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Author Topic:   Using your common sense to solve a physics problem.
coffee_addict
Member (Idle past 476 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 1 of 188 (143823)
09-22-2004 2:25 AM


I've been thinking about using common sense rather than skills you learn in school to solve real life problems. I'm not going to say who (you know who you are), but someone has been claiming that common sense alone is enough to allow a person to know as much, if not more, than any "jerk scientist". This got me thinking. Can common sense really bring a person that far ahead? Is the person in question's common sense trustworthy?
The following is a simple physics problem that was on my entrance exam quite a while ago. What I will post will come in two parts. Part one will only have what they usually give you on a problem. Part two I will give all the equations, constants, and whatnot necessary to solve the problem. You don't need to know anything about physics to be able to complete the problem once you've read part 2. What you will need is a really good common sense.
The intention of this is to allow you, or anyone, to test out whether common sense is as great as it seems. If you like a challenge, do not read part two. However, I think it is more fair that part two is included simply because you are not a physicist and I don't expect you to know this off the top of your head. After you've read part 1 and 2, you only need a calculator and common sense to solve the problem. Everything you need is there. By the way, we rarely ever get that much info on our physics problems.
Part 1
An automobile driver traveling down an 8% grade slams on his breaks and skids 30 meters before hitting a parked car. A lawyer hires an expert who measures the coefficient of kinetic friction between the road and the tires to be 0.45. Is the lawyer correct to accuse the driver of exceeding the 25 MPH speed limit? Why or why not? (Assume that an 8% grade means that the road raises 8 meters vertically for every 100 meters traveled horizontally. Also, drag is negligible.)
Part 2
Uk = coefficient of kinetic friction = 0.45
tan(d) = a/b where d is an angle of a right triangle.
2*a*(x2 - x1) = v2^2 - v1^2 where a = acceleration, v1 = initial velocity, V2= end velocity, x1= initial position, and x2 = end position.
F = m*a where F is force, m is mass of the car, and a is acceleration.
F(fric) = Uk*m*g*cos(d) where F(fric) is friction force and g is 9.8 m/s*s
F(grav) = m*g*sin(d) where F(grav) is the force on the car due to Earth's gravity.
Added by edit:
For those that are also frustrated by the term "jerk scientist", please please please do not get involve. Yes, I am sticking my neck out really far by giving handing him/them the answer on a silver platter.
Added by a second edit:
1 mile = 1609 meters
This message has been edited by Darth Mal, 09-22-2004 03:24 AM

The Laminator
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Replies to this message:
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Nighttrain
Member (Idle past 3993 days)
Posts: 1512
From: brisbane,australia
Joined: 06-08-2004


Message 2 of 188 (143832)
09-22-2004 3:53 AM
Reply to: Message 1 by coffee_addict
09-22-2004 2:25 AM


Just clarifying---Is the offending driver a fundy? Is the lawyer?

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


Message 3 of 188 (143839)
09-22-2004 4:25 AM
Reply to: Message 2 by Nighttrain
09-22-2004 3:53 AM


The offending driver is a fundy. The lawyer is not a fundy, obviously.

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

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


Message 4 of 188 (143893)
09-22-2004 12:49 PM


Bump
Yup, that sure was a bumpy ride to Pumpkin Fest.

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

Replies to this message:
 Message 5 by Percy, posted 09-22-2004 5:39 PM coffee_addict has replied

Percy
Member
Posts: 22388
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 5 of 188 (143932)
09-22-2004 5:39 PM
Reply to: Message 4 by coffee_addict
09-22-2004 12:49 PM


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

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Ooook!
Member (Idle past 5814 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 6 of 188 (143935)
09-22-2004 5:55 PM
Reply to: Message 1 by coffee_addict
09-22-2004 2:25 AM


What a way to make me feel stupid Lam. I thought that 'what is the next number in the sequence' thread was bad enough!
I blooming hate physics
But as a more pertinent point:
Your problem made me think about the nature of science and its relation to common sense. I came to the conclusion that almost every scientific discipline I could think of (apart from the obviously anti-intuitive ones like quantum physics) was the application of common sense once you know the details.
Of course that last bit in bold is the tricky bit, and takes a bit of time.
I think this is especially true when you look at my field (biology) - in my opinion its' all about knowing the facts and applying that knowledge. The closest thing I can think of to your physics tester is those genetic phenotype diamond things I remember from my school days (he says sounding older than he is) - and they definitely can be done with pure common sense, and not much practice.

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


Message 7 of 188 (143945)
09-22-2004 7:06 PM
Reply to: Message 5 by Percy
09-22-2004 5:39 PM


Re: Bump
What Percy said.

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

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


Message 8 of 188 (143950)
09-22-2004 7:29 PM
Reply to: Message 6 by Ooook!
09-22-2004 5:55 PM


