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/sci/ - Science & Math

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File: 379 KB, 1230x447, frictionquestion.jpg [View same] [iqdb] [saucenao] [google]
6505304 No.6505304[DELETED]  [Reply] [Original]

Dear /sci/ I'm making a game and therefore writing some physics. I never had any formal physics education but I've tried to get into
classical mechanics reading books and articles. I have this problem with trying to account for my forces regarding friction.

My physics book states that the kinetic friction of a moving object on a flat surface is the coefficent of kinetic friction times the normal force.
This causes some strange behaviour for me and really don't sit well with my intuition, in my thinking the contact area per unity time must grow larger as velocity increases,
What am I missing /sci/ like pic related describes, are the force of friction on the block really the same in both cases?

>> No.6505309

Depends on how far it moves

>> No.6505311

>>6505309
Let's say the velocity is sustained, how many newton is either experiencing?

>> No.6505313

yes, the force of friction is the same regardless of speed within a single experimental system

now, you can alter the system to change friction - for example, if you're lubricating the block moving over the surface and the heat from friction boils away the lubricant, then you're going to experience more friction.

it's more likely that you're running into some weird edge case relating to how your physics engine handles friction.

>> No.6505315

energy, or work is force times distance. so friction is lost work, or lost energy. if and object is moving faster it is losing energy faster therefore it takes more power (energy per time) to move a fast object than a slow object with the same coefficient of friction.

>> No.6505322
File: 16 KB, 221x267, concussion[1].jpg [View same] [iqdb] [saucenao] [google]
6505322

>>6505315
>energy, or work
lolwut

you really shouldn't be trying to explain physics if you're going to fuck up basic terminology like that

>> No.6505323

>>6505322

please kill yourself in shame

>> No.6505332

>>6505322
I have no problem with the fact that you're wrong. My problem is that you're a dick.

>> No.6505337

>>6505323
>>6505332
energy is needed to perform work, but energy is very definitely not the same thing as work. anon asserted they were the same thing, which is very wrong.

also, it takes the same amount of energy to overcome friction an object no matter how fast it's going.

>> No.6505342

>>6505304
What strange behaviour are you seeing?

It's probably easier to just start there

>> No.6505343

>>6505337

>maximum autism

are you trying to rustle jimmies here? work and energy are the same thing in this context. And even if it's not technically correct according to standard model of quantum mechanics it works pretty damn well so it's not "very wrong" as according to you.

>> No.6505353

>>6505343
>work and energy are the same thing in this context.
no, they're not, just by the very basic definition of the term. yes, there is a direct proportional relationship between them, but they are not the same thing

op was asking a basic physics question - it's important to be precise with terminology when teaching

>> No.6505360

>>6505353

please, explain how they are very different

>> No.6505371

>>6505360
You lose a lot of energy by friction, and it does not contribute to work, at all.

>> No.6505373

>>6505353
In all fairness, I haven't yet found a single undergraduate textbook or professor that didn't royally fuck up not only terminology but often equations and signs in this area of basic physics.

in many cases if you keep track of what the book is saying well enough, you'll find contradictions.
Textbooks use those terms interchangeably, and sometimes they even inadvertently switch "doing work" with "having work done on it" so that everything has positive signs


Just saying, you can't really get on someone's case when the way it's taught is almost universally shitty.
As a TA sometimes I'm even afraid to correct all of those errors and misconceptions because it might actually just do more damage to them when the exam comes

>> No.6505374

>>6505371
even though it's pointless or wasted, work is still being done

>> No.6505375

>>6505371

wtf? The friction is doing work.

>> No.6505378

>>6505342
I have a block with a mass of 100kg, to this block I have attached a rocket engine that accelerate it with a constant force of 500 newton.
The block and the ground have a coefficent of kinetic friction set to 0.5 and gravity of the world is 9.8 newton.
so it takes 490 newton to overcome the friction once it starts moving leaving a force of 10 newton that slowly keep accelerating
the block at increasing velocity for miles before my drag equation produces the 10 newton of force needed to stop the acceleration.
It just seemed so utterly weird to me, that I felt compelled I must have gotten it wrong, and that it should grind to a halt in acceleration and
keep it's velocity a lot earlier due to increase in friction.

>> No.6505379

OP why don't you ignore the pointless argument and tell us what's wrong with your program and what you'd like it to do.

Anyway if I were you I'd just find a way to make the physics do what you want. Who cares if it's unrealistic.

>> No.6505380

>>6505378
What's weird about it? Aerial drag increases with speed, but solid-solid friction doesn't.

>> No.6505381

ITT: people try to apply a basic undergrad friction equation to all systems

this is almost as bad as calling all conductors ohmic. I am ashamed, /sci/

>> No.6505382

>>6505381
What do you propose?

