I don’t know where you are as you are reading this. You could be sitting on a bus, in your room, at a cafe, in a park, or even lying on your bed. Whenever you may be, I am sure if you look around or listen carefully to your environment, you will note some hints of motion. Perhaps a bug buzzing around, a sound of a passing car, birds singing outside, a distant siren, or a screech of a chair. In any case, I can guarantee there is motion all around you.
But for the sake of this article, let us suppose that you are reading this from deep space, so deep that when you lift up your eyes to look around through the glass helmet of your spacesuit, all you see is pitch darkness. No stars or light of any kind; not a single dot in the bare dark sky.
Then suddenly you catch a faint sight of a red flashing light that appears to be getting closer. When it finally gets close enough, you can see that it’s a beacon light attached to the spacesuit of another space-drifter, Amy. She slowly passes by, waving her hand at you as she does. And just as she appeared, she recedes into the darkness of space and is lost out of sight.
Where is she going? You wonder as you get back to your reading.
Amy on the other hand, analyses this encounter as follows: From her perspective, like you, she sees a tiny pulse of flashing light getting closer and closer to her. With time, she is able to deduce that the light is coming from a beacon on your spacesuit. You turn your head to look at her as you pass by, and she in turn, waves at you. Then off you disappear into the darkness of space. Leaving her with the familiar question…
Where is he going?
And here’s our first wrinkle: who is moving?
(The aforementioned argument is only valid for bodies exhibiting uniform or force-free motion, more on this later)
For more discussion on the motion of bodies, read: What is motion in physics?
Introducing the principle of relativity
Although you and Amy cannot agree on who is moving and who is stationary, the principle of relativity declares that you are both right. According to this principle, motion is subjective; an observer can only perceive motion in relation to or by comparison with other objects in their vicinity. Another observer in a slightly modified condition may perceive the same “motion” differently. And both observers would be right of their own accord.
You see just as beauty is in the eyes of the beholder, so is motion in the eyes of the observer – it depends on who you are asking (and their state of sobriety)!
In its simplest form, the principle of relativity says that the laws of physics are identical in a moving* or a static system. Put in another way, there’s no physical law you can perform that’ll let you know whether you are moving or not.
Although the principle of relativity is synonymous with Albert Einstein, it has its roots in the works of Galileo Galilei back in the 15th Century. Isaac Newton developed the concept from Galileo in his study of the motion of bodies and used it to deduce his first law of motion, also known as the law of inertia. Albert Einstein further refined the concept and used it as the basis of his special theory of relativity in 1905. However, the principle of relativity is beyond the scope of Newton’s first law of motion, which is the objective of this article. So we will not delve much deeper into it.
Read: Principle of Relativity (Wikipedia)
The early ideas on motion
There isn’t a universal way of looking at motion. In fact, when we try to do so, we ran into some serious logical problems. The ancient Greeks for example, tried to understand absolute motion and it didn’t end very well for them.
You see, ancient Greek philosophers grappled with fundamental questions on the nature of motion. Two of such questions were these: what is the natural state of matter? Is it at rest or in motion?
The general consensus was that the natural state of all matter was at rest. They had observed that objects in motion would eventually come to a halt. For instance, when a loose stone was given a kick, it traveled for a while and eventually came to a halt. And so did everything else that wasn’t continually pushed or pulled. So their philosophy on motion went something like this, “a force is necessary for a body to stay in motion – even if the body maintains the same speed and direction”.
Let us now revisit our original scenario of two deep space drifters in light of the ancient Greek philosophy on the motion:
So you are out there chilling in deep space and you see Amy drifting past. The ancient Greeks would tell you, “Hey, see Amy moving over there? That means there’s a force continually pushing her!”
“But I don’t see any force pushing on her; the jetpacks on her back aren’t firing” You point out.
“No, there’s definitely something pushing her, you can’t see it from here but trust me; it’s there. Look! She’s even waving at you”
Meanwhile, Amy is also watching you drift past. She also surmises there must be a force pushing you past her although she can’t put a finger on it. So we are forced to conjure up forces from thin air (although we’re in deep space!) to explain motion.
So the notion that a force is necessary to keep a body moving as proposed by the early Greek philosophers doesn’t hold up to the test. Of course, it takes a lot more than a thought experiment involving imaginary space-drifters to overthrow a scientific hypothesis, but scientists have done plenty of experiments over thousands of years dating back to Galileo in the 15th Century to disapprove the Greek’s philosophy on the motion.
Today, we know that a force isn’t necessary to keep a body moving. In fact, the entire concept of motion is subject to the observer’s own frame of reference or perception.
For an interesting satire involving the clash of ideas between the ancient Greek philosophy on motion and modern concepts, check out Aristotle Vs Isaac Newton.
