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Author Topic:   Using your common sense to solve a physics problem.
Rei
Member (Idle past 7013 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 16 of 188 (144181)
09-23-2004 5:40 PM
Reply to: Message 15 by Percy
09-23-2004 5:21 PM


Heh, I had to deal with rigid body kinematics recently. Here's a good paper that covers all of the angular momentum stuff:
Rigid Body Simulation I - Unconstrained Rigid Body Dynamics
The biggest question that I had when working (addressed by this paper) was as follows: Picture a long, lightweight bar with a small, dense, heavy weight on its center. If you push on the center, all of the work that you apply goes into accelerating the object forward. If you apply the force to the end of the rod, almost all goes into angular acceleration. However, if you apply the force in-between, how much goes to angular acceleration and how much to linear momentum? Also, given that 100% of the work goes into angular momentum if you push on the end, what happens if you push at both ends at once? It would seem like you would get two angular accelerations in opposite directions, they would cancel out, and there would be no momentum - but we know that it will instead gain linear momentum.
The error was based on a false assumption that I had, that seems so simple in retrospect: that the work is the same in all cases. It isn't; work is force times distance, and you push for a longer distance if you push along the end of the rod. It's not an issue of "some percent of work as linear force, and the rest as torque"; it is the full linear force plus the full torque, always.

"Illuminant light,
illuminate me."

This message is a reply to:
 Message 15 by Percy, posted 09-23-2004 5:21 PM Percy has not replied

Replies to this message:
 Message 18 by riVeRraT, posted 09-23-2004 5:53 PM Rei has replied

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


Message 17 of 188 (144184)
09-23-2004 5:48 PM
Reply to: Message 1 by coffee_addict
09-22-2004 2:25 AM


Before I start, or even think about this problem, which I have absolutly no experience in, and you can say this is for me, cause we all know it is. I don't have a problem with that.
Why haven't you given me the mass of the car?
For me to figure it with common sense I might need this?
After all you gave me F=m*a. I don't know why you would give me this unless your just trying to throw me off.
Before I start with the numbers, just glancing at it, I would take G and add the force of the incline to it, or possbly subtract it from the Uk. I will work the numbers, but I think I need the cars mass first. since F(grav)=m*g*sin(d) which would give me the force on the car plus the effect of the incline.
Just so I can figure it out in my mind. Don't give me any more clues, I want to figure this out on my own.

This message is a reply to:
 Message 1 by coffee_addict, posted 09-22-2004 2:25 AM coffee_addict has not replied

Replies to this message:
 Message 19 by DrJones*, posted 09-23-2004 6:00 PM riVeRraT has replied
 Message 29 by Melchior, posted 09-23-2004 6:44 PM riVeRraT has replied
 Message 35 by Percy, posted 09-23-2004 7:13 PM riVeRraT has replied

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


Message 18 of 188 (144186)
09-23-2004 5:53 PM
Reply to: Message 16 by Rei
09-23-2004 5:40 PM


I'm not sure what you mean by if you push at the end of the rod that 100% of the force goes into angular momentum.
For me it would require less effort to push the end of the bar, but you would have to push it further. If you pushed from the middle of the bar, isn't 100% of the effort going into the angular momentum still? It would require more effort, but you would have to push it less distance?

This message is a reply to:
 Message 16 by Rei, posted 09-23-2004 5:40 PM Rei has replied

Replies to this message:
 Message 20 by Rei, posted 09-23-2004 6:11 PM riVeRraT has replied

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


Message 19 of 188 (144189)
09-23-2004 6:00 PM
Reply to: Message 17 by riVeRraT
09-23-2004 5:48 PM


Why haven't you given me the mass of the car?
As any jerk scientist would tell you, do the math and you'll find the answer to this question.
edited to add: took me 5 minutes, but then I'm only a jerk engineer.
This message has been edited by DrJones*, 09-23-2004 05:05 PM

*not an actual doctor

This message is a reply to:
 Message 17 by riVeRraT, posted 09-23-2004 5:48 PM riVeRraT has replied

Replies to this message:
 Message 23 by riVeRraT, posted 09-23-2004 6:22 PM DrJones* has replied
 Message 40 by riVeRraT, posted 09-23-2004 11:34 PM DrJones* has replied

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


Message 20 of 188 (144191)
09-23-2004 6:11 PM
Reply to: Message 18 by riVeRraT
09-23-2004 5:53 PM


The bar looks like this:
---------------O---------------
If you push on either end, almost all the work will go into angular momentum. If you push on the weight in the center (the O), all of the work will go into linear momentum. If you push on both ends at once, the angular acceleration cancels out, but the bar accelerates linearly.

"Illuminant light,
illuminate me."

