A question about opposite and equal reactions

[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?

 

[Archosaur]

When will this thread die?

Good question, me-from-5-months-ago.

In fact, this question is even more pertinent today, considering that this mediocre thread has been sporadically active for... roughly 9% of my life.

*Sigh*... see you all in another 5 months.

 

[DaveC426913]

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?

You are being far too literal, and you are assuming the laws are applied literally too.

We know the context in which the laws hold, and we know that they do not hold everywhere. You don't need to go to the centre of a BH to find that.

Two objects moving toward each other do not simply add their velocities, they use relativistic velocity addition, even at human speeds. However, the effect is so vanishingly small that, unless we are dealing with objects moving at near relativistic velocities (or time spans where it's relevant, such as GPS navigation), we do not have to factor it in.

It does not mean we just throw the laws out the window.

 

[Bloodthunder]

A Scientific Law is sort of defined to be an observation that will always occur when under specific conditions. So... we don't throw laws out of the window, we have a new set of them to use under different conditions.

 

[Libohove90]

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).

No, momentum is not force. The rate of change of momentum is force.

Suppose you had an object that is 10 kg and moving at 10 m/s.

It's momentum is (10 kg)(10 m/s) = 100 kg m/s.

Force is defined as the rate of change of momentum. This moving object is not accelerating, it's just moving at a constant velocity in a straight line. So what's its force?

F = ma = (10 kg)(0 m/s^2) = 0 N.

This object exerts no force, but has momentum of 100 kg m/s. Momentum and force are completely different.

 

[sophiecentaur]

Some of you guys really crack me up. You seem to argue against some of the most fundamental ideas in basic Physics when you just haven't done your homework. Go back to basics and learn them. Only then should you feel qualified to 'question' stuff. This isn't a subjective subject. It's hard and as objective as it can possibly be.