# How did Newton come up w/ 1st law? Why does friction exist?

• Matt2411
In summary, the conversation discusses the history of Newton's laws of motion and the concept of inertia. It is mentioned that the first law of motion, also known as the law of inertia, was based on Galileo's observations and has not been contradicted by any evidence. However, it is acknowledged that scientific theories are not absolute truths and are subject to change as new evidence is discovered.

#### Matt2411

So I'm trying to understand Newton's laws, and I know that friction is supposed to be the force that stops an object in movement. But I don't really understand why this is the case, as the force of friction is not something that can be perceived intuitively (unlike, for example, the force I could apply to a box in order to move it around).

In fact, if I remember correctly, it used to be believed (before Newton's time) that objects needed a constant force to move. There was no such thing as "friction", things were supposed to naturally come to a halt after being applied a force. Why did this notion change?

Basically, what I'm trying to ask is, how did Newton come up with this idea? Because it's not an empirical finding: after all, he was the one to define what a force was in the first place. Or is it considered just to make inertia and the first law valid? And if so, how do we even know inertia is truly a real property of objects?

My apologies if I haven't expressed myself well or posted in the wrong section of the forums (wasn't sure where I should write this, because it isn't a homework exercise or anything of the sort).

Matt2411 said:
Basically, what I'm trying to ask is, how did Newton come up with this idea? Because it's not an empirical finding: after all, he was the one to define what a force was in the first place. Or is it considered just to make inertia and the first law valid? And if so, how do we even know inertia is truly a real property of objects?

My apologies if I haven't expressed myself well or posted in the wrong section of the forums (wasn't sure where I should write this, because it isn't a homework exercise or anything of the sort).
The first law was probably first stated by Galileo. It was based on observations done in experiments of rolling a ball down the side of a bowl and observing it roll up the other side. He observed that as the angle of the other side bowl declined the object tended to rise always to the same height. He suggested that if you could eliminate any resistance to rolling it would always roll to the same height and, therefore, if you made the angle 0 it would keep going for ever. So he made a postulate: the law of inertia which Newton made into the first law of motion. No one has ever been able to find any evidence that contradicts that first law.

AM

Andrew Mason said:
The first law was probably first stated by Galileo. It was based on observations done in experiments of rolling a ball down the side of a bowl and observing it roll up the other side. He observed that as the angle of the other side bowl declined the object tended to rise always to the same height. He suggested that if you could eliminate any resistance to rolling it would always roll to the same height and, therefore, if you made the angle 0 it would keep going for ever. So he made a postulate: the law of inertia which Newton made into the first law of motion. No one has ever been able to find any evidence that contradicts that first law.

AM

I see. So is the law of inertia an axiom then? In the sense that you start building the laws from the hunch that there's such a thing as "inertia"?

And when you say there's no evidence that contradicts the law, are you also saying there IS specific evidence that makes the first law valid? The only example I can think of is launching spacecraft into outer space, where an initial force is enough to give the object a constant velocity through time (as there's no "friction" there, according to my book) and never making it stop. Examples that concern bodies on Earth don't seem to indicate anything though, at least to me: the body will eventually stop because of this "force of friction", which can only be empirically estimated (thus not being very refutable in an experiment I suppose).

Thank you for the help so far :)

Matt2411 said:
I see. So is the law of inertia an axiom then? In the sense that you start building the laws from the hunch that there's such a thing as "inertia"?

And when you say there's no evidence that contradicts the law, are you also saying there IS specific evidence that makes the first law valid?
It is a generalization made from observation. Whether it is true in every sense is not known. Science does not provide absolute truth. Science provides models which seek to explain observations and allow us to make predictions. If they do that, we use them.

We have found that the law of inertia and the other laws of motion provide the least complicated and most useful explanation for our observations (eg. why the planets keep going around the sun in elliptical paths) and enables us to make predictions about the physical world. Those predictions are usually very, very accurate. So we use the theory. If a theory leads to a result that can be disproven by experiment, we look for a better theory. General relativity was found to explain what Newton could not and changed how we view inertia. Someone, someday, may find evidence that the theories of both Newton and Einstein are not quite correct.

AM

Matt2411 said:
... are you also saying there IS specific evidence that makes the first law valid?
Not completely sure what you mean here, but keep in mind things are only "proven" in math, never in physics. The best we can ever do is have a theory or law or whatever that so far has never been invalidated by empirical evidence, but that is not a "proof" of "truth", it's just the best we know at the moment. Newton's Law of Gravity was long widely believed to be "true" in the sense that you probably mean but was shown by Einstein to be a subset (a limiting case) of a wider truth and we already know that his Theory of General Relativity has to be a subset of some still wider truth that will encompass the area where it doesn't work, and this is a good example of why we don't believe in absolute truth or proof.

Delta2
Galilei came up with the idea, and Newton enunciated it as his first law. Aristotle said force must be continuously applied to keep a body in motion, and indeed that appears to agree with everyday experience. However, Galilei hypothesized air resistance and friction obscured what was really going on, and devised experimental apparati that reduced friction to negligible levels.

