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One of my friends and I had an argument over this topic. He stressed the fact that in real life many forces exist, whereas in physics we deal only with ideal situations. He put the following arguments:-

  1. Newton's First Law is invalid because friction exists in real life.
  2. Newton's second law is invalid due to the same reasons.
  3. Newton's third law is invalid because in a trampoline, there is excessive reaction.

In defence, I put forward the following arguments:-

Newton's laws are true but the equations have to be modified to take into account the other forces in real life.

For example, if a force $F$ is applied on a body of mass $m$, and $f_s$ is the force of friction, then, the equation becomes $F - f_s = ma$. Thus, we have just modified the equation $F = ma$.

So basically I mean to say that we have to adjust the laws to suit our purpose.

In the end, there was a stalemate between us. Even now I am confused after this argument. Please clarify my doubt.

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closed as unclear what you're asking by ACuriousMind, Carl Witthoft, CuriousOne, Emilio Pisanty, David Z Jul 31 '16 at 13:32

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Why has been my question downvoted? I was just clarifying my doubt. $\endgroup$ – Mriganka Parasar Jul 28 '16 at 2:17
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    $\begingroup$ It's being down voted because it basically answers itself in the question. All forces must be accounted for, and friction is a force. It seems like you know the answer already so why ask the question? At least that's my analysis of the down votes, maybe I am wrong. $\endgroup$ – Max von Hippel Jul 28 '16 at 3:55
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    $\begingroup$ I've deleted some nonconstructive and/or obsolete comments. $\endgroup$ – David Z Jul 29 '16 at 17:09
  • $\begingroup$ @DavidZ: Then you deleted the only physically relevant comments. :-) $\endgroup$ – CuriousOne Jul 29 '16 at 18:32
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    $\begingroup$ It's being downvoted because it has an incendiary title coupled to a poorly researched question. The answer is in every decent highschool textbook. $\endgroup$ – Emilio Pisanty Jul 30 '16 at 20:13

10 Answers 10

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Regardless of relativistic effects:

  1. Newton's First Law is invalid because friction exists in real life.

False, the first law talks about the case when no forces are present, if forces are present go to the second law.

  1. Newton's second law is invalid due to the same reasons.

False, you add friction to the total force.

  1. Newton's third law is invalid because in a trampolin, there is excessive reaction.

False, why do you think there is excessive reaction?

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  • $\begingroup$ I've deleted some nonconstructive and/or otherwise inappropriate comments. $\endgroup$ – David Z Jul 31 '16 at 13:34
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Newton's First Law is invalid because friction exists in real life.

Let's review what Newton's first law says:

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a net force.

Your friend is right that friction exists in real life. But your friend is wrong that the first law is invalid because of it. The friction is that net force being acted upon the object. The law holds out in space where there is (virtually) no friction, and it holds places on earth where there is a lot of friction.

Newton's second law is invalid due to the same reasons.

Again, let's review the second law:

In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration vector a of the object: F = ma.

Once again, friction is a force that goes into that vector sum. So like he said, for the same reasons. Except it's valid for the same reasons, not invalid.

Newton's third law is invalid because in a trampoline, there is excessive reaction.

Just bought a trampoline. There isn't any excessive reaction. There's just more bounce. Say you jump from the ground. You push off with some force, go up in the air, hang for a sec, and fall back down. At the top, you have lots of potential energy and at the bottom you've got lots of kinetic energy. And because the earth is made of very rigid rock it doesn't flex very much and absorbs the energy you put into it. Probably your knees and ankles absorb some too.

On a trampoline, this isn't what happens. Your potential energy turns to kinetic energy as you fall as before, but instead of that energy going into the ground with a thud, it goes into the trampoline as potential energy. At the bottom of the trampoline bounce, the resistance of the trampoline overcomes your motion and starts pushing you back up.

So the difference between the ground and the trampoline is that on the ground your energy from jumping goes into shockwaves in the ground, while jumping on a trampoline goes into potential energy that's used to push you back up.

If you want to try this, try taking a rubber bouncy ball and a stuffed animal. Throw them both at a wall and see which one bounces back more. Same energy going into both of them, but one is able to turn that kinetic energy into potential energy and reverse the flow while the other just kinda flops. Newton's laws account for both scenarios quite well.

