A question about opposite and equal reactions

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Newton's third law of motion states that for every action, there is an equal and opposite reaction, which applies to interactions between objects. When a rock is thrown at a wall, it bounces back due to the equal and opposite force exerted by the wall. However, when the rock hits a window, the window may not exert enough force to reverse the rock's momentum, resulting in the rock breaking through instead. The forces are still equal and opposite, but the material properties of the window limit its ability to respond like a wall. Understanding these interactions clarifies that Newton's law remains valid across different scenarios, despite variations in material resilience.
  • #61


vin300 said:
Let's think. The moment the mass is thrown, it encounters gas molecules all over its front hemisphere(if it is spherical) and each of it exerts a force equivalent to what it experiences according to its distance from the centre of the hemispherical surface, in exactly the opposite direction so the forces cancel out and there is no net force so no motion possible anywhere in the atmosphere and all gravity has no result except the experience of weight the motion of anything that comes in contact with anything else is retarded light does not propogate neither does sound.No electricity.Even inertia means absolute opposite force by the molecules in the system itself.
Comically incorrect. :rolleyes: The equal and opposite forces involved in Newton's third law never cancel out--they act on different bodies.
Does this new theory based on Newton's third law look good? It violates his other laws too, because there is no force.
Completely futile work,definitely.
Futile? Yes. Based on Newton's third law? Not even close.
The actuality is this:
Some of the energy is imparted to the air molecules so they are set in motion and the energy of the mass decreases by a negligible amount.If there was exactly equal and opposite reaction, the scenario doesn't quite fit to observe the conventional action-reaction.
It's true that energy is transferred from the mass to the surrounding air, but that says nothing about how Newton's third law operates.
 
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  • #62


Doc Al said:
Comically incorrect. :rolleyes: The equal and opposite forces involved in Newton's third law never cancel out--they act on different bodies.

Futile? Yes. Based on Newton's third law? Not even close.

It's true that energy is transferred from the mass to the surrounding air, but that says nothing about how Newton's third law operates.
That still means the mass exerts force on an envelope of air ahead of it, and the latter exerts an equal amount of force on the mass, which can be lesser not to nullify the motion of the body only if the force by the body was less, which is contradictory.
 
  • #63


vin300 said:
That still means the mass exerts force on an envelope of air ahead of it, and the latter exerts an equal amount of force on the mass,
That's true.
which can be lesser not to nullify the motion of the body only if the force by the body was less, which is contradictory.
That's gibberish.
 
  • #64


what is being discussed by vin300 and Newtype has been moving further and further away from force reactions and resultants and closer and closer to energy and momentum. I think both of you need to do some reading up on conservation of momentum (mass multiplied by velocity) and conservation of energy.
 
  • #65


Doc Al said:
That's true.

That's gibberish.
If I say the body exerts 5N force on the envelope of air ahead of it you agree that the envelope of air exerts an equal 5N force on the body exactly in the opposite direction. Be it the envelope of air or a denser wall, the force exerted by the body on any of them and the reaction force remain the same, still the body is reflected by the wall and not by air simply because the air is free to move and the wall isn't.
If Newton's third law is to be obeyed, where is the result of the 5N force exerted by the air on the body?
If you say that energy is lost due to air resistance that is not the satisfactory explanation of the law.
 
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  • #66


vin300 said:
If I say the body exerts 5N force on the envelope of air ahead of it you agree that the envelope of air exerts an equal 5N force on the body exactly in the opposite direction.
True.
Be it the envelope of air or a denser wall, the force exerted by the body on any of them and the reaction force remain the same, still the body is reflected by the wall and not by air simply because the air is free to move and the wall isn't.
No. The body is reflected by the wall because the wall exerts a greater force on the body than does the air.
If Newton's third law is to be obeyed, where is the result of the 5N force exerted by the air on the body?
The result of the 5N force exerted by the air on the body is given by Newton's 2nd law. That force accelerates the body, slowing it down.
If you say that energy is lost due to air resistance that is not the satisfactory explanation of the law.
That has nothing to do with Newton's 3rd law.
 
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  • #67


Doc Al said:
True.

No. The body is reflected by the wall because the wall exerts a greater force on the body than does the air.
So the wall exerts a greater force than the air while the action forces are the same. Isn't that a violation of Newton's third law?

