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## #1 2023-05-05 06:52:18

paulb203
Member
Registered: 2023-02-24
Posts: 241

### Newton's 3rd Law of Motion

If I push a door the door pushes me, yes?

This seems strange to me. When I push a door I am making an effort to do something, it is a form of exertion. I am trying. I will burn a calorie or two in the process. I am actively doing something.

The door, it seems to me, is not making an effort, not trying, not exerting itself, not burning any calories, not actively doing something, it’s just standing there (I think?)

The same goes for me pushing a car (I think?).

All the inanimate objects/bodies in the textbook physics examples are, it seems to me, just sitting there, or standing there, while the person in the example is actively doing something.

Q. Is the kind of push ‘done’ by the door/car/etc, a different kind of push to the one a person does?

Q. And if a force is an interaction between objects involving a push or a pull, often with one or more of the objects being inanimate, might it be more helpful to think of the push and pull in terms of repulsion/attraction, given the implications of human pushing and pulling?

Q. After I push the door I am changed, for example, out of breath, an ounce or two lighter, etc; is the door any different than it was, having pushed me?

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## #2 2023-05-06 05:00:44

Bob
Registered: 2010-06-20
Posts: 10,503

### Re: Newton's 3rd Law of Motion

hi paulb203

It does seem odd; I understand your confusion.  If you put a bag of sugar on some kitchen scales you'll get a reading which may be expressed in kilograms or pounds weight, but is actually a measure of the force the bag is exerting on the scales.  It's a force because if you could repeat this test on the Moon you'd get a lower value even though the mass of sugar is unchanged.

As the bag isn't moving downwards there must be equilibrium at the point of contact.  That means there must be two, not one, force acting here; the force of gravity downwards and what is called the reaction of the scales acting upwards on the bag. If you put something squishable between the bag and the scales (say some soft putty) it would be deformed top and bottom ... ie the upwards acting force is also squishing it.

So where does the energy to resist you come from when you push a door? Well let's separate into two cases: (1) the door is open; (2) the door is closed.

(1) Here F = ma (2nd law) comes into play. If you knew the weight of the door you could work out it's acceleration. (2) The door cannot move so your efforts are transmitted through to the door frame and the wall.

If you've got some bathroom scales you could try this experiment. Open the door a bit and wedge the scales sideways between the door and the frame. When you push now the scales will measure the force the door exerts on the frame. (note: don't blame me if you break the scales though!!)

Is the door any different after you have pushed it?  I tried this and asked the door but it refused to answer.  If you pushed really hard you might break it. I expect every time you open and shut a door the hinge screws cut a little more into the wood until eventually you have to put in some larger ones.

Newton's laws are a model for explaining certain behaviours of objects, moving and static.  They work so well that you can explain the movement of the planets in the solar system and astronomers were able to discover new planets using the model.  But it's just a model. At an atomic level the door is mostly open space with neutrons etc hanging about inside. Newton's laws are not consistent with Einstein's theories nor with quantum mechanics so my advice is use them when appropriate but don't get too hung up on why they work.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

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## #3 2023-05-09 01:44:00

paulb203
Member
Registered: 2023-02-24
Posts: 241

### Re: Newton's 3rd Law of Motion

Thanks, Bob.

“If you put a bag of sugar on some kitchen scales you'll get a reading which may be expressed in kilograms or pounds weight, but is actually a measure of the force the bag is exerting on the scales.  It's a force because if you could repeat this test on the Moon you'd get a lower value even though the mass of sugar is unchanged.”

Is that why a person weighs less on the Moon than they do on Earth?

“As the bag isn't moving downwards there must be equilibrium at the point of contact. That means there must be two, not one, force acting here; the force of gravity downwards and what is called the reaction of the scales acting upwards on the bag.”

So for the force of gravity downwards it would be;

F=ma. F=1kgx10m/s^2. F=10N downwards?

Would the reaction force of the scales acting upwards on the bag match/mirror this?

