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Author Topic:   Using your common sense to solve a physics problem.
DrJones*
Member
Posts: 2284
From: Edmonton, Alberta, Canada
Joined: 08-19-2004
Member Rating: 6.8


Message 76 of 188 (144468)
09-24-2004 3:23 PM
Reply to: Message 64 by riVeRraT
09-24-2004 8:52 AM


You mean I will role down a hill at the same speed regrdless of the slope? you obviously didn't understand how I was applying the grade to the formula
See you're mixing things up here. Gravity will always apply a force on an object that is directly straight down. The object will apply forces to the surface that its on. The direction and magnitude of these forces will depend on the angle of the surface. It's vector math. These resultant forces will control your acceleration down the hill.
And V=(2Uk*g*d) is correct.
No its not. The units do not match up, it cant be correct. Its also not the formula you originally gave.
g=m/s2
d=m
so 2Uk*g*d=(unitless)(m/s2)(m)=(unitless)(m2/s2)
m2/s2 is not the unit that we measure velocity with. Your formula is absolutely not right.
This message has been edited by DrJones*, 09-24-2004 02:52 PM

*not an actual doctor

This message is a reply to:
 Message 64 by riVeRraT, posted 09-24-2004 8:52 AM riVeRraT has replied

Replies to this message:
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coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
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!

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riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 78 of 188 (144504)
09-24-2004 4:59 PM
Reply to: Message 75 by Melchior
09-24-2004 3:17 PM


Yes I must have expressed it wrong. But through common sense I realize what the slope is doing to the length of the skid marks.
I guess the slope alters the Uk?
Do I subtract from the Uk, or change the rate of gravity to figure out the length of the skid on the slope?

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Replies to this message:
 Message 79 by DrJones*, posted 09-24-2004 5:04 PM riVeRraT has replied
 Message 84 by Percy, posted 09-24-2004 5:32 PM riVeRraT has replied

DrJones*
Member
Posts: 2284
From: Edmonton, Alberta, Canada
Joined: 08-19-2004
Member Rating: 6.8


Message 79 of 188 (144507)
09-24-2004 5:04 PM
Reply to: Message 78 by riVeRraT
09-24-2004 4:59 PM


to figure out the length of the skid on the slope?
The length of the skid is given as 30m.
I guess the slope alters the Uk
The Uk is a constant 0.45.
change the rate of gravity
You can't change the rate of gravity. Gravity is a constant 9.81 m/s2 here on earth.
edited to add: I'll type out a complete step by step solution tonight after I see Shaun of the Dead.
This message has been edited by DrJones*, 09-24-2004 04:07 PM

*not an actual doctor

This message is a reply to:
 Message 78 by riVeRraT, posted 09-24-2004 4:59 PM riVeRraT has replied

Replies to this message:
 Message 90 by riVeRraT, posted 09-24-2004 9:12 PM DrJones* has replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 80 of 188 (144518)
09-24-2004 5:20 PM
Reply to: Message 71 by Rei
09-24-2004 1:21 PM


Do you honestly think that if we built buildings without structural analyses, that they wouldn't be collapsing all over the place?
Of course they would, we have learned everything the hard way. That is where I get my common sense from.
I think your story proves my point. The engineer can miss something, where in that situation, if I was on the job, I might have caught it. I would have just looked at it, and said, "this ain't gonna work". Thats due to the common sense, or natural understand of pyhsics that God gave me, plus years of experience.
when I say jerk scientist (and I never said jerk physists, or engineer) I mean the ones that tell us how we should live based on some survey. You know the surveys that change everyday on the news.
I mean if your not a jerk scientist, you should know the difference if your not, why get upset at what I said? You know dam well there are jerk scientists out there. everyone has admitted that.
Its only because I'm Christian, that you choose to target me.
The fact that I can even approach to solve these problems on paper, and I was a high school drop out, should tell you something.
I am more for figuring things out in the real world. Not to say I do not appreciate the math involved.
I am really thankful for Lam challenging my brain, I enjoyed it tremendously. I look to understand it further, on paper. I also will say again, I do respect the scientist/engineers/pyhsists who aren't jerks, and you know who you are.