The result of the 5N force exerted by the air on the body is given by Newton's 2nd law.

That's kiddish.
 
  • #68


vin300 said:
So the wall exerts a greater force than the air while the action forces are the same.
What do you mean by "the action forces are the same"? When the body hits the wall, the body and wall exert equal and opposite forces on each other. Same deal when the body hits the air. Of course, the forces involved are different in each case.
Isn't that a violation of Newton's third law?
No.

As russ_watters emphasized in post #53, you really need to go back to the points I raised in post #42. You have some basic misconceptions about motion and forces.
 
  • #69


Doc Al said:
What do you mean by "the action forces are the same"? When the body hits the wall, the body and wall exert equal and opposite forces on each other. Same deal when the body hits the air. Of course, the forces involved are different in each case.

No.

As russ_watters emphasized in post #53, you really need to go back to the points I raised in post #42. You have some basic misconceptions about motion and forces.

I got it.when the body hits the wall it loses momentum in very less impact time to the wall so the force is too much, and the wall reacts with equal force. When it hits a gas molecule only little of the momentum is imparted to it and considering the impact time, the force to the molecule is much lesser and it reacts only with this lesser force.
I had worked on this several times before, but every next time I make mistakes and I need to do it again. That's ridiculous!
Could you get some help for my "stationary shifts"problem?
 
  • #70


I still don't understand. Momentum is basically force. Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).
 
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  • #71


Newtype said:
I still don't understand. Momentum is basically force.
No, they have different units.


Newtype said:
Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).
Yes, this is correct and contradicts your other statement.
 
  • #72


It doesn't contradict my other statement because I've been saying throughout this topic that momentum (force) of a glass window is insufficient to stop a brick thrown at it.

Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.
 
  • #73


Newtype said:
It doesn't contradict my other statement because I've been saying throughout this topic that momentum (force) of a glass window is insufficient to stop a brick thrown at it.
As already pointed out, you are confusing momentum and force. Nonetheless, perhaps you mean something like: "If you throw a brick through a window, the force that the window exerts on the brick was insufficient to stop the brick." Well, that's true. So what? It's still true that the force that the window exerted on the brick was "equal and opposite" to the force that the brick exerted on the window.

Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.
I think part of your confusion is your use of the term 'reaction'--it doesn't mean what you think it means. "Equal and opposite reaction" is an old-fashioned term that means in physics "equal and opposite force". If I jab you in the arm (or eye!) with a nail, the nail and your arm (or eye!) exert equal and opposite forces on each other. Of course the consequences of that equal force are vastly different; that same force destroys your eye while the nail is unperturbed. Same thing with the brick and the window. The window smashes while the brick is unscathed--nonetheless they exerted equal and opposite forces on each other.
 
  • #74


Newtype said:
It doesn't contradict my other statement
It does contradct it. If two things have different units they cannot be the same.
 
  • #75


Newtype said:
Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.
Are you being serious with this example?
 
  • #76


The wall and the glass are made from different materials and I guess have different thickness so they can exert some maximal pressure before they brake. If that force is not enough to stop the brick, to bring the bricks momentum to zero ( in perfect inelastic case ), then the stone will continue to travel with the rest of his momentum whatever it is.

So we know from the material the obstacle is made, what maximum force it can oppose to an external pressure ( because very important is the area on which its acted but if you use the same brick we will suppose that is the same area and it will cancel. ).
For example the maximum force the glass can exert is 10 N for that area of the brick. If you throw the brick with some force, you give some momentum on that brick, the glass does not care how much, when there is contact between them, the glass can oppose ( it can feel ) only that much (10 N ) so the stone will feel 10 N, so the momentum of the stone will change. If its enough to stop the brick or it will continue to travel with that smaller momentum, depends on the previous momentum of the brick.

Same for the wall.

The guys before me did excellent job trying to explain this, so i don't know if this will help but I but i tried.
 
  • #77


vin300 said:
So the wall exerts a greater force than the air while the action forces are the same. Isn't that a violation of Newton's third law?

This is the problem. The action forces are not the same. The force the ball exerts on the air is much, much smaller than the force the ball exerts on the wall. The force is not an intrinsic property of the ball's motion - it is a property of each interaction, and the force will be different depending on what the ball is hitting.