Ooook! writes:
Your problem made me think about the nature of science and its relation to common sense. I came to the conclusion that almost every scientific discipline I could think of (apart from the obviously anti-intuitive ones like quantum physics) was the application of common sense once you know the details.
Science is just taking common sense and discipline it to a point where one can relate past events to explore new ones.
For example, it's common sense that I can describe how fast I am walking by saying something like "I walked all the way to... wonderland in 10 minutes." Well, if one thinks about it, by purely using common sense, one can apply such a concept to something else more complicated, like how long does a horse take to get to wonderland. If it takes a horse only 1 minute to get to wonderland, one now has the basic concept of relativity, that the horse is relatively faster than me.
If wonderland is 1 mile away, one can use his common sense to think, "well, gee, Lam's speed is 1 mile per 10 minutes and the horse's speed is 1 mile per 1 minute. Therefore, the horse is... let me see... that's 1, 2, 3, 4... the horse is 10 times faster than Lam."
Just by pure common sense, I've got the basics for the concept of velocity.
So yes, I really think that science is just disciplining common sense to a point where one can apply past experiences to predict and explain future events.
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.
Going back to my problem, everything that you need to solve the problem is there. You don't even need to know much about physics to be able to solve it. All you need to know is that the total force equals the addition of the individual forces involved.
For example, say that F(total) is the total force, F(1) and F(2) are 2 individual forces at work, the equation would look something like:
F(total) = F(1) + F(2).
There, I've given away the answer. This would take me about half a minute to figure out, but I am well trained in this kind of physics. I wouldn't expect an average person to recognize it this fast, although I would expect anyone with a good enough common sense to be able to put together the very obvious puzzle.

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

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Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 9 of 188 (143952)
09-22-2004 7:53 PM
Reply to: Message 8 by coffee_addict
09-22-2004 7:29 PM


quote:
So yes, I really think that science is just disciplining common sense to a point where one can apply past experiences to predict and explain future events.
Unfortunately, it's not so simple as "disciplining" common sense. For example, common sense told people that objects fell with a velocity in proportion to their weight. It told people that the outer planets moved forward, then stopped and turned backwards, then moved forwards again. It told people that if you have a tall, thin pipe filled with water stopped off by a cork at the base, that it will put less pressure on the cork than if you had a big tank full of water of the same height. Etc.
The problem is accumulated knowledge. Only through application of accumulated knowledge can physics be solved accurately. Data must be acquired, and fit into a common framework; then one can work within the framework. It was only through huge amounts of experimentation conducted the world over that we managed to reach the state of modern physics. Even kinematics (like your problem), which we now take for granted, took a long time - and a lot of testing - to quantify.
One of the big problems with creationists is that they assume the framework must be wrong. Mind you, they don't attempt to fit the centuries of accumulated evidence into a different framework; they just assume, using their "common sense", that there must be a different framework out there - something, anything - that works instead. They furthermore assume that had this framework simply been considered, all of these people who worked so hard in their evidence collecting throughout the centuries would have found that their data fit into it - all of the billions of datapoints out there.
This is the antithesis of science, and it is very flawed. One of the biggest flaws is that the standard bible-literalist framework *was* the default framework used for a long time. Scientists tried and tried, to no avail, to keep it. Look at the early scientists in the fields of geology, physics, chemistry, biology, etc, in Europe - they were almost all Christians. Even today, the majority of western scientists are Christians - and a fair number of them were brought up in creationist households. Far from never being considered it, these people would undoubtedly *love* to have the bible being proven literally true. They believe in God, they believe in Christ, and just like the creationists on this board, they want to find any evidence possible that the words within that book are literally true as much as possible.

"Illuminant light,
illuminate me."

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crashfrog
Member (Idle past 1466 days)
Posts: 19762
From: Silver Spring, MD
Joined: 03-20-2003


Message 10 of 188 (143954)
09-22-2004 8:05 PM
Reply to: Message 8 by coffee_addict
09-22-2004 7:29 PM


Going back to my problem, everything that you need to solve the problem is there. You don't even need to know much about physics to be able to solve it. All you need to know is that the total force equals the addition of the individual forces involved.
For example, say that F(total) is the total force, F(1) and F(2) are 2 individual forces at work, the equation would look something like:
F(total) = F(1) + F(2).
Doesn't force have both amplitude and direction, though? Doesn't that make it vector addition, not scalar addition?
Or is that just implied?

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


Message 11 of 188 (143956)
09-22-2004 8:09 PM
Reply to: Message 10 by crashfrog
09-22-2004 8:05 PM


Look at the picture.

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

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Nighttrain
Member (Idle past 3993 days)
Posts: 1512
From: brisbane,australia
Joined: 06-08-2004


Message 12 of 188 (143999)
09-23-2004 1:19 AM
Reply to: Message 11 by coffee_addict
09-22-2004 8:09 PM


Once lawyers are involved, you can throw away commom sense. Should the formula factor in the mass of the vehicle? Cadillac or Beetle? Mass of driver? Pre-lunch or after? Was the sun in driver`s eyes. Did his biorhythms affect his ability? Were there any signs of a Great Flood? All valid lawyer tricks.

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


Message 13 of 188 (144000)
09-23-2004 1:31 AM


I'm going to give another big hint tomorrow if noone that's an advocate of the common sense versus "jerk scientist" tries to solve it by then.

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

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


Message 14 of 188 (144168)
09-23-2004 4:53 PM


Ok, here comes another big clue.
I stated before that F(total) = F(1) + F(2). Here is the hint. F(1) = F(grav) and F(2) = -F(fric). If you look at the picture again, F(fric)'s arrow is pointing the opposite direction of F(grav). Therefore:
F(total) = F(grav) - F(fric)
I'm getting a little irritated. What ever happened to common sense?

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

Replies to this message:
 Message 15 by Percy, posted 09-23-2004 5:21 PM coffee_addict has replied

Percy
Member
Posts: 22388
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 15 of 188 (144174)
09-23-2004 5:21 PM
Reply to: Message 14 by coffee_addict
09-23-2004 4:53 PM


Lam writes:
I'm getting a little irritated. What ever happened to common sense?
No one's engaged the problem yet - who are you irritated at? Do you often find yourself arguing with department store mannikins?
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.
--Percy

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