>> No.6505386

>>6505378

Your rocket should be producing a constant power, not a constant force

>> No.6505387

>this thread
i know /sci/ isn't very smart and are very much full of themselves but fucking hell come on this is some highschool physics shit

>> No.6505395

>>6505386
this is a good point,
I second this

>> No.6505399

>>6505379
>Who cares if it's unrealistic.
Ultimately this is of course true for a game, but physics of the real world is a good place to start to get things to feel convincing.
There's also a general curiosity with the subject I suppose, There's an elegant beauty to how real physics operate.

>>6505380
How long it kept accelerating, I would never expect that if I could do the experiment in reality.
I felt something had to be wrong with it, like the coefficent of friction somehow altering depending on the velocity.

>>6505386
So my friction is correct but my engine is too naive?

>> No.6505401

>>6505382
not being high schoolers :^ )

linear relationships are usually very approximate, and as such care needs to be taken when you say LOL IT'S A LINE

>> No.6505405

>>6505401
Again, what do you propose? You don't sound nearly as smart as you probably think you are.

>> No.6505407

>>6505401

Jesus Christ what faglord

>> No.6505410

>>6505380
because the drag force is dependant on the air density (which increases as it's compressed) and its not like you can just run into something faster and harder and not expect it to push you back more. this is different than the block because you aren't applying any extra force for the friction to counter. making it heavier is about the only way to change the frictional force without lubing it or something

>> No.6505411
File: 38 KB, 508x595, 1346015013205.jpg [View same] [iqdb] [saucenao] [google]
6505411

>>6505304
>>6505337

Qualitatively, friction is a constant deceleration of a moving object. If anything, it seems more intuitive to think of it like that than as a decreasing force because of this:

If friction decreased as the object got slower, it would become harder and harder to slow it down the slower it got. The brakes would work better and better the faster the object is moving, but hitting the brakes on a slow-moving object would hardly make a difference because the friction isn't there. That's not how friction works, though; it brings things to a complete stop.

It's just that when you have an object moving fast enough, it seems to decelerate faster because of the effects of its kinetic energy being dispersed.

KE = (1/2)mv^2

for m = 2, KE = v^2

When this object slows down from 1 to 0, its kinetic energy goes from 1 to 0, a loss of 1 joule. When it slows from 2 to 1, its kinetic energy goes from 4 to 1, a loss of 3 joules. As the object gets faster, more and more energy must be expended to slow it down, and that energy manifests as, say, the flare of a meteor. Even though the deceleration is the same, the energy expended on a faster object is larger. This gives the illusion that the frictional force is getting stronger, when, in fact, it's just expending more energy at a time since the object is covering more distance at a time.

>> No.6505417

>>6505410

the air density doesn't noticeably begin to change until your start approaching the speed of sound, i don't think OP is trying to model a supersonic system here

>> No.6505421

>>6505417
either way the third law still holds

>> No.6505424

>>6505411
Thank you Mr. Obama, that actually made me a bit wiser.

>>6505417
>i don't think OP is trying to model a supersonic system here
Correct, I realize the 1000km/h block is approaching the speed of sound but I made no considerations for the compression of the air
It was only meant as a stark illustrative example for the contact forces.

>> No.6505427

>>6505417
what i mean to say was that the pressure changes a lot and thats a major cause of drag, not density

>> No.6505442

>>6505304
Why not using a physics engine?

>> No.6505525

>>6505371
work can both be positive and negative

E=+-work*distance

>> No.6505545

>>6505410
doesnt an increase in velocity contribute to an increase in mass?

>> No.6505566

>>6505545
technically yes, but the mass increase is so negligible we gain no useful information at this scale by including it

>> No.6505607

>>6505343
>work and energy are the same thing in this context
>in this context

work is NEVER the same as energy, you stupid faggot.

its not autism, its science, it doesnt work if you fucking handwave and say make up magic words.

>> No.6505630
File: 31 KB, 576x765, Mount Stupid.gif [View same] [iqdb] [saucenao] [google]
6505630

>>6505607

Please just stop. You are on Mount Stupid.

>> No.6505635

Generally physics in games are nothing like real physics and are done by feel and intuition. You can use any function (analytic or otherwise) for anything you want

>> No.6505643

>>6505411
Aren't there also systems where friction becomes negligible at high speeds?

>> No.6505644

>>6505371

I think you mean gravity/normal force, dude. Friction does negative work.

>> No.6505653

>>6505304

If you're just talking about the approximate formula, then no, friction has no dependence on speed once the object is moving. However, more accurate descriptions of friction actually state that the force decreases to some extent as velocity increases. The reason for this is that the molecular surfaces of the object and the floor or whatever don't have time to form weak intermolecular bonds. That's why the static coefficient of friction is always larger than the kinetic coefficient.

>> No.6505663

>>6505643
Space.

>> No.6505688

>>6505304
Fr=uR
It seems as though Friction is independent of velocity.
Viscous drag isn't though.

>> No.6505715

>>6505643
Coloumb friction doesn't rely on velocity.
If you go fast enough it will be negligible