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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 106 of 188 (144595)
09-25-2004 2:17 AM


The solution
Starting from Part 2.
Uk=0.45 Coefficient of kinetic friction.
tan(d) = a/b in this case a= 8 and b= 100
so tan-1 (8/100)= d
d=4.57o
(1) 2*a*(x2 - x1) = v22 - v12
the acceleration of the car a=is currently unknown
initial position x1=0
end position x2=30m
initial velocity v1= currently unknown, also the final answer we are looking for
final velocity v2=0
So looking at equation (1) we see that we need to find the acceleration of the car during its braking before we can find the initial velocity.
We now need a formula that incorporates acceleration.
(2) FT=ma
FT= force exerted on the car during its braking. This force will be the sum of the frictional force and the component of the gravitational force that is acting along the surface.
m= mass of the car
a= the acceleration of the car during the braking
We now need to figure out the forces on the car. Which are:
The force of gravity FG=m*g acting directly downwards
This force can be broken down into a component acting along the surface and a component acting against the surface.
Force acting along the surface (3) FP= m*g*sin(d)
Force acting against (90o to the surface this is also known as the Normal Force) the surface FN=m*g*cos(d)
The frictional force will act along the surface opposite to the direction of motion. When determining the frictional force the force acting against the surface is multiplied by the coefficient of friction (either static or kinetic), so:
(4) FF=Uk*FN=Uk*m*g*cos(d)
So now we have the forces we need to figure out FT.
If we set the direction that car is travelling as positive we find that:
FT=FP-FF
And here begins the counterintuitive stuff. We don't need to find the value of FT. We can sub in equation (2). So we get
m*a=FP-FF
We then sub in (3) and (4) for FP and FF and we get:
m*a=m*g*sin(d)-Uk*m*g*cos(d)
The mass of the car just drops out (say A-2=5 and 10*A-10*2=10*5 once you do the math you'll find that A=7 in both equations), this is why you didn't need to know the mass of the car.
a=g*sin(d)-Uk*g*cos(d)
we now can find the acceleration of the car during braking. We throw in the appropriate values and get:
a=-3.62 m/s2
We now can go all the way back to equation (1) and find v1.
(1) 2*(-3.62)*(30-0) = 0 - v12
and doing the math we get
v1=14.73 m/s=32.95 mph.
The guy was speeding when he hit the brakes.
This message has been edited by DrJones*, 09-25-2004 01:19 AM
This message has been edited by DrJones*, 09-25-2004 01:23 AM

*not an actual doctor

Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 107 of 188 (144604)
09-25-2004 5:23 AM
Reply to: Message 99 by Rrhain
09-25-2004 12:03 AM


I blooming hate word games too
Actually what I had in mind would be something from other sciences like chemistry, or biology.
Of course it depends what Lam wanted to demonstrate in this thread. If he wanted to show that application of accumulated knowledge was a more efficient method of reasoning than 'common sense' then not only has made the point already (Riverrat refused to use the accumulated knowledge that Lam supplied, and struggled * ), but it could be re-emphasised by providing other examples.
Take a classic Mendelian experiment crossing tall and short in-bred pea plants:
What would be the resulting ratio of tall/short plants when you cross tall and short pea plants?
What would be the resulting ratio of crossing two individuals from that first generation of hybrids?
I'm prepared to say that without the knowledge of genes and dominance and recessiveness then it would be nigh-on impossible to get the answer to that question by common sense alone.
Naturally if the point is to set problems and provide all of the information needed to solve them then the above problem is childs play (that's biology for you), and not really worth discussing to any length.
Of course, I would have had no problem with your puzzle but (gosh darn it) Jar went and did it (spelling mistakes permitting) before I got a chance
* No offense meant to Riverrat on this point, I'm relatively sure he could have beaten me to the answer if he had used the equations provided

This message is a reply to:
 Message 99 by Rrhain, posted 09-25-2004 12:03 AM Rrhain has replied

Replies to this message:
 Message 115 by Rrhain, posted 09-25-2004 4:49 PM Ooook! has replied

riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 108 of 188 (144614)
09-25-2004 8:57 AM
Reply to: Message 94 by NosyNed
09-24-2004 9:42 PM


Re: Probably
You'll have to stop assuming that you know a lot more than you actually do.
You know, I argue stuff, but I do not claim to really know anything. I love to argue, because sometimes the truth comes out when we do. If there is one thing in life I have learned, is how little I/we really know. Also how it can change over time, as we see things around us.
Thanks for the compliment ned.