If you throw a ball, and it hits a pillow, the force on the ball will be lower than the force on the ball if it hits a brick wall. If you throw the ball and it hits a piece of glass, the glass will push back on the ball just as hard as the ball pushes on the glass. The glass may break, because the glass is more fragile, but that doesn't negate Newton's 3rd law. It just means that the glass took less force to break than would be required to stop the ball.
 
  • #78


Gokul43201 said:
Are you being serious with this example?

Yes. Is there any way to physically measure a glass window exerting 10 N on a brick thrown at it and exerting 10 N on it? If so, then how?

And here's another example: a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1, but I'm guessing scale2 can't because that boulder smashes scale2 to pieces.
 
  • #79


Newtype said:
And here's another example: a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1, but I'm guessing scale2 can't because that boulder smashes scale2 to pieces.
And what does that have to do with anything? Just because you've managed to smash the scale to pieces says nothing about the fact that whatever hit the scale and scale itself exerted equal and opposite forces on each other.

You're still confusing the exertion of a force with the effect of the force.
 
  • #80


Newtype said:
Then clarify the distinction between an exertion of a force and effect of a force.
You seem puzzled by the fact that a brick smashing through a plate of glass can exert equal and opposite forces on each other. I can only imagine it's because the same force can have very different effects on different objects. A force X might smash the glass to bits, but the same force X might just barely slow the brick down.

And where's the opposite and equal reactions during Yogic Flying?

:smile: OK, now we know you're joking. At least I hope so! All forces involved in "Yogic Flying", a.k.a. bouncing on the ground, comply with Newton's 3rd law. That video is complete crackpottery.
 
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  • #82


Jasongreat said:
A pane of glass can only exert so much force on a brick before it breaks, but the brick does not have to exert its full force on the glass to break it. If a brick has 10N of force, and the glass can only withstand 2N of force, before breaking, the brick can only exert 2N of the 10N of force it carries on the glass, the floor exerts the other 8N of force on the brick to stop it when it lands. Is this correct? I am trying to understand this myself. Thanks.
No, not really correct. Don't think of a brick as 'having force'. A moving brick doesn't 'have force', but it does have mass and momentum. Only when the brick interacts with something else are forces involved. And depending on what it interacts with will determine the size of those forces. For example, if the brick hits a wad of cotton, the forces produced will be small. But if it the brick hits a solid wall, then the forces will be much greater.
 
  • #83


Newtype said:
And where's the opposite and equal reactions during Yogic Flying?


The action of their knees on the ground is opposed by the action of the ground on their knees. If it weren't, their knees would go through the ground.

Also, this is ridiculous. It's upsetting that National Geographic even entertains this. Show me that they're accelerating at anything less than g, and then you've got something, but I don't pay my cable bill just so I can watch people in their pajamas jump around on their knees.

If you want to learn about force pairs, fine, but bringing stuff like this into the thread is a great way to get it locked.
 
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  • #84


Newtype said:
a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1
Exactly correct. The two scales will measure the same force (obviously assuming that the forces are in the range that the scales can measure). This is what the 3rd law means.
 
  • #85


DaleSpam said:
Exactly correct. The two scales will measure the same force (obviously assuming that the forces are in the range that the scales can measure). This is what the 3rd law means.

You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.
 
  • #86


Assuming the scales are identical, they'll either both get crunched, or both not get crunched. What makes you think the scale attached to the rock wouldn't get crunched? It has the rock on top of it just as much as the scale attached to the ground does.
 
  • #87


Newtype said:
You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.
What happens to each scale depends on the net force on it, among other things. Nonetheless, the forces two objects exert on each other are equal and opposite.
 
  • #88


Newtype said:
You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.
Many variations of this test have been performed. Your theory is wrong. Scale 1 will measure the same as scale 2.
 
  • #89


So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible.

And getting back to that Yogic Flying video, what's the opposite and equal reaction of that guy who is simply hovering in midair with his legs crossed?
 
  • #90


Newtype said:
And getting back to that Yogic Flying video, what's the opposite and equal reaction of that guy who is simply hovering in midair with his legs crossed?

It's called an iron-type rod poised secretly under the garments.
No magic at all, just deception.
 

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