math question (daughter needs help)

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May 17, 2002
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any math whizzes out there?

My 15 year old daughter has this math problem, she can't seem to figure it out.

"Here is a cylinder. The formula for the volume of a cylinder is pie r2 height. What is it's height? The diameter is 22.5 centimeters. The mass is 45.4 grams and the density is .13 grams per cubic centimeter."
 
Mass = density * volume (grams = grams/cm^3 * cm^3)

Volume = pi * R^2 * height (height is unknown) (cm^3 = cm^2 * cm)

R = diameter / 2

Mass = density * pi * (d/2)^2 * h (grams = grams * grams/cm^3 * cm^2 * cm)

(mass)/(density*pi*(d/2)^2) = h (grams / (grams/cm^3 * cm^2 * cm) = cm

(edited to add dimensional analysis - everything checks out!)
 
"Here is a cylinder. The formula for the volume of a cylinder is pie r2 height. What is it's height? The diameter is 22.5 centimeters. The mass is 45.4 grams and the density is .13 grams per cubic centimeter."

V = PI times R square times H

V = 45.4 gm divided by .13 gm/cm cubed = 349.23 cm cubed
PI = 3.1416
R = 22.5 diameter divided by 2 = 11.25
R squared = 126.56

349.23 = 3.1416 times 126.56 times H
349.23 = 397.61 times H
349.23 divided by 397.61 = H
.88 cm = H
 
Allow me to correct one misconception. Many people believe that pie are squared. Wrong! Pie are round.

Sorry, I'll go now...:p :foot:
 
Don't forget to tell your daughter to put acknowledgements in her asssignment.
 
tarsier said:
Allow me to correct one misconception. Many people believe that pie are squared. Wrong! Pie are round.

Sorry, I'll go now...:p :foot:


... Boy! What they been teaching ya at there school? Pie are round - corn bread are square!
 
thanks! She actually knew how to write out the formula, but got hung up by the grams over centimeters. (Something to that effect)

Centaur said:
Mass = density * volume (grams = grams/cm^3 * cm^3)

Volume = pi * R^2 * height (height is unknown) (cm^3 = cm^2 * cm)

R = diameter / 2

Mass = density * pi * (d/2)^2 * h (grams = grams * grams/cm^3 * cm^2 * cm)

(mass)/(density*pi*(d/2)^2) = h (grams / (grams/cm^3 * cm^2 * cm) = cm

(edited to add dimensional analysis - everything checks out!)
 
excellent, thanks for your help! :)



Esav Benyamin said:
"Here is a cylinder. The formula for the volume of a cylinder is pie r2 height. What is it's height? The diameter is 22.5 centimeters. The mass is 45.4 grams and the density is .13 grams per cubic centimeter."

V = PI times R square times H

V = 45.4 gm divided by .13 gm/cm cubed = 349.23 cm cubed
PI = 3.1416
R = 22.5 diameter divided by 2 = 11.25
R squared = 126.56

349.23 = 3.1416 times 126.56 times H
349.23 = 397.61 times H
349.23 divided by 397.61 = H
.88 cm = H
 
Ah damn, I just went over that stuff. I feel like I'm wasting my time in Alg (2nd time around) I should start cutting and go to a geometry class. :cool:
 
Gary007 said:
"Here is a cylinder. The formula for the volume of a cylinder is pie r2 height. What is it's height? The diameter is 22.5 centimeters. The mass is 45.4 grams and the density is .13 grams per cubic centimeter."

This problem is a very important problem, IMHO, and teaches many important lessons... and not just in math.

At first, this problem seems very complicated with the height and the mass and stuff that's per cubic centimeter and all. It seems like a very complicated, complex problem.

However, as several people here have well-demonstrated, it's really two problems that have been intertwined. Once you separate the two smaller problems, neither is very difficult.

A lot of "higher" math is this way. It's just a lot of simple problems that have been intertwined. Here's an interesting fact: if you can add, subtract, multiply, and divide, then you can do all math operations because all higher operations are definded based on those four simple operations. And when you consider the fact that muliplying is just repeated adding and dividing is just repeated subtracting, well you see that from about third grade on, math is just increasing intertwining and even the most seemingly-complex problem breaks down into simple operations.

Life is often like this too. Seemingly huge, complex problems break down into just a series of little problems that are intertwined. Huge tasks break down into just a series of simple tasks.

It's important to learn that when you face a big problem -- be it a math problem or a marriage problem -- don't be intimidated by the seeming complexity or enormity of it. Just break it down into little pieces.



The other thing this problem teaches is an approach to problem solving. The first step in my previous assertions is to separate the intertwined problems. The approach to learn is to write down what you know and start filling in the blanks. This is how a detective often solves a mystery. It's how many problems in life in general can be approached. Just start writing down what you know and see how the pieces fit together.
 
Centaur said:
Mass = density * volume (grams = grams/cm^3 * cm^3)

Volume = pi * R^2 * height (height is unknown) (cm^3 = cm^2 * cm)

R = diameter / 2

Mass = density * pi * (d/2)^2 * h (grams = grams * grams/cm^3 * cm^2 * cm)

(mass)/(density*pi*(d/2)^2) = h (grams / (grams/cm^3 * cm^2 * cm) = cm

(edited to add dimensional analysis - everything checks out!)