We may have been able to find scenarios where Newton's laws don't apply (i.e. relativity) but the laws remain fundamental to engineering and have very, very real world applications to this day.

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Wolphram johnny gave good explanation but for trampoline case there can be more explanation

Whenever you jump from earth, according to "third law", the force you give to the earth is equal to the force (reaction force) given by earth to you which makes you go off from the ground, the force you gave to earth actually moves earth (negligibly due to its high mass) the reaction force is the force which makes you jump... So i guess you are clear about where the third law acts now.

Coming to the trampoline case when you jump and land on trampoline your kinetic energy is stored due to its spring like property, so when you jump again the force is exerted by you and trampoline, both so you have more acceleration (newton second law) this net force is higher compared when you jump from the ground alone (when you jump and land on ground the energy is not stored like in that of trampoline and no extra force). Here the net force is more and the reaction force given to the earth is more and the earths moves more (yet negligible).

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As the saying goes: "All models are wrong, but some of them are useful."

For me, laws of physics are actually just models, i.e., simplifications of reality that give satisfactory answers to certain questions. Newton's laws are useful to build bridges, build skyscrapers, land rockets on the Moon, etc.

You could also argue that:

  • In reality, a force is never perfectly constant (e.g., combustion engines produce force in bursts)
  • In reality, mass is never constant (e.g., even the prototype kilogram loses mass)

Then again, how often do you "care" in reality about a change of $50µg$? If you do calculus of a car, you might decide to model its mass as being constant during its trip. As long as the model gives you are reasonable answer it is useful. Otherwise, you need to adjust or change the model (e.g., add fictions forces, relativistic effects, etc.)

P.S. No clue who had the idea of giving models in physics such a solemn name as "laws". Scientific marketing, I guess. :D

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    $\begingroup$ This answer is misleading. Newton's laws become wrong at high velocities, small distances or high densities (or your favorite combination of those). Forces not being constant, masses not being constant, the presence of friction, or any other complications you might encounter in "real life" can be handled by Newton with zero problems. $\endgroup$ – Javier Jul 28 '16 at 22:02
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    $\begingroup$ "Become wrong" is a bit subjective: For example, below certain speeds we choose to ignore relativistic effects (but if you really want super-precision, nothing stops from taking them into account). Maybe I'm splitting the hair, but Newton's law can handle non-constant quantities by essentially splitting time or space in infinitesimal units, in which the quantities can be assumed constant. $\endgroup$ – user1202136 Jul 28 '16 at 22:32
  • $\begingroup$ As an example of why I think Newton's laws are essentially a model: To make them work inside a non-inertial frame of reference, one needs to add fictious forces. $\endgroup$ – user1202136 Jul 28 '16 at 22:34
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    $\begingroup$ @Javier It absolutely is misleading. Newton's laws cope perfectly well with non-constant forces and non-constant masses: you just have to use the calculus with which Newton (and Leibniz) conveniently also provided us. $\endgroup$ – David Richerby Jul 29 '16 at 14:13
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    $\begingroup$ And, by the way, you absolutely do not want to model the mass of a plane as being constant through its trip. Fuel can be more than half the take-off weight of a large jet departing on a long flight. $\endgroup$ – David Richerby Jul 29 '16 at 14:14
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Newtons laws are a good approximation for how the world works when the velocity is less than 1% the speed of light, the gravity isn't too strong, when the number of elementary particles an object is composed of isn't too small as an object needs to be large enough for quantum uncertainty to be insignificant, and the amount of space is small compared to the observable universe so that the effect of the expansion of space is insignificant. Friction doesn't disprove Newtons Laws as Friction itself is a force. Because we live on a planet where we tend to be unable to get objects off the ground for long periods of time without technology friction tends to effect everything in our daily lives to the point that until people performed proper experiments on motion it wasn't obvious that friction was effecting objects, which is why it wasn't obvious that moving objects would continue to move in the absence of any force.

Friction does not disprove the second law as friction has to be taken into account to find the total force acting on an object.

