A question about opposite and equal reactions

[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.

 

[DaS Energy]

Hello Newtype,

A point of rock throwing the equal opposite effect was in play.

At point of rock touching window equal opposite forces came into play.

The fact the rock did not bounce back but passed through did not take away the equal opposite force as the rock first contacted the glass

Cheers

Peter

 

[Dale]

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.

Have you any experimental evidence to support that assertion?

 

[Newtype]

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

Yes.

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.

So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and opposite reactions onto his hand enough to break it?

 

[Dale]

Don't be stupid. It's movie special effects, not experimental data.

 

[Newtype]

I'm not being stupid. I simply responded and then asked a question. Watch that movie Sidekicks if you don't believe me. In that movie, Jonathan Brandis uses his hand to karate break stacks of bricks.

 

[Dale]

You are being completely stupid if you think that everything which is depicted in a movie is real and scientifically plausible. Have you never watched "Mythbusters"?

Get some real data, not a movie reference.

 

[sophiecentaur]

Clearly the force that broke the bricks just wasn't enough to break his hand. There are no special dispensations for karate exponents to violate N3. You must look elsewhere for your explanation.

 

[Dale]

Here is a series of questions for you Newtype.

A 1 kg toy boat is floating in a still bucket of water.
1) How much force does the toy hull exert on the water?
2) How much force does the water exert on the hull?
3) How much did the water deform?
4) How much did the hull deform?

 

[Newtype]

Brandis's hand wasn't broken in that movie.

 

[AlephZero]

So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and
opposite reactions onto his hand enough to break it?

The reason is (and I'm not just being facetious) that bricks are hard lumps of baked clay with lots of tiny internal cracks in them, and hands are fairly soft squidgy thngs.

A large force applied for a very short amount of time is enough to join up the cracks inside a brick and split it into two. If you hadn't done any training beforehand, the same force applied to your hand for the same length of time would hurt and bruise you, but it wouldn't "break" anything.

It hurts a lot more if you pull the punch and don't break the brick, because then the force between your hand and the brick continues for a much longer time than if the brick "instantly" breaks.

The only "trick" involved in this is to convince your brain that you can actualy do it. That may take years of training, of course.

 

[sophiecentaur]

@newtype
You may find it more fruitful not to look for flaws in accepted Science before you have actually understood what it tells you. When you are a 'big boy' like Albert I or Isaac N you may be in a position to shake the foundations but Not Yet, I think.

 

[JaredJames]

Yes.

Then you are looking at and comparing the wrong forces. They are completely independent. This is where your misunderstanding stems from.

Your links so far are "Yogic Flying", "Earth stopping" and "movie science". Do you want people to take you seriously?

 

[Doc Al]

So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and opposite reactions onto his hand enough to break it?

Two points (for real life, not just movie effects):
(1) The force exerted by the bricks on the hand is exactly equal and opposite to the force the hand exerts on the bricks.
(2) Just because the force is equal doesn't mean the effect of the force is equal. (I've broken bricks and wood with my hands--my hands didn't break.)

 

[JaredJames]

I don't think there is any way to put this differently for the OP, but here goes:

You throw a brick with force X, the brick exerts is force back on your hand (this is one equal and opposite force pair).

The brick hits a window with force Y and the window exerts force Y back on the brick (this is another separate equal and opposite force pair). Because the window breaks, force Y = the maximum force the window could take.

Force X that the brick was thrown with does not have to equal force Y. It can equal it, such as when it hits a wall, but in the case of a window smashing X doesn't equal Y.

This is a key point that you are missing.

The force the brick imparts on the window does not have to be equal to the force it was thrown with. They are separate events and not connected with Newton's third law.

 

[satya98]

I find that, as a general rule of thumb, if you ever think one of Newtons laws is wrong, don't tell anyone.

Is it the 3rd law


What the laws which we declared as universal are not applicable at all the portions of the universe. With out knowing at least 1% about the universe, how we can declare a law as universal.

How many laws are applicable at the center of the black hole?