To quote Homer Simpson..could you dumb it down a bit? :confused:
 
I've got a question.

Does anyone know for sure what the answer is?

I mean, Chuck is right. Thinking your way through these problems is the whole idea of the exercise, not some number that pops out at the end. But I haven't looked at problems like this for years. :foot:
 
Gollnick said:
A lot of "higher" math is this way. It's just a lot of simple problems that have been intertwined. Here's an interesting fact: if you can add, subtract, multiply, and divide, then you can do all math operations because all higher operations are definded based on those four simple operations.
If "higher maths" means something like "high school maths" you're completly right. If not you're wrong. Try some algebra, and you'll see these are use less. Actually "higher maths" if such a thing exists go beyond this.

Esav's answer is basically correct. If you're daughter want to make herself look smart she could point that proper term is "mass-density" rather than "density".
 
Gollnick said:
This problem is a very important problem, IMHO, and teaches many important lessons... and not just in math.

At first, this problem seems very complicated with the height and the mass and stuff that's per cubic centimeter and all. It seems like a very complicated, complex problem.

However, as several people here have well-demonstrated, it's really two problems that have been intertwined. Once you separate the two smaller problems, neither is very difficult.

A lot of "higher" math is this way. It's just a lot of simple problems that have been intertwined. Here's an interesting fact: if you can add, subtract, multiply, and divide, then you can do all math operations because all higher operations are definded based on those four simple operations. And when you consider the fact that muliplying is just repeated adding and dividing is just repeated subtracting, well you see that from about third grade on, math is just increasing intertwining and even the most seemingly-complex problem breaks down into simple operations.

Life is often like this too. Seemingly huge, complex problems break down into just a series of little problems that are intertwined. Huge tasks break down into just a series of simple tasks.

It's important to learn that when you face a big problem -- be it a math problem or a marriage problem -- don't be intimidated by the seeming complexity or enormity of it. Just break it down into little pieces.



The other thing this problem teaches is an approach to problem solving. The first step in my previous assertions is to separate the intertwined problems. The approach to learn is to write down what you know and start filling in the blanks. This is how a detective often solves a mystery. It's how many problems in life in general can be approached. Just start writing down what you know and see how the pieces fit together.

THAT IS SO TRUE. A lot of people think maths is difficult. I've always loved maths and statistics. My first degree was actuarial science which was basically a math/stats degree.

Anyway, it's problems like these that are the most important 'cos it actually combines many concepts together. Even in much higher maths this type of problems will help you the most. There is no use in doing 100's of the same type of maths problems. Once you do a couple of these intertwined questions which use several concepts at once, you're set to go. Once you understand the method that's all you need to do.

But 1 mistake that i've made and a lot of people make during exams is concentrate too much on the complicated stuff and forget the simple stuff. Stuff like remembering the method but forgetting the formula of the cyclinder.

I actually couldn't answer 1 big question in my final year of my forensic science degree because of it. I needed to calculate some chemistry equations and i knew the method and all but actually forgot the formula for ammonium hydroxide or something like that. I just had a mental block and just couldn't remember it or even piece it together. I nearly had a nervous breakdown in the exam room. I was so desperate i actually tried asking one of the invigilators for the formula. hehehe. In the end he just suggested putting whatever i thought it was. So i did. And it was wrong. But they only took away marks for the wrong formula but not for the whole question. In the end i got top marks for that particular exam i think.

I think my biggest problem with maths now is that i'm too dependent on my calculator. I've been using the same one for 7 years and i think when it conks out i won't be able to use any other 'cos i'm already too familiar with it. Now i'm even thinking of getting a new calculator to start training on in case this one dies on me.
 
Ravaillac said:
If "higher maths" means something like "high school maths" you're completly right. If not you're wrong. Try some algebra, and you'll see these are use less. Actually "higher maths" if such a thing exists go beyond this.

Esav's answer is basically correct. If you're daughter want to make herself look smart she could point that proper term is "mass-density" rather than "density".


Higher degree maths is actually quite similar. Normal maths, not algebra. You just need the ability to approach the question in smaller steps to understand it. Although it's not just addition and subtraction. There's stuff like triple integration. ALGEBRA ON THE OTHER HAND IS JUST PLAIN STUPID. I NEVER PASSED ALGEBRA. Couldn't get my brain to understand it. The module i did in university just consisted of equations of greek alphabets and proving it. It just didn't make sense. I still have the workbook and have been trying to sell it on amazon but nobody wants it.
 
Point44 said:
Higher degree maths is actually quite similar. Normal maths, not algebra. You just need the ability to approach the question in smaller steps to understand it. Although it's not just addition and subtraction. There's stuff like triple integration. ALGEBRA ON THE OTHER HAND IS JUST PLAIN STUPID. I NEVER PASSED ALGEBRA. Couldn't get my brain to understand it. The module i did in university just consisted of equations of greek alphabets and proving it. It just didn't make sense. I still have the workbook and have been trying to sell it on amazon but nobody wants it.
Topology is maths too. It's useful (by the way algebra is useful too) and there's not a single addition or subtraction. Series are maths and they can work with "distance" concept only, without any addition nor substraction. Logic is maths too. No addition there either.
"Divide to reign" is an efficient principle... in its limited scope. Ability to change point of view, is in my opinion far more important.
 
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