A question about opposite and equal reactions

[Dale]

I still don't understand. Momentum is basically force.

No, they have different units.

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.

 

[Newtype]

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.

 

[Doc Al]

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.

 

[Dale]

It doesn't contradict my other statement

It does contradct it. If two things have different units they cannot be the same.

 

[Gokul43201]

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?

 

[vlado_skopsko]

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.

 

[cjl]

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.

 

[Newtype]

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.

 

[Doc Al]

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.

 

[Doc Al]

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?

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.

 

[MagnetDave]

I hereby invoke the Moon Principle:

https://www.physicsforums.com/showthread.php?t=182413&page=9

It is no longer possible to explain this any more clearly.

 

[Doc Al]

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.

 

[Archosaur]

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.

 

[Dale]

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.

 

[Newtype]

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.

 

[cjl]

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.

 

[Doc Al]

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.

 

[Dale]

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.

 

[Newtype]

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?

 

[pallidin]

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.

 

[cjl]

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.

Yep - the force of the glass on the brick would be the same as the force of the brick on the glass (and both would be equal to the breaking strength of the glass, since the glass broke).

 

[Doc Al]

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?

Yes.

I'd call that impossible.

Why?

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?

There was only bouncing, no hovering. All rather silly. (There was a still photograph of someone in mid-air, though. )

Nonetheless, as palladin points out, it's easy to fake such things.

 

[sophiecentaur]

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.

We are confusing Force with Impulse (change of Momentum).
There is no reason at all why the two forces should be the same. Your force on the brick can last much longer than the force of the glass on the brick. What could be the same would be the force times the time for which the force was acting. In any case, the brick through window scenario is not a good one to discuss because it does not specify at what speed the brick emerges on the other side. It will probably still have some momentum left so there's no answer yet.

 

[sophiecentaur]

That Yogic thing. The equal and opposite forces when the guy is in the air are 1. the force pulling HIM down and 2. the force pulling THE EARTH up. Gravity is supplying the 'piece of string' between them.

 

[Doc Al]

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.

Perhaps there's some confusion as to what forces you are talking about. Are you, as sophiecentaur suggests, comparing these two forces:
(1) the force that your hand exerted on the brick when you threw it with
(2) the force that the brick exerts on the glass when they collide

Those forces are not connected by Newton's 3rd law (they are are not 'equal and opposite reactions') and have no simple relationship to each other.

The Newton's 3rd law pairs would be:
For (1): The force that your hand exerts on the brick is equal and opposite to the force that the brick exerts on your hand.
For (2): The force that the brick exerts on the glass is equal and opposite to the force that the glass exerts on the brick.

 

[sophiecentaur]

Yes yes yes

 

[vin300]

Your force on the brick can last much longer than the force of the glass on the brick. What could be the same would be the force times the time for which the force was acting. .

The first part isn't right, the force on the glass can exist only as long as that on the brick, because only an action can cause a reaction.From what follows,the second statement is true.

 

[sophiecentaur]

I wasn't, perhaps, clear enough. By "your force", I was referring to the force you exerted on the brick and not 'yer force', in the Irish sense. If I had meant the force from the glass on the brick, then I would have said it was equal to the force of the brick on the glass.
But my intro emphasised the difference between force and impulse - so you could have inferred what I really meant.

(Edit) When you throw a brick, the brick isn't making contact with the glass at same time so the forces can be different.

 

[Newtype]

And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?

 

[Archosaur]

And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?

A force pair formed at the point of contact between "What the f%$k" and "are you talking about?!"

Yogic flying?!
The Earth stopping?!

When will this thread die?

Also, what newspaper do you get?

 

[Doc Al]

And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?

Don't believe everything you read in the tabloids.

 

[Newtype]

CNN covered the story of how scientists detected how Earth stopped spinning during the recent Chilean earthquake (it was for 1.26 microseconds).
http://www.google.com/search?hl=en&...ed+rotating+Earth&aq=f&aqi=&aql=&oq=&gs_rfai=

 

[Archosaur]

That is not at all what that article says.

 

[cjl]

CNN covered the story of how scientists detected how Earth stopped spinning during the recent Chilean earthquake (it was for 1.26 microseconds).
http://www.google.com/search?hl=en&...ed+rotating+Earth&aq=f&aqi=&aql=&oq=&gs_rfai=

The earthquake didn't stop the Earth for 1.26 microseconds and then start it again. That would be impossible, and it would also cause incredible devastation if it were possible. What it did is it changed the rotation rate of earth, so days are (permanently) 1.26 microseconds shorter than they were before. This didn't require any force, since the Earth's angular momentum didn't change. What did change was the Earth's mass distribution, which changed the moment of inertia.

Oh, and linking a google search on a topic does NOT count as evidence for it, especially when none of the top search results actually support what you are saying. Link directly to the page making the claims please.

 

[Doc Al]

And here's another question:

Let's stick to your 'brick through the window' example, which has the benefit of being real. Please respond to my questions in post #95.