The trampoline example doesn't disprove Newtons third law because it states that for every force there is an equal and opposite force, which is different from the effect being equal. Because F=ma the acceleration of a lighter object will be greater than that of a heavier object even when the same force is applied to the lighter object than the heavier object.

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    $\begingroup$ I wouldn't say it's about the strength of gravity or the velocity, but rather, their relative values with respect to the objects you care about. E.g. relativity is important when considering ISS versus the surface of the Earth, but not versus an object floating inside of the ISS. $\endgroup$ – Luaan Jul 28 '16 at 9:49
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Your friend is wrong, and seems to suffer from a serious case of anti-intellectualism, not to mention arrogance and authority issues.

It's true that the models of physics we know are approximations, but the reason we're using them - after centuries of research - is that they're extremely good approximations.

To suggest that our physical laws can't cope with something as simple as a trampoline is utterly ridiculous.

So your friend doesn't understand the finer points of how a trampoline works. That's fine, many people don't. The anti-intellectualism lies in what he does with this lack of knowledge. A healthy approach is to be curious, to try to analyze the phenomenon, to try to understand why it works they way it does, to wonder if Newton's laws mean what he thinks they mean, and to ask questions along the way. To realize that if physicists - many of whom have seen trampolines in their lives - haven't modified the models to accommodate them, it's because they already do, for reasons he needs to investigate.

Instead, he chooses to assume that trampolines are magic, that they are outside the natural order of things, that they cannot be explained. That physicists are stumped in the face of trampolines. "We've placed a man on the moon, but we just can't figure out the damn trampoline thing!".

As for specifically how trampolines work - that's the smaller issue, but I'll address it anyway.

  1. Your friend seems to confuse force with energy/velocity. He looks at a person gaining energy by jumping on the trampoline, and for some reason assumes the trampoline exercises more force on it than the other way around. That's completely baseless.

  2. Your friend seems to assume trampolines give you energy for free. They don't. It's not like the video game jumping blocks that you just step on and get flung in the air. If you stop moving while jumping on a trampoline, you'll lose energy, speed and height until you come to a stop. To bounce higher, you need to use your muscles to generate energy - the trampoline helps you convert your downward velocity to upwards velocity, and you gain more energy with your muscles, so you're able to jump higher than before.

As for the friction thing, this was a completely inane argument. Friction is a force. It's not some mysterious "non-force force".


Edited to add based on smci's comments:

Based on the OP's account (which is the only thing I know of said friend), what he displays is not healthy skepticism. It's ok not to get how friction or trampolines work. It's not ok to deduce that physics isn't applicable to real-life, or that it can't explain these two things. "Magic" is the appropriate word to describe the friend's stance because he insists science can't explain these things. The friend's approach is defeatist rather than inquisitive - he doesn't try to reconcile and explain his counterexamples, he just assumes they cannot be explained.

It's true that skepticism can lead to new discoveries, but you have 1,000,000 people who misunderstand something for every person who discovers a genuine deficiency in the current paradigm. So some level of humility is required. Furthermore, you need to understand the current paradigm very well before you can find that it's wrong. If the friend "gave the system a chance" and sought answers, he would have found them. Or, if the confusion happened to be that once-in-a-million time where he actually discovered a mistake, he'd continue investigating until he could demonstrate the error convincingly. But the friend didn't do any of that, he just rage-quit and adopted his own pet theory that "Physics doesn't apply to real life". If young Richard Feynman had had an approach like this, he wouldn't have become the Feynman we know.

As I wrote originally, reconciling friction and trampolines with Newton's laws (and whether the textbooks do a good job with the explanations or not) isn't the main issue here, and explaining those things in the answers doesn't address the main issue. The main issue is the friend's approach, so I focused on that in the answer, and I'm calling a spade a spade. Even more so if their age is 13 - that's a good age to start adopting a healthy approach to science.

