Physics Student Struggling to Explain Conservation of Linear Momentum

[ShayanJ]

Sometimes people ask «what is the physical reason for...»
You know,for example when they read about the increase of mass due to motion in SR they come and ask how it happens or why it happens and when I tell them its because of conservation of linear momentum,they say no I want a physical reason.
You know what I mean?
I,as a physics student,can understand that mathematical reasons are convincing enough but can't explain it to others
What would you do?

 

[Simon Bridge]

If you have answered in terms of physical principles or laws of nature then you have provided the "physical reason" and it's not your fault they don't realize that.

Note: for your example, the physical reason is that the moving object still gains kinetic energy. Mass does not actually increase.

But I do know what you mean - merely being right does not help you be convincing. You need to ask them what sort of explanation they are looking for. Can they give you an example?

Often they want one in terms of forces, or in terms of objects striking or pushing on each other. They are just negotiating the language for the communication.

Sometimes they want to know in terms of "purpose" - you get that a lot arguing with creationists for example. These are "why" questions and science does not do those. When they hear that they think they've caught you out in some way.

 

[ShayanJ]

I know what they want
They think that all physics can be explained in sentences and without equations and when I tell that this mathematical statement implies such a behavior,they say "no,I want the physical reason"
I think they want to know sth that we may call an underlying theory
sth like maxwell's equations with which you can justify the empirical laws like snell's law
In that case I simply say"no one knows why that happens"
Like when they ask "how time dilation happens?"

 

[Simon Bridge]

Time dilation is a consequence of time being another dimension of space... it only looks strange because we are not equipped to measure things that accurately. It's geometry.

"How" is a question you can usually answer.

The trick is usually to get the level the answer is needed for ... and if they only want words it does get hard to do the more subtle ideas justice.

Snells law - you can go for the "least time" explanation at several levels ... most people are happy with the life-guard on the beach having to get to a drowner in the water in the shortest time coupled with light just having to be the fastest thing around.

"least time" is a key idea that will work well for deeper descriptions.

Of course there is always the Discordian answer: "it's because of the banana ..."

 

[lisab]

I know what they want
They think that all physics can be explained in sentences and without equations and when I tell that this mathematical statement implies such a behavior,they say "no,I want the physical reason"
I think they want to know sth that we may call an underlying theory
sth like maxwell's equations with which you can justify the empirical laws like snell's law
In that case I simply say"no one knows why that happens"
Like when they ask "how time dilation happens?"

Years ago I took a horrible class - "Philosophy of Science". Nearly all the students in the class were philosophy majors, specializing in epistemology. I could not have cared less as they debated why gravity exists...they would just go on and on :zzz:. What a waste of time, IMO.

Science is good at explaining "how". Leave the "why" to philosophers.

 

[wuliheron]

I point what they are really asking for are metaphysical explanations which physics cannot provide. If they want some sort of ultimate physical explanation for gravity or life, the universe, and everything they need to see a mystic, priest, or medium.

 

[dydxforsn]

I know what they want
They think that all physics can be explained in sentences and without equations and when I tell that this mathematical statement implies such a behavior,they say "no,I want the physical reason"
I think they want to know sth that we may call an underlying theory
sth like maxwell's equations with which you can justify the empirical laws like snell's law
In that case I simply say"no one knows why that happens"
Like when they ask "how time dilation happens?"

You can explain why time dilation happens by Einstein's two postulates of special relativity and a light clock in a moving car as viewed by one in the car versus an observer on the sidewalk. These people are looking for these kind of explanations. Usually specific simple ("real life") situations that convey the salient points.

Years ago I took a horrible class - "Philosophy of Science". Nearly all the students in the class were philosophy majors, specializing in epistemology. I could not have cared less as they debated why gravity exists...they would just go on and on . What a waste of time, IMO.

lol, that sounds incredibly painful..

 

[BobG]

Sometimes people ask «what is the physical reason for...»
You know,for example when they read about the increase of mass due to motion in SR they come and ask how it happens or why it happens and when I tell them its because of conservation of linear momentum,they say no I want a physical reason.
You know what I mean?
I,as a physics student,can understand that mathematical reasons are convincing enough but can't explain it to others
What would you do?

I understand what they mean.

For example: Why does the Earth's rotation rate gradually slow down and the Moon gradually move further away from the Earth?

Conservation of energy really isn't any kind of answer. If the Earth's rotation rate stayed constant and the Moon's distance from the Earth stayed constant, you'd still have conservation of energy. If the Earth's rotation sped up and the Moon's distance decreased, you'd still have conservation of energy.

Without explaining the tides (both the high tide closest to the Moon and the high tide opposite the Moon), you really haven't explained anything by just saying you have conservation of energy.

 

[ShayanJ]

No BobG,what your telling is an incomplete explanation which obviously is the explainer's fault

 

[Evo]

No BobG,what your telling is an incomplete explanation which obviously is the explainer's fault

I have absolutely no idea what you are talking about.

 

[ShayanJ]

I have absolutely no idea what you are talking about.

BobG mentioned that conservation of energy alone doesn't imply the gradual slow down of the Earth's rotation
So if someone says that "earth's rotation slows down gradually because of conservation of energy"he is missing something and the explanation is incomplete,whether he's explaining to another physicist or a layman.
If you read my posts,you understand that's not what I mean