This message is a reply to:
 Message 71 by Rei, posted 09-24-2004 1:21 PM Rei has replied

Replies to this message:
 Message 81 by Rei, posted 09-24-2004 5:24 PM riVeRraT has replied
 Message 82 by nator, posted 09-24-2004 5:28 PM riVeRraT has replied

Rei
Member (Idle past 7013 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 81 of 188 (144521)
09-24-2004 5:24 PM
Reply to: Message 80 by riVeRraT
09-24-2004 5:20 PM


quote:
I think your story proves my point. The engineer can miss something, where in that situation, if I was on the job, I might have caught it.
But... the engineer *DID* get it right, and *YOU* got it wrong. The engineers conducted a structural analysis and determined how much the nut could hold. *They were right*. You, however, just used your common sense, figured that the structure would be safe given the modification, and completely failed. Your concept that there'd only be the extra stress of a few inches of pipe and an extra nut was *wrong*. If this had been your job, you would have been responsible for the deaths of these people.
This message has been edited by Rei, 09-24-2004 04:24 PM

"Illuminant light,
illuminate me."

This message is a reply to:
 Message 80 by riVeRraT, posted 09-24-2004 5:20 PM riVeRraT has replied

Replies to this message:
 Message 91 by riVeRraT, posted 09-24-2004 9:31 PM Rei has replied

nator
Member (Idle past 2170 days)
Posts: 12961
From: Ann Arbor
Joined: 12-09-2001


Message 82 of 188 (144524)
09-24-2004 5:28 PM
Reply to: Message 80 by riVeRraT
09-24-2004 5:20 PM


quote:
I mean the ones that tell us how we should live based on some survey. You know the surveys that change everyday on the news.
The scientists aren't telling you anything, rat.
The NEWS REPORTERS are telling you, usually really poorly, the results of some study.
If you decide to change your behavior based upon hearing a soundbite you heard on the evening news, that's your problem.
Don't blame the scientists for poor science reporting.
Please also do not accuse scientists of telling you how to live. All the scientists do is give the results of a study. That's all.
It is usually people in the medical profession (who are NOT scientists) who take those findings and reccommend changes or whatever.

This message is a reply to:
 Message 80 by riVeRraT, posted 09-24-2004 5:20 PM riVeRraT has replied

Replies to this message:
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riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 83 of 188 (144526)
09-24-2004 5:30 PM
Reply to: Message 76 by DrJones*
09-24-2004 3:23 PM


Sorry I left out the squared the second time.
I think Idid write it wrong, but figured it correctly.
g=32.2, without the squared. I put the squared outside the brackets to clear things up for me.
The answer is correct for level ground?

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Replies to this message:
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Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.3


Message 84 of 188 (144528)
09-24-2004 5:32 PM
Reply to: Message 78 by riVeRraT
09-24-2004 4:59 PM


riVeRraT writes:
Do I subtract from the Uk, or change the rate of gravity to figure out the length of the skid on the slope?
Once you've been provided the data and the equations, the rest is just common sense. There's more than one way to approach the solution, but they're all just common sense.
I can't tell if you're expressing things in a funny way or if you're misunderstanding something, so I'll just say this:
  • There's a component of the force of gravity parallel to the slope pulling the car down the slope.
  • There's a force resisting the car's progress down the slope that is equal to the kinetic coeeficient of friction multiplied by the component of the force of gravity that is normal to the slope.
  • The difference between these two forces is the net amount of force on the car. If the net stopping force is larger, then the car will decellerate, while if the falling down the slope force is larger then the car will accelerate.
  • If you solve the problem symbolically before plugging in the values, you'll find that mass drops out and isn't a factor.
That last point is counterintuitive, isn't it? That's why this problem is such a wonderful example of the insufficiency of common sense for much of problem solving. You've discovered that your common sense has failed you, and that your lack of knowledge has become a significant obstacle. You've been reduced to slogging your way through what is actually an extremely simple problem in mechanics. And you can't even cite your lack of college training on this one, because this is basic high school physics.
You've obviously learned a lot and come a long way on just your own inner resources, and you deserve lots of credit for that. But human intelligence is often given far more credit than it deserves. We're definitely the smartest creature on the planet, but a lot of our intellectual advantage derives from the base of knowledge we've accumulated over the last 10,000 years. Take away our ability for each generation to pass its knowledge on to the next and our big advantage diminishes almost completely. That's why for most of human existence it has been bare subsistence.
So while your accomlishments deserve praise, keep in mind that as much as Newton you are standing on the shoulders of those who came before.
--Percy
This message has been edited by Percy, 09-24-2004 04:35 PM