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    $\begingroup$ Your first sentence is really unhelpful. A healthy level of skepticism and searching for counterexamples is always good. Sure the guy is mistaken, but the answers are educational. "chooses to assume that trampolines are magic"... "inane" is very disparaging. Would you apply the same language to a young Richard Feynman? I bet you wouldn't. $\endgroup$ – smci Jul 30 '16 at 21:55
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    $\begingroup$ Wrt "Newton's laws are true but the equations have to be modified to take into account the other forces in real life", high-school students typically don't get taught the vector formulation of Newton's laws, or the crucial clarification that it's "net force". Everyday situations do indeed involve lots of reactions and friction forces, so the textbook emphasis on one single force externally applied is indeed misleading. There's really no need to be disparaging. We also don't know whether they're age 13 or what. $\endgroup$ – smci Jul 30 '16 at 22:01
  • $\begingroup$ @smci: I've written some comments but they're actually best as an appendix to the answer, stay tuned. $\endgroup$ – Meni Rosenfeld Jul 31 '16 at 7:28
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Short Answer: Your friend is wrong because our models of friction (within certain parameters) are derived from Newton's laws. Tidal forces, for example, are are a form of friction.

The argument that Newton's "laws" are "invalid" in an pure philosophical sense, better rest on Newton simply pulling the concepts of gravity, force and inertia out of thin air in order to give meaningless names to parts of his highly predictive geometric and mathematical models.

One can also argue that since they remain accurate only within a specific range of measurements, they capture any real truth about reality but just approximate it to some degree.

It's very, very important to remember that scientific "laws" or "models" do not predict concrete reality, they predict measurements. New scientific laws/rules/models arise when the old laws/rules/models failed to be able to reproduce new measurements.

TL;DR unless you've got time to kill

The core scientific method is take a large number of measurements and create a huge data set. Then create a geometric, mathematical (or as some now argue, computational) system that will, given one part of the set, be able to reproduce another part of the set. Scientific laws are really mere convinces to compress vast number of observations down to mathematical equation, geometric ratios or (as some argue today) a computation.

That scientific laws reproduce sets of measurement data and not reality is seen easiest in statistics, which is really a second layer of math/modeling that tells us how much off from reality or measurements might be e.g. no one has 2.1 kids.

But... the equations don't always reproduce the observed measurement sets unless we cheat a bit an cram in some arbitrary numbers i.e. numbers not derived from measurement, don't seem to represent any observable or measurable phenomena, but which nevertheless, make the equation predictive of its data set.

The great-grandday of them all is Newton's gravitational constant. Cavendish just measure fine gravitational deflections, then started punching in numbers till he found one reproduced the deflection in the data.

There are many constants crammed into scientific equations for which we have no explanation of why the constants are needed or what they represent. A large number of constants in the math of a system is strong predictor that the model will fail once superior measurements become available.

One of the sources of the great friction between Leibniz and Newton was that Newton just made things up. Things like "gravity", "force", "inertia". Leibniz following the French Cartesian idea that all motion in the universe resulted from the collisions of particles and that positing a mysterious "force" called "gravity" instantly began pulling to objects towards each other, or that objects kept moving unless acted upon by another object or "force" all bordered on outright mysticism.

Newton replied that his geometric and mathematical rule-sets/models made vastly better predictions than anything Leibniz had come up with and that naming the parts of the model was just a conceptual connivence and that he would even try to guess what inertia, forces or gravity were, or were caused by, he merely asserted that using the concepts worked very well in making predictions.

But the idea that forces, gravity and inertia were just arbitrary names, didn't catch on and most people still talk about them as if they exist and have some concrete reality we understand.

Because scientific models reproduce measurements and not reality, we often evoke metaphors, call them "as-if" fantasies we use to explain what phenomena we can't observe and measure causes the phenomena we can.

Thus Newton modeled gravity between two objects "as-if" they were instantly connected by a contracting elastic band ( or a non-streching string on the axis of an ellipse. Likewise, Einstein modeled gravity "as-if" nothingness was elastic, bent and curved and caused objects follow the bends and curves.

Newton modeled gravity as a "force" that transmitted instantly between objects. Later, scientist showed that it propagated at the speed of light. Einstein, then modeled gravity not as a "force" like an elastic band between objects, but as each object "bending" space around it such that any object traveling through the bent space found itself deflected toward the initial object.

We're still doing it. The Higg's particle isn't a particle but a convenient way to model changes in the Higg's gauge field (I think.)

But it bears repeating that scientific laws/rules/models reproduce/predict measurements, not the reality that scientist attempt to measure. Since we can't measure with perfect accuracy, our scientific models will always be off a bit from reality.

