the Copernican model, recognized these problems with the then-accepted nature of motion and, at least partially as a result, included a restatement of Aristotle's description of motion in a void as a basic physical principle:
Galileo's concept of inertia would later come to be refined and codified by Isaac Newton as the first of his Laws of Motion (first published in Newton's work, Philosophiae Naturalis Principia Mathematica, in 1687):
Note that velocity in this context is defined as a vector, thus Newton's constant velocity implies both constant speed and constant direction (and also includes the case of zero speed, or no motion). Since initial publication, Newton's Laws of Motion (and by extension this first law) have come to form the basis for the almost universally accepted branch of physics now termed classical mechanics.
The actual term inertia was first introduced by Johannes Kepler in his Epitome Astronomiae Copernicanae (published in three parts from 1618-1621); however, the meaning of Kepler's term (which he derived from the Latin word for idleness or laziness) was not quite the same as its modern interpretation. Kepler defined inertia only in terms of a resistance to movement, once again based on the presumption that rest was a natural state which did not need explanation. It was not until the later work of Galileo and Newton unified rest and motion in one principle that the term inertia could be applied to these concepts as it is today.
Nevertheless, despite defining the concept so elegantly in his laws of motion, even Newton did not actually use the term inertia to refer to his First Law. In fact, Newton originally viewed the phenomenon he described in his First Law of Motion as being caused by innate forces inherent in matter, which resisted any acceleration. Given this perspective, and borrowing from Kepler, Newton actually attributed the term inertia to mean the innate force possessed by an object which resists changes in motion; thus Newton defined inertia to mean the cause of the phenomenon, rather than the phenomenon itself. However, Newton's original ideas of innate resistive force were ultimately problematic for a variety of reasons, and thus most physicists no longer think in these terms. As no alternate mechanism has been readily accepted, and it is now generally accepted that there may not be one which we can know, the term inertia has come to mean simply the phenomenon itself, rather than any inherent mechanism. Thus, ultimately, inertia in modern classical physics has come to be a name for the same phenomenon described by Newton's First Law of Motion, and the two concepts are now basically equivalent.
Physics and mathematics appear to be less inclined to use the original concept of inertia as a tendency to maintain momentum and instead favor the mathematically useful definition of inertia as the measure of a body's resistance to changes in momentum or simply a body's inertial mass. This was clear in the beginning of the 20th century, when the theory of relativity was not yet created. Mass, m, denoted something like amount of substance or quantity of matter. And at the same time mass was the quantitative measure of inertia of a body. The mass of a body determines the momentum P of the body at given velocity v; it is a proportionality factor in the formula:
By this formula, the greater its mass, the less a body accelerates under given force. Masses m defined by the formula (1) and (2) are equal because the formula (2) is a consequence of the formula (1) if mass does not depend on time and speed. Thus, mass is the quantitative or numerical measure of body’s inertia, that is of its resistance to being accelerated.
This meaning of a body's inertia therefore is altered from the original meaning as a tendency to maintain momentum to a description of the measure of how difficult it is to change the momentum of a body.