This message is a reply to:
 Message 18 by riVeRraT, posted 09-23-2004 5:53 PM riVeRraT has replied

Replies to this message:
 Message 22 by riVeRraT, posted 09-23-2004 6:15 PM Rei has replied

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


Message 21 of 188 (144192)
09-23-2004 6:14 PM
Reply to: Message 8 by coffee_addict
09-22-2004 7:29 PM


I just read down. But I want to understand it further, and then explain it to you.
However, what I don't like is people claiming that undisciplined common sense gives a person the same skills as someone that has been disciplining his common sense for decades.
I never said that common sense would replace knowledge. Given my common sense, and add some more knowledge, I would be smarter.
My common sense would tell me however that the problem can be solved to a certain degree of accuracy. However it could never be totally accurate, as there aer many variables to consider, besides the ones you mentioned. but it would be close enough to say, yes he was speeding, or no he wasn't. This holds true for many things in science, and sometimes we tend to say yes or no, when we really don't know the answer. If you can understand that, then you will know where I'm coming from.
But I really want to solve the problem, and show you my work, and describe how I came about my answer. Just so you won't think I am a bumbling idiot.
You play chess? I would love to play you a game. Haven't played in a while, but I would be up for it.
Hey, check this out, tell me what happened in this video, lol. Lets calculate the force required to stop that plane. I was the one flying the blue one, that I built myself. It flys again. 8ft wings 50cc 2 cylinder menthanol engine.
Forbidden
Forbidden
Forbidden

This message is a reply to:
 Message 8 by coffee_addict, posted 09-22-2004 7:29 PM coffee_addict has replied

Replies to this message:
 Message 26 by coffee_addict, posted 09-23-2004 6:35 PM riVeRraT has replied
 Message 63 by Dr Jack, posted 09-24-2004 6:26 AM riVeRraT has replied

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


Message 22 of 188 (144193)
09-23-2004 6:15 PM
Reply to: Message 20 by Rei
09-23-2004 6:11 PM


Oh, ok, so its pivoting around O?
I got it now.
It won't accelerate lineraly if the bar has flex in it.
This message has been edited by riVeRraT, 09-23-2004 05:16 PM

This message is a reply to:
 Message 20 by Rei, posted 09-23-2004 6:11 PM Rei has replied

Replies to this message:
 Message 24 by Rei, posted 09-23-2004 6:23 PM riVeRraT has not replied

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


Message 23 of 188 (144194)
09-23-2004 6:22 PM
Reply to: Message 19 by DrJones*
09-23-2004 6:00 PM


I would hope that you could, but thats not my point.
If I had no training in any of this, which I don't or forgot what I did learn, I would be starting from scratch. That makes it harder.
I don't know why you would get defensive, if your really not a jerk engineer. Tell me you haven't ever meet one?
I've seen some things designed by engineers, and just laughed. When we (the mechanic) open our mouth to say it doesn't work, we used to get yelled at. So we were told to put it up the way it was designed. When it doesn't work, then it all has to come down again. This is where your tax dollars go. They hire $30,000 a year engineers, to tell $80,000 a year mechanics how to put something up.
Besides all the paper work in the world, and formulas, you might still miss something, that I would see through my common sense, honed by 24years of mechanical hands-on experience. I pity the engineer that doesn't respect that.
*edited to clarify something*
I believe it is through the combined effort of both engineer and hands -on experience that something can go together right. I do not want to get rid of any engineer. But when this doesn't happen, it just sucks.
This message has been edited by riVeRraT, 09-23-2004 05:23 PM

This message is a reply to:
 Message 19 by DrJones*, posted 09-23-2004 6:00 PM DrJones* has replied

Replies to this message:
 Message 27 by coffee_addict, posted 09-23-2004 6:39 PM riVeRraT has replied
 Message 28 by DrJones*, posted 09-23-2004 6:39 PM riVeRraT has replied
 Message 30 by coffee_addict, posted 09-23-2004 6:48 PM riVeRraT has replied
 Message 38 by Rei, posted 09-23-2004 8:13 PM riVeRraT has replied

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


Message 24 of 188 (144196)
09-23-2004 6:23 PM
Reply to: Message 22 by riVeRraT
09-23-2004 6:15 PM


That's why we're dealing with "Rigid body kinematics"

"Illuminant light,
illuminate me."

This message is a reply to:
 Message 22 by riVeRraT, posted 09-23-2004 6:15 PM riVeRraT has not replied

coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 25 of 188 (144200)
09-23-2004 6:31 PM
Reply to: Message 15 by Percy
09-23-2004 5:21 PM


Percy writes:
Except for the tedium of going through the actual steps, your problem is too simple. I'm more interested in the solution to Crash's problem. I can't remember the equations for rotational momentum or energy, I couldn't find them on the web, and now it's bothering me, I want the answer.
That's just it. The frog's problem wasn't really fair, because you can't use common sense for it. You have to have the formula for it to be able to solve it. My problem is designed specifically so that all you need is common sense and a calculator. My original intent is to test out the common sense of those that claim their common sense to be more productive than the years of experience of scientists.
Now really, how the hell are we suppose to believe that their common sense is enough to solve such a big puzzle as the age of the earth when they can't even solve something as simple as this?
This message has been edited by Lam, 09-23-2004 05:36 PM

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

This message is a reply to:
 Message 15 by Percy, posted 09-23-2004 5:21 PM Percy has not 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 26 of 188 (144201)
09-23-2004 6:35 PM
Reply to: Message 21 by riVeRraT
09-23-2004 6:14 PM


the rat writes:
My common sense would tell me however that the problem can be solved to a certain degree of accuracy. However it could never be totally accurate, as there aer many variables to consider, besides the ones you mentioned. but it would be close enough to say, yes he was speeding, or no he wasn't. This holds true for many things in science, and sometimes we tend to say yes or no, when we really don't know the answer. If you can understand that, then you will know where I'm coming from.
For a start, could you name some of the variables not already mentioned that would throw off the result?
No, you do not need the mass of the car. This is also a really big hint.