F=ma. F=1kgx?. What would the acceleration upwards be? If you removed the 1kg bag of sugar would the top part of the scale spring up at 10m/s^2?

Nb; I’m using 10m/s^2 as an approximation for g.

“Is the door any different after you have pushed it?  I tried this and asked the door but it refused to answer.”

“If you pushed really hard you might break it. I expect every time you open and shut a door the hinge screws cut a little more into the wood until eventually you have to put in some larger ones.”

I saw a Youtube video about the normal force. There was a diagram of a box on a floor. It was given as an example of Newton’s 3rd Law but a few people in the comments said it wasn’t, that it was more about elasticity than about Newton’s 3rd Law. Is that correct? Was I incorrect in thinking that my example of me pushing on a door was an example of Newton’s 3rd Law? Is me pushing on a door different from a box resting on a floor?

“But it's just a model. At an atomic level the door is mostly open space with neutrons etc hanging about inside. Newton's laws are not consistent with Einstein's theories nor with quantum mechanics so my advice is use them when appropriate but don't get too hung up on why they work.”

By model do you mean an approximation? They are roughly correct, and work (in engineering etc), but when we look more closely they’re actually incorrect?

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## #4 2023-05-09 03:20:40

Bob
Registered: 2010-06-20
Posts: 10,503

### Re: Newton's 3rd Law of Motion

hi paulb203

Is that why a person weighs less on the Moon than they do on Earth?

Yes, that's it exactly.  Scales and spring balances measure the attraction due to gravity although they may be calibrated in units of mass.  If you've got a very accurate way to measure this you can detect places on Earth where the gravitational pull is more or less; eg up a mountain.

F=10N downwards? yes.

Would the reaction force of the scales acting upwards on the bag match/mirror this?

yes.

F=ma. F=1kgx?. What would the acceleration upwards be? If you removed the 1kg bag of sugar would the top part of the scale spring up at 10m/s^2?

That's a tricky question to answer. First you've got to instantly remove the bag in no time; then allow for the restorative properties of the spring ... which will vary as the spring returns to its normal uncompressed state.  I think too hard to analyse.

I saw a Youtube video about the normal force. There was a diagram of a box on a floor. It was given as an example of Newton’s 3rd Law but a few people in the comments said it wasn’t, that it was more about elasticity than about Newton’s 3rd Law. Is that correct?

I don't see what elasticity has got to do with it. Do you have a link to the vid?

Was I incorrect in thinking that my example of me pushing on a door was an example of Newton’s 3rd Law? Is me pushing on a door different from a box resting on a floor?

I think they are both N 3rd L.

By model do you mean an approximation? They are roughly correct, and work (in engineering etc), but when we look more closely they’re actually incorrect?

Not really.  Newton's laws work pretty well.  They're good enough to land a man on the Moon and to predict the existance of planets that cannot be seen just by looking up. Uranus and Neptune were discovered by analysing the movement of the 'known' planets; noting that there were small discrepancies in their orbits; guessing that these were caused by unknown planets rather than the theory not working; using the laws to predict where such a planet might be; and finally pointing a good telescope at that bit of sky and lo! there's a new planet. Uranus was found, I think, straight away after the predicted orbit. Neptune took a little longer. Still the orbits suggested another planet. It took 30 years of searching before Pluto was found and it doesn't 100% account for the discrepancy so there's still a chance of another large object (it has been decided that Pluto isn't big enough to 'earn' the title planet so it's been downgraded to a minor planet.

Maths is used all the time to make models of the real world. The shape of graphs can be 'explained' by the calculus model.  The rules of arithmetic provide a model for keeping track of how many sheep you've got and how much is in your bank account.  Arithmetic doesn't work in some cases though.  1 + 1 = 1 when you're talking about piles of sand!

I spent a term learning the axioms for projective geometry and all the theorems. The lecturer said it was a closed theory as we had covered everything that can be deduced from those axioms.  I was very disappointed to hear that and occasionally think I ought to re-visit the theory to see if I think he was right.

