Newton's 1st law
Newton's first law of motion; revolutionary in its day and often not understood or unappreciated in ours. Yet this first of three laws make up a large part of the foundation of science and technology enjoyed today.
So, what is it?
A tale of two situations
Consider two cases.
Case one: You're driving down a long, straight and level road. You have the media of your choice playing and life couldn't be better.
All of a sudden, the engine quits running and you cannot restart it. What will happen to the car?
OK, pretty obvious, it comes to a stop by the side of the road.
So the next question is, why? OK, again you might say “isn't it obvious? The engine stopped.”
True, it was the engine that kept the car going. But there is one question that then needs to be asked. If the engine running was the reason the car was in motion, and its stopping was the reason the car came to a stop, why didn't the car instantly come to a stop the moment the engine stopped? Why did it take a while to come to a stop?
Case two: The voyager space probe was launched from Earth in the late 1970s.
The rocket engines that launched it were turned off and abandoned about 40 years ago. And yet, the probe is flying merrily through space, visiting several planets along the way and is now flying out and away from the solar system without any sign of stopping
So, we have two similar cases with very different results. How can this be?
Keep on, keeping on
OK, confession time. Like a magician, I used some “smoke and mirrors.”
The two cases are not the same. Can you tell what's different?
The difference (among other things) is that while in motion the car had to push itself through the air, which gives the car a resisting force. On the other hand, the probe doesn't. So the probe provides with our first, pure and clean, view as to how motion behaves.
In the absence of forces on an object, its motion keeps on doing the same thing. Never changing.
But how about the first case and the question that went with it? Is there nothing to be learned?
Absolutely.
Consider the differences between the two cases. The first case involves a resisting force, the second does not. In the first case, there is a change in motion (the car slows down), the second does not.
Let's draw a conclusion.
Forces cause a change in motion.
This is an important discovery. The car engine did not cause the car to move, it originally caused the car to change its motion from sitting still to being in motion in the first place.
But this is not the whole story, is it? Because you can still have a car engine driving the car forward and drive at a constant speed. What's up with that?
In this case, the forward force caused by the engine and the resisting air force push against each other like a tug-of-war.
If the forward engine force and the backward air resistance force push with the same force (imagine a tug-of-war with both sides pulling the same amount) then the two forces neutralize each other; it is the same as if there were no force. So we've learned something new.
It is the combined action of all forces that determines the change in motion. Forces that neutralize each other are the same as no force at all.
Not just a good idea, it's the law
All this brings us to Newton's 1st law of motion or law of inertia:
“An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.”
Or perhaps put another way in light of the above discussion:
Non-neutralized forces cause a change in motion. In the absence of forces, or non-neutralized forces, an object's motion continues doing whatever it is doing, or not doing.
Two ways to think of motion
Lastly, it is useful to introduce two ways to describe rates of motion:
Speed – We are familiar with this term. It describes how much distance an object moves over time. A car traveling at 75 miles per hour will move 75 miles for every hour it is in motion.
Velocity – Velocity is speed + direction. More descriptive than speed, it also tells you where you are going. So a car with a velocity of 75 miles per hour north will move 75 miles north for every hour it is in motion.
Velocity is better than speed for use with Newton's laws. It would then be accurate to say that force causes a change in velocity. Any change in velocity (speed or direction) must be caused by a force. Some examples:
A car speeding up requires a force caused by the engine.
A car slowing down requires a force from the air or brakes.
A satellite with it's changing velocity (the direction of its motion must change as it goes around the Earth) requires a force. In this case its gravity.
Newton's 1st law is a very good way to describe the reasons why motion changes and the role force plays in this process. Still, it may not at first seem obvious to the casual observer that this is how motion behaves.
But, isn't the pursuit of knowledge and the challenges new knowledge can bring part of what makes discovery worthwhile? This is what makes science worthwhile.
On the web
Let's Chat about Newton’s First Law of Motion!
A fun animation describing Newton's first law in the context of hitting baseballs.
Physics - Newton's First Law of Motion
Another fun video presentation describing Newton's first law.
See Two Spacecraft Journey to Outer Reaches of Solar System
An illustration of Newton's first law. This video follows the path of the two Voyager spacecraft from launch as they move through the solar system. True, their motions are never constant due to the Sun's gravity; their paths do curve. However, as they encounter planets and their gravity, the paths suddenly change and they move in a very different direction. Fun to watch in a geeky sort of way.
They Might Be Giants - Speed and Velocity
Video showing the difference between speed and velocity, done as a type of music video.
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