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

This message is a reply to:
 Message 21 by riVeRraT, posted 09-23-2004 6:14 PM riVeRraT has replied

Replies to this message:
 Message 41 by riVeRraT, posted 09-23-2004 11:40 PM coffee_addict has replied

coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 27 of 188 (144206)
09-23-2004 6:39 PM
Reply to: Message 23 by riVeRraT
09-23-2004 6:22 PM


the rat writes:
Besides all the paper work in the world, and formulas, you might still miss something, that I would see through my common sense, honed by 24years of mechanical hands-on experience. I pity the engineer that doesn't respect that.
I pity the mechanics that don't respect 200 years of scientific inquiries.
Edited to change "doesn't" to "don't." I'm a moron and I still don't use singular and plural stuff right by instinct.
This message has been edited by Lam, 09-23-2004 05:44 PM

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

This message is a reply to:
 Message 23 by riVeRraT, posted 09-23-2004 6:22 PM riVeRraT has replied

Replies to this message:
 Message 42 by riVeRraT, posted 09-23-2004 11:43 PM coffee_addict has not replied

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


Message 28 of 188 (144207)
09-23-2004 6:39 PM
Reply to: Message 23 by riVeRraT
09-23-2004 6:22 PM


Besides all the paper work in the world, and formulas, you might still miss something, that I would see through my common sense, honed by 24years of mechanical hands-on experience. I pity the engineer that doesn't respect that
There isn't anything wrong with hands on experience. But you should understand that your first sentence works both ways. For all of your 24 years of hands on experience and common sense you might still miss something that I would see through my knowledge, honed by 4 well 7 but I meandered into history and anthropology as well years in a university. I pity the person that doesn't respect education.
Tell me you haven't ever meet one
Sure I've met jerk engineers, but they're vastly outnumbered by jerk tradespeople/regular citizens who think the "college boy" doesn't understand whats going on. Common sense is great, but unfortunatley isnt all that common.
They hire $30,000 a year engineers
Well then you're getting substandard engineers if they're working for that much. I don't know any engineer who started at less than $45K a year and thats in Canadian dollars.
Have you figured out why the mass of the car wasn't given yet?
This message has been edited by DrJones*, 09-23-2004 05:46 PM
This message has been edited by DrJones*, 09-23-2004 06:33 PM

*not an actual doctor

This message is a reply to:
 Message 23 by riVeRraT, posted 09-23-2004 6:22 PM riVeRraT has replied

Replies to this message:
 Message 36 by Percy, posted 09-23-2004 7:35 PM DrJones* has replied
 Message 43 by riVeRraT, posted 09-23-2004 11:53 PM DrJones* has not replied

Melchior
Inactive Member


Message 29 of 188 (144211)
09-23-2004 6:44 PM
Reply to: Message 17 by riVeRraT
09-23-2004 5:48 PM


riVeRraT, a bit of maybe not so common sense advice when dealing with equations.
As long as you can, try NOT to put in any numbers in the equations. If it says that the mass of the car is 'm', use 'm' as long as you possibly can.
It is also perfectly reasonable, and often prefered, to give an answer that isn't a number but just the final equation that you can just stick everything into.
That means less mess with calculators and rounding off and such. Quite a lot of people seem to think numbers are easier to work with, but it's usually the other way when it comes to equations.
If there is any step you feel you don't understand, or you get stuck, I can help you out if you send a PM. I don't think posting answers in this thread is the intent of the thread maker.

This message is a reply to:
 Message 17 by riVeRraT, posted 09-23-2004 5:48 PM riVeRraT has replied

Replies to this message:
 Message 31 by coffee_addict, posted 09-23-2004 6:50 PM Melchior has replied
 Message 44 by riVeRraT, posted 09-23-2004 11:58 PM Melchior has not replied

coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 30 of 188 (144215)
09-23-2004 6:48 PM
Reply to: Message 23 by riVeRraT
09-23-2004 6:22 PM


the rat writes:
They hire $30,000 a year engineers...
Hehehe. I just noticed this. All my brothers and sisters are engineers of some sort and none of them started their career with such a low salary. The lowest among them when they started was almost 50,000, and that was over 6 years ago. You're getting some crappy engineers.

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

This message is a reply to:
 Message 23 by riVeRraT, posted 09-23-2004 6:22 PM riVeRraT has replied

Replies to this message:
 Message 47 by riVeRraT, posted 09-24-2004 12:16 AM coffee_addict has not replied

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