But "a little bit off" is just fine when your calculating the tossing of a horseshoe blowing something up with nuke.

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Your friend actively misunderstands the Newton laws and the ideas behind it. He is about to argue that formula $E=mc^2$ is invalid because it states that Youngs modulus ($E$) is equal to mass ($m$) times hypotenuse squared ($c$). And he is supporting his arguments by several argument fouls.

First law states that when there's no forces applied to the body it remains still or move with constant velocity. The argument by friction is foul, because friction is force and must be included to the equation. It also states that if the velocity is not changing there is zero net force - all applied forces cancel each other completely.

Second law describes the relation between net force and change in net momentum of point mass*. For real bodies we have to introduce angular momentum ($E_{rot}=J\omega^2$). Note that $F=\frac{dp}{dt}$. For constant mass (non-relativistic universe) we can derive $F=ma$.

Third law states that for every force (action) there is equal force (reaction) in opposte direction. In case of the trampoline (idalised) there is gravity ($G=mg$), the force of the trampoline pushing you ($F_{tr}=-k\Delta y$) and the force your body that pulls the trampoline down ($F_{body}=ma=m\ddot {\Delta y}$). If you draw proper image you will see that $G+F_{tr}+F_{body}=0$, that means $F_{tr}=-(G+F_{body})$. Call whatever side action, the other side is reaction.
We can also state that third law is result of laws of conservation of momentum and energy. It just say that if the net momentum and net angular momentum does not change, the net force is zero.


  • I could not find proper english word to czech "hmotný bod", german "Massepunkt", polish "punkt materialny". If such word(s) exist, feel free to edit.
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  • $\begingroup$ Point mass? $\endgroup$ – deltab Jul 29 '16 at 23:28
  • $\begingroup$ Whoa you know the term in 3 three languages but not English?! Fascinating. @deltab It's kinda weird those entries are not connected on wiki. Although, Czech entries have it in "See also sections". $\endgroup$ – luk32 Jul 30 '16 at 4:50
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Newton's laws are valid for all situations where velocities are small (compared to the speed of light, ie relativity is not important) and where quantum effects are negligible (mostly where objects are much bigger than elementary particles). The problem with your argument is that you and your friend are using idealized expressions for Newton's laws, not their most general form. That is completely understandable because the more general forms require mathematical concepts that are mostly restricted to physicists and mathematicians (in fact, Newton invented the calculus in order to formulate these laws). Rest assured that the laws are not just valid for those idealized situations that are expressed in terms of elementary math.

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    $\begingroup$ Thank you very much. I know that newton's laws are not valid at or near the velocity of light. But guess what? That friend of mine believes that einstein is wrong. I think I will leave him to his own thoughts. $\endgroup$ – Mriganka Parasar Jul 28 '16 at 2:32
  • $\begingroup$ I was just making the most general statement that I could regarding their validity. I thought your question was a good one so I up voted it. As for leaving your friend to his own thoughts, that seems advisable. $\endgroup$ – Lewis Miller Jul 28 '16 at 2:38
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    $\begingroup$ @MrigankaParasar Thinking Einstein is wrong is fine. We should not give too much extra consideration to authority figures in science. If your friend thinks relativity is wrong, please ask him to explain what experimental evidence makes him think that relativity is wrong. $\endgroup$ – DanielSank Jul 28 '16 at 3:41
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    $\begingroup$ If expressed in the correct form (for example $F=dp/dt$ and not $F=ma$), Newton's laws are valid in spacial relativity (and notice that Newton never wrote $F=ma$ in the Principia...). Also, the problem is not that his friend is not using "their most general form". The usual middle school textbook form is perfectly fine to explain every one of the situations described by the OP. $\endgroup$ – valerio Jul 28 '16 at 7:00
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Remember the sum $\sum$ symbol: $$\sum \vec F=0$$ $$\sum \vec F=m\vec a$$

A very important detail.

It's not about "adjusting" anything to make the laws work, it's just about including all forces. Which might be tricky in real life, but that's another story.

And by the way, you can tell your friend that friction is not at all always present - space is a quite good example.

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