Kevin has edited encyclopedias, taught history, and has an MA in Islamic law/finance. He has since founded his own financial advice firm, Newton Analytical.
A Classical Problem
Imagine for a moment the scientific understanding of the Ancient Greeks. While they got a great deal right, notably the idea of the atom, there were some things that they utterly missed. One of those is the idea of motion. According to the Ancient Greeks, all objects naturally seek a pre-designated location. Heavy items fall to the Earth quickly, since they are made of materials that come from the Earth, for example.
Obviously, this is not the case, or else so much of modern life would be impossible. Can you imagine the headlines: 'A Rocket Launch Fails Because the Metal in the Fuselage has Decided to Return to an Iron Mine?' Luckily, we have the benefit of Sir Isaac Newton's three laws of motion. In this lesson, we're going to look at his first law of motion, which states that an object in motion stay in motion, while an object at rest stays at rest until a force acts upon it. And no, the desires of the metal are not the types of forces we are talking about.
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The Idea of Inertia
Have you ever been driving someplace a little faster than you should have been? Go ahead, be honest. And chances are someone may have pulled out in front of you so you had to slam on the brakes. The car skidded, your passenger looked at you with a combination of annoyance and some uncertainty, but after a few feet the car comes to a stop. But wait! Didn't you hit the brakes back there?
Simply put, the car still had inertia. Inertia is the term given to an object's resistance to any change in motion. Your brakes changed your motion, but they did not do so with enough force to overcome inertia. That's probably not a bad thing, as had you not slowed down over at least some space, chances are that anything unsecured in the car would have inertia that the brakes couldn't stop.
Inertia at Work
Inertia is a pretty powerful force, as anyone who has ever been thrown against their seatbelt can attest to. After all, it keeps the planets in orbit, helps cars maximize their energy output from engines, and carries long-distance runners the last few steps to the finish line. Inertia even helps you kick the dirt off your shoes. Really - when you kick your shoes against the frame to dust them off, the leather and sole of your shoe is firmly attached to your foot, so it doesn't go anywhere. Meanwhile, the dirt on your shoes is not so attached, so it flies off freely.
The First Law in Practice
For these reasons, many scientists will often just define Newton's first law as 'inertia.' It's a pretty succinct answer, and in those few letters can refer to a concept much too large to cover in one breath. So, let's have some fun with inertia.
Say that you were at a demolition derby watching cars crash into each other. A car slams into another car and both move. Some of the inertia from the colliding car transfers to the car that got slammed into. In fact, enough inertia was transferred to cause the hit car to overcome any force of inertia to remain still. There's plenty of math to explain this sort of conclusion, but this will suffice for now.
So, what about gravity? Let's test the inertia of you versus gravity. Jump into the air. Go ahead, jump up, no one is watching. On the way up, your upward inertia overcomes any downward inertia caused by gravity. However, that upward inertia runs out of steam, meaning that the downward inertia carries you back. Again, there's plenty of math to back that up, but what is important now is that you get this concept.
In this lesson, we looked at the idea of Newton's first law of motion, commonly referred to as inertia. In short, the idea is simple: an object in motion tends to stay in motion, and an object at rest tends to stay at rest. Any change to the motion or rest of an object must overcome the inertia of that object to stay put. Inertia is the reason that planets stay in orbit around the sun, as their motion to move straight is perpetually curved by the gravitational pull of the sun. Meanwhile, inertia also carries us up when we jump and cleans our shoes when we kick them against a post.
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Overview of Newton's First Law of Motion
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