This message is a reply to:
 Message 78 by riVeRraT, posted 09-24-2004 4:59 PM riVeRraT has replied

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 Message 93 by riVeRraT, posted 09-24-2004 9:40 PM Percy has not replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 85 of 188 (144529)
09-24-2004 5:32 PM
Reply to: Message 82 by nator
09-24-2004 5:28 PM


What about discovery science channel, whats your opinion of that channel. I hear a lot of things on there that I don't agree with.
I mean these are the only things that people like me who only casually obcerve science through TV, and popular science, and discover mag, are going to give us information.
All these things are bad?

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 Message 82 by nator, posted 09-24-2004 5:28 PM nator has replied

Replies to this message:
 Message 87 by NosyNed, posted 09-24-2004 8:02 PM riVeRraT has replied
 Message 88 by nator, posted 09-24-2004 8:07 PM riVeRraT has replied

Melchior
Inactive Member


Message 86 of 188 (144542)
09-24-2004 7:02 PM
Reply to: Message 83 by riVeRraT
09-24-2004 5:30 PM


The answer for flat ground, Uk=0.45, d=30 meters is;
v^2 = 2*Uk*g*d = 264.6 m^s/s^2
v = 16.267 m/s
16.267 m/s = 16.267 / 1609 * 3600 mph = 36.396 mph
Given that there are 1609 meters in a mile, and 3600 seconds in an hour.

This message is a reply to:
 Message 83 by riVeRraT, posted 09-24-2004 5:30 PM riVeRraT has not replied

NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 87 of 188 (144548)
09-24-2004 8:02 PM
Reply to: Message 85 by riVeRraT
09-24-2004 5:32 PM


Good and bad
The various popular science magazines, TV shows and the like aren't all bad or good. It depends on who is doing it, the topic and what audience they aim for.
Generally, there is a big problem with complex issues that are at the leading edge. The producers have to aim at a middle of the road audience with a limited attention span. How well they simplify varies a lot. Somethings when simplified aren't really right any more. How close they can get to conveying the right message is tricky.
It also depends on what you bring to it. If you have an extensive back ground you might get more of the "truth" out of a show or be able to see where they have over simplified. An example might be the global warming issue.
There might be an argument presented between two scientists with differing views. One might say "we have shown that global warming is partially caused by man". The other might be shown saying "it isn't proven".
This may appear to be a big difference when they might, if brought together, end up saying the same thing.
"we have shown" might be short hand for "the correlation between what we observe and the expected effects of the measured CO2 increases make it relatively unlikely that the change is a part of natural variation"
"not proved" might mean "the direct linkage to carbon emissions and precise changes in climate have not been shown. We still don't have all the information on the carbon cycle as it is currently running."
With the above translations both claims could be right. However, you'd have to know something that the show isn't going to have time to air and know a lot about the nature of scientific thinking before you could suss that out.
Without the background you'd just come away thinking that there is a total disagreement with only a black and white decision to be made. This would be terribly misleading and you would have no real idea of what is actually going on.
This is one reason why popularizations can be iffy in complex areas.
Add to that the need to jazz up a show by making it appear that there is controversy when there might not be and the rush to air that some shows must do to keep costs down and there is risk in taking them as your source of information.
The magazines that offer a chance for questions and disagreement to be published as letters to the editor seem to me to be more trustworthy. If you follow a topic from initial publication through a round of leter writing you get a bit better an idea of just how well accepted an idea is.
I like "New Scientist" myself. It isn't at too high a level but not as low a level as "Scientific American" as settled to. Discovery seems to be ok but I don't read it all that often. It is a bit lighter as well.
A better option is to get more than on book out of the library on a topic. If real experts in the field right them for a popular audience you get a much more detailed insight. Then read any differing views you can find. By the time you've done that you probably have as good a grasp as you're going to get.
The only real source is primary literature or books written that intend to be at that level. They don't all have to be unreadable. (It just seems that way )
With the resources here (at evcforum) you can probably get reasonable views on anything you see on Discovery channel or read somewhere. The real scientists here do have some expertise and insight that should be helpful.