This message is a reply to:
 Message 94 by NosyNed, posted 09-24-2004 9:42 PM NosyNed has replied

Replies to this message:
 Message 110 by NosyNed, posted 09-25-2004 11:39 AM riVeRraT has replied

jar
Member (Idle past 393 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 109 of 188 (144618)
09-25-2004 9:54 AM
Reply to: Message 105 by Rrhain
09-25-2004 1:16 AM


"Other" than that, how did I do? I never could spelll worth a damn.

Aslan is not a Tame Lion

This message is a reply to:
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NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 110 of 188 (144626)
09-25-2004 11:39 AM
Reply to: Message 108 by riVeRraT
09-25-2004 8:57 AM


Arguing on no grounds
There can be a serious problem with arguing without having the basic knowledge on which to base the discussions.
It tends to produce suggestions which, to the more knowledgable, are clear nonsense. Eventually they get tired and write the person off as a total crackpot. Then your learning opportunities are gone.
Combine that tendancy with one that is stuggling to support a pre conceived idea and you aren't likely to learn much of anything.
No add in a suspicion of those who are expert and a tendancy to be unhappy with having to take somethings just because those who are expert say so and one will find themselves in a very narrowly defined little box without a way out.
This message has been edited by NosyNed, 09-25-2004 10:41 AM

This message is a reply to:
 Message 108 by riVeRraT, posted 09-25-2004 8:57 AM riVeRraT has replied

Replies to this message:
 Message 113 by riVeRraT, posted 09-25-2004 12:03 PM NosyNed has replied

riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 111 of 188 (144630)
09-25-2004 11:53 AM
Reply to: Message 95 by Rei
09-24-2004 9:43 PM


Well as a general rule of thumb, for the last 23 years I have been hanging ductwork in buildings throughout the tri-state area, I never hang something from something else.
I only hang things indepentantly of each other. So if I was there, I would have never hung the second catwalk from the first. The hangers must meet load requirments and fire ratings. So that in the event of a fire, it won't be the first thing to go.
Yes, I would have never done what they did.

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 Message 95 by Rei, posted 09-24-2004 9:43 PM Rei has replied

Replies to this message:
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riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 112 of 188 (144632)
09-25-2004 11:55 AM
Reply to: Message 98 by DrJones*
09-24-2004 11:09 PM


No, I will when I have the time come up pwith the correct solution. I will not learn anything if you do it for me.
Besides showing everyone, I like to learn things.

This message is a reply to:
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Replies to this message:
 Message 116 by tsig, posted 09-26-2004 12:03 AM riVeRraT has replied

riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 113 of 188 (144635)
09-25-2004 12:03 PM
Reply to: Message 110 by NosyNed
09-25-2004 11:39 AM


Re: Arguing on no grounds
Understood.
I need to learn to express myself better, thats always been a problem for me.
Being in this forum as helped me greatly, and has actually prepared me more for the real world, and teaching people about God, and how I should go about it, so as to do it with love, and understanding, and without sounding like a crackpot. I only mean well.

This message is a reply to:
 Message 110 by NosyNed, posted 09-25-2004 11:39 AM NosyNed has replied

Replies to this message:
 Message 114 by NosyNed, posted 09-25-2004 12:11 PM riVeRraT has replied

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


Message 114 of 188 (144637)
09-25-2004 12:11 PM
Reply to: Message 113 by riVeRraT
09-25-2004 12:03 PM


Teaching about God
Being in this forum as helped me greatly, and has actually prepared me more for the real world, and teaching people about God, and how I should go about it, so as to do it with love, and understanding, and without sounding like a crackpot. I only mean well.
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.
A better, deeper understanding of what the Bible is really about is a solid foundation that can only produce a growth in faith as new things are learned.
This is the opinion of the majority of Christians. You might want to consider, not the scientific side of it, but the theological issues involved.
If you wish to spread the word of God and do mean well,then think carefully about the real impact your actions may have.
To the religious you may be seen as dangerous. To those of us without faith you might just confirm the idea that it is all nonsense if you are careless and spout things which we do know are nonsense. That's not going to win any hearts and minds, now is it?
This message has been edited by NosyNed, 09-25-2004 11:13 AM