Introducing the idea of force-free motion
As the name suggests, force-free motion is the type of motion that takes place in the absence (or with a net-zero) of external influence or force. (Yes, force-free motion is a thing and legend has it ancient Greeks are still punching the air over this)
Force-free motion is also known as uniform motion. Bodies exhibiting a force-free or uniform motion have the property of maintaining the same speed and the same direction in space throughout their entire tenure of motion. A good example of such motion is the space drifters we introduced at the beginning of this article. Being in completely empty space, far from other celestial bodies, the space drifters were floating force-free in space – never to stop.
At this point, the ancient Greeks would ask, “Hey, but why does everything we set in motion come to a halt then”?
And we would answer, “because those motions weren’t force-free”.
Whatever experiments the early Greeks made on motion, they were made on planet Earth. And Earth has a wide array of forces at play when we try to move objects along it: air resistance, gravity, and friction are the most common. And there are more than just forces, for instance, you’ll have to account for the rotation of the Earth, which in principle affects the uniformity of motion along with it. This is why it was important to set our thought experiment at the beginning of the article somewhere in deep space; far away from all the confusion and chaos that Earth adds to the motion.
However, force-free conditions may be achieved on planet Earth if we introduce the idea of a net-zero force – or zero resultant force.
What is a net force?
What happens when you have more than one force simultaneously acting on an object, such as that of a rope being pulled by two puppies below?
Nature says a single force can represent all the forces (in magnitude and direction). That single force would have the same effect as the combination of the individual forces; we call it the resultant force or net force.
The implication is that if the net or resultant force is zero, then the object in question will behave as though no force acts on it all i.e. it’ll exhibit a force-free behavior.
A wise man once said, “Nature takes delight in simplicity, and nature is no dummy”.
The combination of forces is a vectorial addition technique known as the parallelogram law. For an in-depth analysis of this interesting property read: Understanding the Parallelogram Law of Vector Additions
Newton’s first law of motion
Newton’s first law of motion comprises two profound statements; the first part is a summary of our little discussion.
“It doesn’t matter whether a body is at rest or moving with a constant velocity in a straight line, it’ll continue to stay that way”.
This first part of the law has a name of its own, the principle of inertia – the tendency of a body to maintain its state of rest or uniform motion.
Newton was able to realize that the behavior of a body in uniform motion was equivalent to that of a body at rest, and if it behaves like a body at rest, then it should continue to remain in motion just as a body at rest continues to remain at rest. It’s a bold extrapolation, but true nevertheless.
The second part of the law follows directly from the first one and continues, “… unless a net external force acts on the body causing it to change that state of motion or rest”. This part would be even trickier to explain to our ancient Greek philosophers.
Since the early Greeks couldn’t reconcile uniform motion and rest as equivalent states of a body, they were forced to categorize motion into different groups. For example, they invented the notion of natural motion, which was thought to be the kind of movement that a body would “naturally” follow, such as objects falling down to Earth or water flowing downhill. This kind of motion required no force – it was assumed to be spontaneous, guided by the ratio of the four elements (fire, air, water, and earth). The other type of motion went by the name “forced motion”; this was the opposite of natural motion. In this case, a body was going against some resistance – such as gravity or friction. There was yet another form of motion, freewill motion, where living organisms would decide where to go based on their will. With these 3 kinds of motion and their combinations, the ancient Greeks could explain all terrestrial motion – but it wasn’t enough.
Newton realized that he could attack the topic of motion effectively by investigating changes in motion rather than analyzing motion in general. For instance, in our earlier example, neither you nor Amy could agree on who’s moving, yet you would both agree on whose motion is changing. Let’s say Amy sped up as she passed you so that she receded from you faster than she approached. Then your assessment would be that Amy sped as she passed me.
In space, Amy has neither the free will nor natural propensity to accelerate on her own. So whatever change in her motion would have been “forced”, and this would likely be something that Amy herself could detect (say the thrusters on her back). So the two of you would agree that Amy sped up.
When Amy waved as she passed you, that was a change in her (hand) motion. And both of you would agree that it was Amy’s hand that was waving and not yours!
This second part of Newton’s first law of motion introduces the concept of cause and effect, that is, changes in motion are a secondary effect of a previous cause – force.
Examples of Newton’s first law of motion
A book sitting alone on a seat of a moving bus will slide forward when the bus suddenly stops. The first part of Newton’s first law of motion, “objects in motion or at rest tend to continue to stay that way” explains the forward motion of the book.
It is also interesting to analyze this scenario from the point of view of (say) a bug who is asleep on top of the book. As far as the bug is concerned, the book hasn’t changed its state of rest. Assuming the seat is polished and has very little friction, it wouldn’t be apparent to the bug that the book is moving at all unless of course the bug wakes up and notices the bus moving or until the book falls off over the edge of the seat.
There are plenty of examples of Newton’s first law of motion in everyday life, I won’t go through them all here but I will link out a resource.