This message is a reply to:
 Message 85 by riVeRraT, posted 09-24-2004 5:32 PM riVeRraT has replied

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nator
Member (Idle past 2170 days)
Posts: 12961
From: Ann Arbor
Joined: 12-09-2001


Message 88 of 188 (144549)
09-24-2004 8:07 PM
Reply to: Message 85 by riVeRraT
09-24-2004 5:32 PM


quote:
What about discovery science channel, whats your opinion of that channel.
The Discovery Channel isn't very good.
You'd be better served by watching National Geographic programs, special presentations, and episodes of NOVA on PBS.
quote:
I hear a lot of things on there that I don't agree with.
I mean these are the only things that people like me who only casually obcerve science through TV, and popular science, and discover mag, are going to give us information.
You could take a basic biology or geology class at a local community college if you wanted to fill in the holes in your knowledge and get a good basic education in these subjects.
You could also do worse than reading through TalkOrigins.
This message has been edited by schrafinator, 09-24-2004 07:09 PM

This message is a reply to:
 Message 85 by riVeRraT, posted 09-24-2004 5:32 PM riVeRraT has replied

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Rei
Member (Idle past 7013 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 89 of 188 (144550)
09-24-2004 8:32 PM
Reply to: Message 88 by nator
09-24-2004 8:07 PM


Probably the worst thing about channels like the discovery channel is sins of omission.
For example, they had a show about Nefertiti, in which they postulated that Nefertiti was in fact the pharaoh Smenkhkare. They present a lot of the very real, very compelling evidence that Nefertiti was Smenkhkare. However, they omitted the information that argues against it - for example, the fact that Smenkhkare married one of Nefertiti's daughters (Ankhesenpaaten), nor discussed that something that is possibly Smenkhkare's body already exists.
I mean, they don't always omit stuff; unfortunately, it happens too often to be a consistant, reliable source of information. The Nefertiti show, for example, was a lot more entertaining the way they presented it as a dedicated archaeologist, who through her efforts, conclusively overturned long-held views about one of Egypt's pharaohs. Presenting the nuance that is found in the real world isn't as entertaining, however.
This message has been edited by Rei, 09-24-2004 07:33 PM

"Illuminant light,
illuminate me."

This message is a reply to:
 Message 88 by nator, posted 09-24-2004 8:07 PM nator has not replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 90 of 188 (144553)
09-24-2004 9:12 PM
Reply to: Message 79 by DrJones*
09-24-2004 5:04 PM


You can't change the rate of gravity. Gravity is a constant 9.81 m/s2 here on earth.
I know that.
I'm saying gravity is straight down. They use the perpendicular pull on gravity relative the surface of the earth to figure the force on the car. If gravity is not exactly perpendicular, its affect is different.
You could take the angle of the slope and figure it in, or like I did, use a percentage to figure it in(I think).

This message is a reply to:
 Message 79 by DrJones*, posted 09-24-2004 5:04 PM DrJones* has replied

Replies to this message:
 Message 98 by DrJones*, posted 09-24-2004 11:09 PM riVeRraT has replied

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