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# Using your common sense to solve a physics problem.

Author Topic:   Using your common sense to solve a physics problem.
Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 46 of 188 (144324) 09-24-2004 12:03 AM Reply to: Message 41 by riVeRraT09-23-2004 11:40 PM

 the rat writes:I would like the mass of the car...

Ok, if you must know, the mass of the car is 1 gram.

 A variable could be the condition of the road on that day. Dry and wet are not the only conditions. Dew points and oil build up before a rain can affect traction conditions.

Irrelevant, because the dry or wet condition or whatever else that could affect the friction is taken into account as the friction coefficient, which I have included as the constant Uk.

 The condition of the shocks and springs in the car can greatly affect how long it takes for a car to stop, as well as air pressure in the tires. Temperature of the tires at the time of lock-up can change a tires Uk. These are common sense things.

Well, your common sense is a little off in this case. None of those things have any significant affect on the way the problem is set up.

Remember that we are assuming that as soon as the driver stepped on the break, the wheels immediately stopped rotating.

The Laminator

B ULLS HIT

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 41 by riVeRraT, posted 09-23-2004 11:40 PM riVeRraT has responded

 Replies to this message: Message 55 by riVeRraT, posted 09-24-2004 2:36 AM coffee_addict has not yet responded

Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 59 of 188 (144370) 09-24-2004 3:38 AM Reply to: Message 52 by riVeRraT09-24-2004 2:14 AM

Well, perhaps my common sense sucks, but I just don't see how you could come up with that. It's wrong, anyway. Just use part 2 and put everything together. That's what I originally intended for you to do. I never expected you to know how to do this. It's like asking me to write a report on the stock market... wouldn't even know where to begin. The point is to see if you could put all the equations together using your common sense... and a calculator.

The Laminator

B ULLS HIT

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 52 by riVeRraT, posted 09-24-2004 2:14 AM riVeRraT has responded

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Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 69 of 188 (144419) 09-24-2004 11:27 AM Reply to: Message 62 by Ooook!09-24-2004 5:39 AM

Re: Another example of common sense...
Go ahead and show your work if you want. We're done as far as I'm concern.

Oh yeah, and don't feel bad about it. I'd take a whole hour too if I were doing something in you field. We all have aour specialties

This message has been edited by Lam, 09-24-2004 10:29 AM

The Laminator

B ULLS HIT

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 62 by Ooook!, posted 09-24-2004 5:39 AM Ooook! has responded

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Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 77 of 188 (144469) 09-24-2004 3:23 PM Reply to: Message 70 by Ooook!09-24-2004 12:46 PM

Re: Ok here goes
Not bad for a jerk scientist.

Yes, if the driver had 30 meters to stop and he still hit the car in front of him, then he must have been going at at least 32.8 mph.

You could work the other way too. If you calculate using the speed limit (25 mph), you could calculate the distance at witch it would have taken him to stop.

v2 / 2*a = x where x is the distance that would have taken him to stop.

If we plug 25 mph (which is 11.2 meter/second), it would have taken him only about 14 meters to stop.

What's the moral of the story? No matter what, there will always be people like me that can calculate the speed that you were traveling. Can't argue your way out of something like this.

The Laminator

B ULLS HIT

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 70 by Ooook!, posted 09-24-2004 12:46 PM Ooook! has not yet responded

Member (Idle past 10 days)
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 Message 119 of 188 (144859) 09-26-2004 3:47 PM Reply to: Message 114 by NosyNed09-25-2004 12:11 PM

 Ned writes:There is something that I have seen expressed here, on other boards and by religious friends that you might want to consider. The concern expressed is the enormous damage that fundamentalist literalism can do to faith. You might want to ask some of those here who were once devote but lost that, in part, because of the literalist nature of what they had been taught.Many religous people see that teaching that, if you don't believe in a literal bible, then you can't believe in Jesus and God is very wrong. If this is really believed by an intelligent person then eventually there is a great danger that they will find out the truth about the world around us and in doing so lose their faith.

I disagree because I am an atheist now.

I was one that grew up with the constant "goddunit" explanation for every question that I asked. Everything was magical, and everything was god's plan.

Now that I think about it, I am very thankful that I was brought up like that. It made me realized just how rediculous religion is. It made me... me. I am no longer bound by the bible.

Even though I am a little annoyed with the rat's anti-science and homophobic attitude, part of me is glad that people like him exist. Otherwise, how can we show the brainwashed what a really really brainwashed look like? Since the really really brainwashed are a minority group, we can use them to save the brainwashed, which are the majority of all the brainwashed.

Make sense?

The Laminator

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 114 by NosyNed, posted 09-25-2004 12:11 PM NosyNed has not yet responded

Member (Idle past 10 days)
Posts: 3627
From: Chicago
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 Message 121 of 188 (144960) 09-27-2004 12:58 AM

I was just talking to a friend... not really a friend but more like someone that I just met in the hall way... about physics and stuff. I had a lot of free time, so we talked for quite some time. It turned out that he didn't really know that there are physics problems that deal with stuff much more complicated than the ones we see in kinematics. He didn't really know that there are problems such as the ones dealing with electromagnetism where it's entirely calculus based. With kinematics, you can pretty much get away with only basic knowledge of algebra. However, it gets a lot more complicated than that in higher disciplines of physics.

This got me thinking. If the rat (please don't take offense in me calling you the rat, I tend to do that with people... like the frog) thought that common sense alone could give you enough skill to solve such a problem as a car breaking to stop, it crossed my mind that may be there are members on this board that are still under the impression that physics is only about solving algebraic equations and dealing with everyday stuff.