When Einstein came up with his theory it showed that Newton is only an approximation but relativity isn't consistent with quantum mechanics so the search is on for a theory that fits both.

Some mathematical models may have no obvious application.  Binary arithmetic was a curiosity for many years.  But it is essential in the operation of computers; suddenly an abstract theory blossoms into life as being really useful.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

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## #5 2023-05-17 10:32:24

paulb203
Member
Registered: 2023-02-24
Posts: 241

### Re: Newton's 3rd Law of Motion

Thanks, Bob

“...places on Earth where the gravitational pull is more or less; eg up a mountain.”
Because we’re further away from Earth’s centre?

“I don't see what elasticity has got to do with it. Do you have a link to the vid?”

Here you go, Bob. https://www.youtube.com/watch?v=57gKnA94Doc  The video doesn’t mention elasticity, it’s someone in the comments. In the first handful of comments 2 people say the Normal force is not an example of Newton’s 3rd Law, and 1 person says it’s a type of elastic force.

“I think they are both N 3rd L.”

Thanks. I watched another video and found out that the Normal force doesn’t only apply to objects on the ground, or on horizontal surfaces, it can apply to, for example, me standing upright pushing on a door.

“They're good enough to land a man on the Moon...”

Did they use stuff like F=ma, and W=mg, etc?

“...Pluto isn't big enough to 'earn' the title planet so it's been downgraded to a minor planet.”

I like telling people it’s a dwarf planet, because they discovered dwarves on it.

“Arithmetic doesn't work in some cases though.  1 + 1 = 1 when you're talking about piles of sand!”

1 pile of sand plus another pile of sand = 2 piles of sand, no? Or do you mean 1 pile piled onto another?

“When Einstein came up with his theory it showed that Newton is only an approximation but relativity isn't consistent with quantum mechanics so the search is on for a theory that fits both.”

So relativity (general relativity) isn’t quite ‘right’ either, similar to Newton’s laws? I’ve heard often that quantum mechanics is spot on, the most accurate theory ever, in terms of predictions, something like that?

“Binary arithmetic was a curiosity for many years.  But it is essential in the operation of computers; suddenly an abstract theory blossoms into life as being really useful.”
Base 2? Ones and zeroes?

Last edited by paulb203 (2023-05-17 10:35:28)

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## #6 2023-05-17 10:36:57

paulb203
Member
Registered: 2023-02-24
Posts: 241

### Re: Newton's 3rd Law of Motion

Here's the comments on the video;

The normal force and its relation to weight, has nothing to do with Newton's third law. It is a coincidence that it is equal and opposite, but it is not the third law counterpart to weight. Its existence has a lot more to do with Newton's second law, as a constraint force to stop the object from accelerating through the surface beneath it. The third law counterpart to your weight, is the weight of Earth in your own gravitational field. The fact that gravity is a two way street, where both the small and large object pull on each other with equal and opposite forces.
*
Careful with invoking 3rd law and saying the normal force is the reaction force to gravity. The 3rd law reaction force to the earth pulling on a mass is the mass pulling on the earth. The normal force is often equal to gravity when a=0 because of Newton's 2nd law
*
Normal force is fun fact a type of elastic force. Normal force is the opposite of pressure force(the F in pressure equation, p=F/A), they are both caused by objects don’t elastic deformation, which our eyes can’t see. So pressure force is caused by your contacting area doing elastic deformation ever so slightly which caused an elastic force and normal force is caused by the other contacting object like the wall doing elastic deformation ever so slightly which also caused an elastic force. If you are wondering, elastic deformation is the cause of elastic force, it is basically a change in an object’s shape caused by a constant force, but will turn back into how it was with the force is not present, example of that is a spring or a rubber band. And elastic force is just a force caused by that. However it also can be explained in energy, it is caused by elastic potential energy turning into kinetic energy.