This message is a reply to:
 Message 113 by riVeRraT, posted 09-25-2004 12:03 PM riVeRraT has replied

Replies to this message:
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Rrhain
Member
Posts: 6351
From: San Diego, CA, USA
Joined: 05-03-2003


Message 115 of 188 (144692)
09-25-2004 4:49 PM
Reply to: Message 107 by Ooook!
09-25-2004 5:23 AM


Ooook! writes:
quote:
I blooming hate word games too
Well, make up your mind!
quote:
Actually what I had in mind would be something from other sciences like chemistry, or biology.
OK:
Suppose we have a single-gene, single-trait characteristic in a diploid organism such that there is one dominant allele and one recessive allele. To put it in a classic Mendelian sense, there is A and a and that's it.
Suppose that the occurrence of the recessive phenotype (that is, people who actually express the recessive characteristic or, in Mendelian genetics, aa) is one in a thousand.
Suppose that we can control breeding such that those who do express the recessive characteritic do not have children.
How many generations would it take to reduce the expression of the recessive characteristic from one in a thousand to one in a million?
Some hints!
Don't get bogged down in individuals. Assume there is a large enough population that everybody can find a mate.
Other than the prohibition on those who are aa reproducing, all other mating is random. Thus, all those who are AA and Aa will mate, but they do not know the genetic makeup of the other.
Think about dividing the gene population. For example, let p be the ratio of A genes in the population and let q be the population of a genes. Thus, p + q = 1. What other equation can we derive? And what is it we're trying to do to that other equation?

Rrhain
WWJD? JWRTFM!

This message is a reply to:
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Replies to this message:
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tsig
Member (Idle past 2908 days)
Posts: 738
From: USA
Joined: 04-09-2004


Message 116 of 188 (144785)
09-26-2004 12:03 AM
Reply to: Message 112 by riVeRraT
09-25-2004 11:55 AM


equations are tools
Riverrat
What whould you think of an HVAC tech who showed up on the job without gages,a voltmeter, or even basic hand tools?

This message is a reply to:
 Message 112 by riVeRraT, posted 09-25-2004 11:55 AM riVeRraT has replied

Replies to this message:
 Message 118 by riVeRraT, posted 09-26-2004 8:45 AM tsig has replied

riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 117 of 188 (144803)
09-26-2004 8:29 AM
Reply to: Message 114 by NosyNed
09-25-2004 12:11 PM


Re: Teaching about God
Well, when first coming in here, I really didn't know what I was up against. Now I have a better understanding. Its been a great test of my faith. I believe there are still people in here who might need to hear a few things, that they haven't heard before.
Everyones got their reasons for not believe, and its educating to learn why. Like I said its its helped me to become a better person believe it or not. We got a long way to go, I'm not going anywhere

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riVeRraT
Member (Idle past 415 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 118 of 188 (144807)
09-26-2004 8:45 AM
Reply to: Message 116 by tsig
09-26-2004 12:03 AM


Re: equations are tools
Again, I will explain, that I am not saying we don't need engineers.
We don't need jerk engineers, can't you see the difference?

This message is a reply to:
 Message 116 by tsig, posted 09-26-2004 12:03 AM tsig has replied

Replies to this message:
 Message 128 by tsig, posted 09-27-2004 7:38 PM riVeRraT has replied

coffee_addict
Member (Idle past 476 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 119 of 188 (144859)
09-26-2004 3:47 PM
Reply to: Message 114 by NosyNed
09-25-2004 12:11 PM


Re: Teaching about God
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:
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Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 120 of 188 (144877)
09-26-2004 5:11 PM
Reply to: Message 115 by Rrhain
09-25-2004 4:49 PM


Oooh that's a good one, genetics with a twist of natural selection.
I'll get back to you when I have time to sit down and think about it, as its been a while since I've done any genetic type problems and on top of that my maths is not diamond sharp (as seen earlier in the thread)

This message is a reply to:
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