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Friction is not a fundamental force but it definitely is measurable.
So in the standard model it either arises from electromagnetism or from gravity, (or interactions of both ??)
(I think we can exclude nuclear forces).
Is there a known relationship between EM and gravity which explains friction?

How is gravity involved in friction?
You will have friction even in the absence of gravity.

nasu said:
How is gravity involved in friction?
You will have friction even in the absence of gravity.
Indeed. The issue is the force pressing two object together as they move against on another. Gravity is certainly one way that force can be applied, but not the only one.

Yes, this Is what I expected regarding gravity - it can contribute to friction and often does, but it isn't required,
So does that mean that friction is an emergent property of electromagnetism?
Are electrons behaving like a kind of velcro in a situation where two rigid bodies are sliding against one another, (or are attempting to do so)?

rootone said:
So does that mean that friction is an emergent property of electromagnetism?

Yep.

Drakkith said:
Yep.

I thought the origin of frictions and the normal force was due to Pauli's principle, but a quick search on internet just gave me contradictory answers.

andresB said:
I thought the origin of frictions and the normal force was due to Pauli's principle, but a quick search on internet just gave me contradictory answers.

The interaction between the atoms of object and the surface is from the electromagnetic force, and friction is a direct result of that interaction. I have no wish to get into a discussion on the end-all-be-all origin of either friction or the normal force.

Thank you guys for all the help! Forgot to get on PF after Sunday, so apologies for being so late.

The only thing that still bugs my mind has to do with what u/rootone said. How is friction to be measured? Thought the best we could do were friction coefficients. And what does it have to do with EM?

Matt2411 said:
Thank you guys for all the help! Forgot to get on PF after Sunday, so apologies for being so late.

The only thing that still bugs my mind has to do with what u/rootone said. How is friction to be measured? Thought the best we could do were friction coefficients. And what does it have to do with EM?
It has everything to do with EM. The reason you don't fall through the chair you are sitting in is because of EM force between the electrons in your pants and those in the chair.

Now rotate you and the chair 90 degrees and have someone push you really hard against the chair. Same thing. Now make 10,000 little bitty you's and 10,000 little bitty chairs and glue all the chairs onto one surface and all the little you's one another surface above it and push the two across each other. Same thing. As you smooth out your butt and angle the chair, it gets easier but to the extent that there is still "friction" it is the same EM force.

So electrons stop you from falling through the chair you are sitting in,
but equally they stop you from floating away from it.
I'm getting there slowly.

rootone said:
So electrons stop you from falling through the chair you are sitting in,
but equally they stop you from floating away from it.
I'm getting there slowly.
Gravity is what keeps you from floating away from it -- unless you are holding hard on the arm rests or have velcro on your butt.

Damn. I was nearly covinced by earlier posts that gravity has nothing to do with it (friction).

Gravity has nothing to do with friction. But it does have something to do with you not rising out of your chair.

rootone said:
Damn. I was nearly covinced by earlier posts that gravity has nothing to do with it (friction).
Friction between two surfaces depends on the characteristics of surfaces and the forces normal to the surfaces. If the normal forces are due to gravity, then gravity does have something to do with friction in that sense.

AM

Same as gravity has something to do with wine.
If the grapes are crushed by stepping on them, some of the force crushing the grapes is due to gravity.

Matt, the beast you call friction is a very large, complicated variable. It has a myriad of different applications for all things that rub...
You have not given us a physics problem, you have sent out a 911 call for 411 findings. Good Luck.
P.S. Go to tricipedia...

## How did Newton come up with the first law of motion?

Newton's first law of motion, also known as the law of inertia, was developed by Sir Isaac Newton in the 17th century. He came up with this law after observing the behavior of objects in motion and studying the works of previous scientists, such as Galileo and Descartes.

## What is the first law of motion?

The first law of motion states that an object at rest will remain at rest and an object in motion will continue to move in a straight line at a constant speed unless acted upon by an external force. This means that objects have a natural tendency to resist changes in their state of motion, known as inertia.

## Why is Newton's first law of motion important?

Newton's first law of motion is important because it laid the foundation for his other two laws of motion and helped explain the behavior of objects in motion. It also helped to disprove the earlier notion that objects needed a constant force to keep moving, as Newton showed that objects will continue moving without a force acting upon them.

## Why does friction exist?

Friction is a force that resists the relative motion between two surfaces that are in contact with each other. It exists due to the microscopic roughness of surfaces, which causes them to interlock and create resistance when they slide against each other. Friction is important because it allows us to walk, grip objects, and drive vehicles, but it can also be a source of energy loss and wear and tear on objects.

## How does friction relate to Newton's first law of motion?

Friction is related to Newton's first law of motion because it is an external force that can act upon an object to change its state of motion. For example, when a person pushes a box across a floor, the force of friction between the box and the floor causes it to resist the motion and eventually come to a stop. This is in accordance with Newton's first law, as the box's natural tendency is to remain at rest unless acted upon by an external force.