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## #7 2023-05-17 19:58:39

Bob
Registered: 2010-06-20
Posts: 10,503

### Re: Newton's 3rd Law of Motion

hi paulb203

Thanks for the video link.  I've watched it in full.  I like the explanations from this guy; clear, simple language and nice animations to assist.  Nothing about elasticity from him.  The commenting guy who brings this up is talking rubbish.  Just shows you have to be careful believing everything you read on the internet.

I get why some are wanting to invoke the 2nd law.  Normally we would only apply law 2 to accelerating objects but it still works if they're not.  Law 1 says an object carries on with a uniform velocity unless acted upon by a force.  If an object isn't moving then the forces on it must be in equilibrium. This means that if you resolve the forces in two perpendicular directions the sum of forces in each of these directions must be zero.

So if a mass is resting on a horizontal surface its weight (the downwards acting force) must be balanced by an upwards acting force (the normal reaction).  Newton's laws are a 'package'; by that I mean you can explain what's happening with all 3.  There's no acceleration so the forces must balance (law 1).  F = ma with a = 0. (law 2).  Normal reaction (law3).

note: 'normal' here doesn't mean "standard; usual, typical, or expected" but rather " at right angles to a given line or surface".

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

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## #8 2023-06-06 06:52:43

paulb203
Member
Registered: 2023-02-24
Posts: 241

Thanks, Bob.

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## #9 2023-07-22 05:52:51

Novice
Registered: 2023-07-22
Posts: 6

### Re: Newton's 3rd Law of Motion

When it comes to the interaction between a person and an inanimate object like a door or a car, there is indeed a fundamental difference in the nature of the push. Let's delve into the physics behind this intriguing phenomenon.

Q. Is the kind of push 'done' by the door/car/etc, a different kind of push to the one a person does?

A. Yes, the push exerted by an inanimate object like a door or a car is fundamentally different from the push carried out by a person. When a person pushes an object, they apply a force through their muscles, creating a mechanical interaction that causes the object to move or respond. The person's action involves a transfer of energy, resulting in a change in both the person and the object's state.

On the other hand, when the door or the car "pushes" back against the person, it is not actively exerting a force. Instead, it is merely responding to the external force applied to it. This is known as Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. So, when a person pushes the door, the door pushes back with an equal force, causing the person to feel the effect of the push.

Q. And if a force is an interaction between objects involving a push or a pull, often with one or more of the objects being inanimate, might it be more helpful to think of the push and pull in terms of repulsion/attraction, given the implications of human pushing and pulling?

A. Considering the implications of human pushing and pulling, it may indeed be more helpful to think of forces in terms of repulsion and attraction. When a person pushes an object, they are applying a force that overcomes the repulsion between the atoms or molecules in the object, allowing it to move. Similarly, when a person pulls an object, they are creating an attraction between the object and themselves, causing it to move toward them.

Thinking of forces in terms of repulsion and attraction can provide a clearer understanding of how objects interact with each other. It also highlights the underlying electromagnetic interactions at the atomic level that govern these forces.

Q. After I push the door, I am changed, for example, out of breath, an ounce or two lighter, etc; is the door any different than it was, having pushed me?

A. After a person pushes the door, they experience changes in their body, such as being out of breath or expending energy. These changes occur because the person is an active and dynamic entity, and pushing the door requires effort and exertion from their end.

On the other hand, the door itself does not undergo any significant changes as a result of pushing the person. The door remains an inanimate object, and its state remains largely unchanged after exerting the equal and opposite reaction force in response to the person's push.

In conclusion, the kind of push exerted by an inanimate object like a door or a car is different from the push carried out by a person. While a person actively applies a force through muscular effort, the object responds with an equal and opposite reaction force as per Newton's third law. Considering forces in terms of repulsion and attraction can enhance our understanding of how objects interact, particularly when it involves inanimate entities responding to external forces. Moreover, after pushing an object, a person experiences changes due to their dynamic nature, whereas the object itself remains relatively unchanged. The interplay of forces and reactions provides fascinating insights into the intricacies of physics and the world around us.