Using your common sense to solve a physics problem.

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

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The Laminator
B ULLS HITFor goodness's sake, please vote Democrat this November!

Well it was 11 years ago. Thats NYC for ya.I have a few architects in my family and one civil engineer, I should have been one of them. I still might one day, officially.Unofficially, I own my own HVAC business, and have to engineer HVAC systems from blueprints. I caculate heat loads, and losses, and size the systems. Then I size the duct work base on a given pressure, and calculate CFM requirements.
I also design radiant heat systems, amd commercial refrigeration systems. So I deal with the coorilation between tempurature, and presure and the latent heat of evoporation on a daily bases.
I also trouble shoot all the electrics that control all this stuff.Then in my spare time I am an ameteur radio operator, and take pictures through my 8" telescope, and build and fly r/c planes.DEspite applying science in my life to all these things, I still found God

Yes I agree with all that.
But without having gone to college, I can look at what happened in the car crash, and I would be able to come up with all that on my own. It would take a long time, but I can do it.I am not so sure that even the most learned engineer, or scientist would be able to do that on his own, with out going to college.. That is my point.

Well, at some point, one of them had to, because how else would we be teaching it in college?Moreover, they realized that it's a huge fucking waste of time to re-invent the wheel every time you want to figure out a problem of kinematics. As far as I'm concerned, that realization makes them way smarter than you.

It would less carrying capcity. I am talking about the carrying capcity of the completed project.The difference would be A-B=C
A being the original carrying capacity
B being the extra wieght of the extra nut and inch or 2 of rod involved.
C being the carrying capacity.The upper rod estends below the upper catwalk, and the rod connecting the upper cat, and lower cat is sticking up an inch or 2, thereby adding wieght to the catwalk, which decreases its carrying capacity.
Plus the extra nut (and washer, you do use washers don't you?).How'd I do teach?

Well there are multiple solutions to this problem depending on the type of Aluminium and what I-beam design you use. The first combinination that worked for me (and I'll admit that I might be off as I'm a mechanical engineer not a civil and I haven't done this type of problem since my second year, which was long long ago):Aluminium 1100-H14
I Beam W310x143This might not be the best or most economical combo, it was just the first one that worked for me. And once again I add the disclaimer that I am not a civil engineer so I fully accept that I might have screwed up somewhere.But I dont think the purpose of this thread is to be a dick-measuring contest (or the appropriate female situtation).And once again you do not require the mass of the car to solve the initial problem. What Lam supplied in steps 1, 2 and 3 is more than enough. Start to set up the problem and you should see why the mass of the car is irrelevant.This message has been edited by DrJones*, 09-24-2004 02:39 AM

Ok, using common sense skills only here is where I am at, and why I need the mass of the car to figure it out.For some of you, you were taught how to apply the coeffecient of kinetic friction to this. I didn't even know what that word meant. I knew what it was, I just didn't know what it was called. I also understand the difference between kinetic friction and static friction.I am going to ignore part 2 for the time being, and just figure out Part 1 using common sense.First I would need to figure out the force on the car by adding the skid with the grade of the hill.So I figure it like this. If at 0 degrees(or level) gravity has an effect of 9.8 m/s^2 Then 90 dgrees would increase it by 100%.The angle of which the car was sliding on was 13.875% increase on the effect of gravity and the Uk.So changing your formula around a little I came up with v=(2Uk*G*D)squared. Where D = the distance skidded. I converted the whole thing to miles, and feet, because Thats what I grew up with. V= the velocity of the car.So the effect of gravity is now 36.67m/s^2, the distance is 98.425 feet, and the Uk is .45So I get 56.992 MPH was the speed lost in the skid. Add that to the force of the impact between the 2 cars, and you'll get the total speed of the car.Crap, I hope this is right.

1 is not a good number to work with for common sense reasons.
Also pounds would be better. But I think I got it anyway.

Its enough if you know what your doing. If you just walk up to the seen and try to figure it out using common sense, I would need the wieght of the car, and some kind of reference for the Uk.
Using common sense, I would test the tires to see first hand how far they would skid.I only asked you that question out of curiousity, because I had a Dock building business. I took an average pontoon boat, and made a crane on the end of it out of wood. I used common sense to design it, and it did exactly what I thought it would do. It lifted a 4000 lb dock out of the water and I could drive it up on land (the dock) and set it down, So I was storing docks for the winter. I added floatation underneath the boat to compensate for the extra wieght, and the leverage factor of the dock being 4ft out in front of the boat.
I also added a water ballast system to the back, by caculating the wieght of water, and how much I would need to balance it.
Then I took a common boat lift motor and rigged it to operate the crane.
Worked quite well, and was the talk of the lake here. But the dock business as a whole sucks, so I gave it up. MY HVAC business is doing to well, and it makes more money.
At one point I wanted to make the crane out of metal. Steel was too heavy for the boat (wieght was a big factor in designing this) So I asked around about aluminum, and what it could handle, because I didn't want to guess. I had figured in my head about a 10" beam, but I really didn't know. The numbers I gave you was a little high, I would have gone a little smaller. Each arm really only neede to carry 2,000lbs, as there were 2 arms off the front of the boat,9 feet apart.The cool thing about the boat, was that it still went 20knots, and I could respond to sunken boat emergencies on the lake. We raised several boats, and I devised many methods to get them out, using my common sense that God gave me.