So, here is a slightly more difficult problem just so you wouldn't think that physicists are fruitcakes. No, I don't really expect any non-physicist to be able to solve this. It's relatively simple, but it's quite beyond everyday common sense, I'd say.

Find the electric field a distance z above one end of a straight line segment of length L, which carries a uniform line charge of λ. What happens if z is approaching infinity?

Here is the equation necessary for this problem.

E = 1 / (4 * pi * ε) ∫ [(λ r')/R2] * R^ dl'

where E = elctric field, ε = permitivity constant, r' = field point, R = distance between a source charge point on the line segment and the field point r', and R^ is the direction of the magnitude.

Here is a picture to make it easier.

As I know it, there are 2 ways to do this. One of them will take you 2 pages to do, and the other will take you about 4 or 5 steps. Not supprisingly, I still have trouble when doing the long way because it's too complicated.

Have fun.

The Laminator

For goodness's sake, please vote Democrat this November!

 Replies to this message: Message 123 by riVeRraT, posted 09-27-2004 3:58 PM coffee_addict has responded Message 124 by riVeRraT, posted 09-27-2004 4:01 PM coffee_addict has not yet responded

Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 127 of 188 (145156) 09-27-2004 7:21 PM Reply to: Message 123 by riVeRraT09-27-2004 3:58 PM

 Lam writes:Find the electric field a distance z above one end of a straight line segment of length L, which carries a uniform line charge of λ. What happens if z is approaching infinity?

In other words, lambda in this case is the uniform line charge.

 the rat writes:Tell me, do you think just because you can figure this out on paper, means that you can see whats going on? You might, but what about the next guy?

Yes, I do really see what's going on. One of the most wildly misconceptions regarding physics is that physicists can't really understand what the equations are saying. That is not true.

The average person might not know what the equations are saying, but if you are studying physics you better know damn well what the equations are saying.

 Isn't there a big difference between being taught formulas, and trying to figure them out on your own?

Actually, I did figure out that equation on my own. The only reason I posted it here is because I didn't expect non-physicists to be able to. It's like me writing a paper on economics... I wouldn't know where to begin.

Again, if you are studying physics and you can't derive this equation on your own, then you have a problem.

This message has been edited by Lam, 09-27-2004 06:22 PM

The Laminator

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 123 by riVeRraT, posted 09-27-2004 3:58 PM riVeRraT has responded

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Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 129 of 188 (145161) 09-27-2004 7:48 PM Reply to: Message 128 by tsig09-27-2004 7:38 PM

Re: Nonresponse
 Observer writes:RiverratYou did not respond to the content of my post.

I'm going to say something for the rat, if I may. I think the rat was talking about egoistic engineers who think that they know everything better than the mechanics who usually deal with these things hands on. Although I am not a big fan of the rat (since he hates people like me), I think that there is some merit to the rat's point.

I've met some freshed out of college know-it-all engineers, and one of them was my brother. However, I've also encountered some know-it-all mechanics in my lifetime *ahem*.

The Laminator

For goodness's sake, please vote Democrat this November!

 This message is a reply to: Message 128 by tsig, posted 09-27-2004 7:38 PM tsig has responded

 Replies to this message: Message 130 by tsig, posted 09-27-2004 9:39 PM coffee_addict has not yet responded Message 134 by riVeRraT, posted 09-28-2004 9:07 AM coffee_addict has not yet responded

Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 144 of 188 (145454) 09-28-2004 6:38 PM

By the way people, I have no intention of expecting anyone to actually solve the problem of electric field. You can have fun trying to solve it if you want. If someone feels strongly about it and must see the answer, tell me and I'll post it. Well, have fun.

Again, please don't feel bad if you can't solve it. There's nothing wrong with that. It's like me trying to write an econ paper or doing a geology homework.

Actually, one time I took a world literature class that was intended for English majors. It took me a week of misery and lots of tears to write a paper about a poem that our prof wanted us to analyze. As far as I know, it only took everybody else like a day or so and their papers were considerably better than mine (I was kinda embarrassed because we exchanged papers around for peer evaluation). After that class, I swore never to put myself through something like that again.

This message has been edited by Lam, 09-28-2004 05:44 PM

For goodness's sake, please vote Democrat this November!

Why? Bush is a right wing nutcase.

Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 146 of 188 (145472) 09-28-2004 7:20 PM Reply to: Message 145 by riVeRraT09-28-2004 7:18 PM

Re: Nonresponse
 the rat writes:So tell me, I live on a mountain where the slope is about 30degrees. How long would it take water to run off? The hieght is about 500ft above everything else around it. The base of it takes up about 400 acres. It would seem impossible that it would ever flood right?Well it does.

I think everybody has realized this part of your "theory." However, what people have a problem with is the world wide flood continuing on for months, which is an impossibility as was proven by Rrhain.

For goodness's sake, please vote Democrat this November!

Why? Bush is a right wing nutcase.

 This message is a reply to: Message 145 by riVeRraT, posted 09-28-2004 7:18 PM riVeRraT has responded

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Member (Idle past 10 days)
Posts: 3627
From: Chicago
Joined: 03-29-2004

 Message 186 of 188 (146354) 09-30-2004 9:39 PM Reply to: Message 185 by riVeRraT09-30-2004 9:36 PM

Re: Nonresponse
Actually, yes. Don't forget about our conversation regarding homosexuality where you tried to used every trick you could make the bible do to satisfy your bigoted view of... me. Now, you're trying to tell us that you actually don't believe that much in the bible, or at least take it literally?

This message has been edited by Lam, 09-30-2004 08:41 PM

For goodness's sake, please vote Democrat this November!

Why? Bush is a right wing nutcase.

 This message is a reply to: Message 185 by riVeRraT, posted 09-30-2004 9:36 PM riVeRraT has not yet responded

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