I am not saying for this problem that any of those variables ar going to make a difference. I am saying in the real world, use your common sense, or just read what I wrote.

Wait!!!
I made a mistake. I thought about it as I was falling asleep. I added the percentage the wrong way. I added it as if the car was going up hill.The answer should be 49.8mph.
You see, I want to solve the problem my own way, and you confuse me, because you want me to solve it your way. so without knowing the exact wieght of the car, I couldn't figure this out.Anyway its late, I am dead, gn.

Percy writes:

quote:

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Perhaps you could start with a simpler question, like, "Is there sufficient information to solve the problem? Explain why or why not."

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This reminds me of a Foxtrot cartoon that I have since used as a training for new tech support staff. We've all seen this kind of word problem setup:A train leaves Station A at 10:00 am and arrives at Station B, 180 miles away, at 2:00 pm.Now the question I have is: What do we need to assume in order to determine the average speed of the train?You see, those of us who have done a slew of these problems will look at the setup, see the appropriate signals (180 miles, 4 hours) and jump to the perfectly sensible answer (divide the miles by the hours and come up with 45 mph.)But what do you do when the person on the other end of the phone says, "It still doesn't work"? The most obvious solution is quite often correct, but every now and then you find you're ignoring something that hasn't been directly stated:Just because the stations are 180 miles apart doesn't mean the track between them is 180 miles long. Perhaps the track is a Euclidean straight line that tunnels through the earth and thus ignores the curvature of the earth. Perhaps the track takes the scenic route and zig-zags back and forth along the way. Perhaps the train went the other way, a journey of about 23,820 miles. And what if it took the scenic route on that journey, too?Are the stations in the same time zone? For that matter, it was never explicitly stated that the train arrived the same day. We're assuming the clocks are working. It might always be "2:00 pm" at Station B. When we said "10:00 am" and "2:00 pm," did that mean clocks at the station or clocks we're carrying with us? If we're carrying the clocks, shouldn't we take relativistic effects into account?See, this is why anecdote is not evidence. The seemingly simplest of questions require tremendous amounts of control in order to make sure that we have even a hope of saying we understand what's going on. The idea that a person can "common sense" his way through something as complicated as the diversification of life is ridiculous. You can't "common sense" your way through something as simplistic as "How fast was the car moving?"As someone more erudite once said, "Common sense is neither common nor sensible."

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Rrhain
WWJD? JWRTFM!

Nope. Acceleration due to gravity is a constant 9.81 m/s² here on earth no matter what angle the object is at. Check the dimensions (ie units of measurement) used in your formula:U_k is dimensionless
G your acceleration due to gravity is measured in m/s²
D is distance measure in m
v is velocity measured in m/sso inputing these units into your formula you getv=(2U_k*m/s²*m)²doing the math you get v=(2U_k m²/s²)²which works out to v=4U_k²(m⁴/s⁴)so your final velocity would have units of m⁴/s⁴ which just aint right.edited multiple times for spelling and making sure all the HTML tags are working.This message has been edited by DrJones*, 09-24-2004 02:45 AM

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*not an actual doctor

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.

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The Laminator
B ULLS HITFor goodness's sake, please vote Democrat this November!

How'd you do?Crash!You see, the people on-site thought they'd only be adding the weight of a bit more rod and an extra nut, too. However, if they had repeated the original structural design analysis, they would have discovered the fact that the nut supporting the upper catwalk has twice the force on it after the modification, and while you don't normally expect a nut to be a failure point, the tensile stress imposed was too much for it. See below:Original:
Modified:
Rethinking it, can you see what's going on? Originally, the *rod only* was bearing the weight of both catwalks, but after the modification, the *nut* that supports the upper catwalk *also* has to support the weight of the lower catwalk.Your "common sense" just killed 114 people. A structural analysis would have told you that the new design would put too much stress on the nut. You can't just ignore all of those tensile strengths and just build things based on looks - to know what materials can take what amounts of stress when forces combine in complex ways, you need to know your physics. Just because a nut looks like it might hold, doesn't mean it will. The only reason that the original design for the catwalk was (relatively) sound to begin with was because it *was* calculated (using physics), and that's how they knew what materials to build with, what diameter rods to use, etc.If you didn't do structrural analyses from the start, *every* building would be killing people because common sense fails way too often. You can't just "common sense" your way to the results of physics problems.That is why we find it annoying when you sit here and insult people who use science in their jobs, as if you somehow know better.

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"Illuminant light,
illuminate me."