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Weight, Length

 
 
Reply Mon 2 Feb, 2015 06:26 pm
I have two problems:

1) A piece of copper wire has the following dimensions:

Thickness - 1/8 inch.

Length - 20 yards

Calculate weight of wire.

Specific gravity of copper - 8.8

Cubic foot of water - 62.5 lbs. = 1000 oz.

What is known:

A piece of wire is copper.

Thickness of wire - 1/8 inch.

Length of wire - 20 yards.

Specific gravity of copper - 8.8

Want to know:

Weight of wire.

My partial solution:

1/8 inch = .125 = .010416 ft.

20 yards = 60 feet

.010416 x 60 = .62496 ft.

Not sure how to proceed.

2) A piece of copper wire has the following description:

Thickness - .2 cm

Weight- 1 kg.

Determine length of wire.

What is known :

A wire is copper.

Thickness - .2 cm

Weight - 1 kg.

Specific gravity - 8.8

Want to know:

Length of wire.

Not sure how to solve.


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neologist
 
  1  
Reply Mon 2 Feb, 2015 09:34 pm
@Randy Dandy,
Treat wire as a skinny cylinder.
Base area Pi*R^2
Times length = volume.
Randy Dandy
 
  0  
Reply Tue 3 Feb, 2015 01:20 pm
@neologist,
1) Check my calculation:

pi x r^2*h
3.1416 x 0.05208 x 0.05208 x 60 = .51126 cu ft.

.51126 x 8.8 (sp. of copper) = 4.499 = 5 lbs. weight of wire

2) .2 cm = .002 m.

1 kg = 1000 gm.

I am not sure how to calculate (2).
engineer
 
  1  
Reply Tue 3 Feb, 2015 01:55 pm
@Randy Dandy,
You need to use this:
Quote:
Cubic foot of water - 62.5 lbs.
Randy Dandy
 
  0  
Reply Tue 3 Feb, 2015 02:25 pm
@engineer,
2) .002 x 1000 = 2 cu. m. (volume)

62.5 x 2 = 125

125 x 8.8 = 1100

I don't know.

I have a question about 1).

I located an answer - 45 oz. = 2.8125 lbs.

Where did I miscalculate ?
0 Replies
 
engineer
 
  1  
Reply Tue 3 Feb, 2015 03:00 pm
@Randy Dandy,
Randy Dandy wrote:

1) Check my calculation:

pi x r^2*h
3.1416 x 0.05208 x 0.05208 x 60 = .51126 cu ft.

Good
Randy Dandy wrote:

.51126 x 8.8 (sp. of copper) = 4.499 = 5 lbs. weight of wire

No. Here is where putting your units down would really help. .51 cubic ft times 8.8 does not magically become pounds.

.511 ft^3 of copper x 62.5 lb water/ft^3 x 8.8 lb copper / lb water = 281 lb copper.
Randy Dandy
 
  0  
Reply Tue 3 Feb, 2015 03:43 pm
@engineer,
How was the answer 45 oz. ( weight of wire) calculated ?
engineer
 
  1  
Reply Tue 3 Feb, 2015 06:05 pm
@Randy Dandy,
Because this statement has a error in it:

Quote:
3.1416 x 0.05208 x 0.05208 x 60 = .51126 cu ft.


A thickness of 1/8" produces a radius of (1/16)/12 = .0052, not .052, so instead of 281 lb, you get 2.81 lb = 45 oz

Randy Dandy
 
  0  
Reply Tue 3 Feb, 2015 06:28 pm
@engineer,
Thanks.

How is (2) calculated ?
engineer
 
  1  
Reply Tue 3 Feb, 2015 07:07 pm
@Randy Dandy,
The exact same way.

Volume = pi r^2 L
Weight = Volume x Density of water x specific gravity

You are given
Quote:
What is known :

A wire is copper.
Thickness - .2 cm
Weight - 1 kg.
Specific gravity - 8.8

Want to know:
Length of wire.


From the second equation, you compute volume then use the first equation to get length.
Randy Dandy
 
  0  
Reply Tue 3 Feb, 2015 08:28 pm
@engineer,
Pi *r^2 * L
3.1416 x .1 x .1 x 1 = .031416 cu. ft.

.031416 x 62.5 x 8.8 =17.2788 meters

I am not sure.
engineer
 
  1  
Reply Tue 3 Feb, 2015 08:38 pm
@Randy Dandy,
Watch your units. You should not be getting cu ft. Try again but this time type out all of your units.

Example 3.14 x 0.1 cm x 0.1 cm ....
Randy Dandy
 
  1  
Reply Tue 3 Feb, 2015 09:11 pm
@engineer,
3.1416 x .1 cm x .1 cm = .031416 cm.

.031416 cm. x 62.5 lbs./cu. ft. of water x 8.8 specific gravity of copper = 17.2788 meters.

If not correct, I don't know. I always get confused with converting units.
Randy Dandy
 
  0  
Reply Wed 4 Feb, 2015 11:54 am
I found an answer - 36.17 meters

Where did I miscalculate ?
0 Replies
 
engineer
 
  1  
Reply Wed 4 Feb, 2015 12:42 pm
@Randy Dandy,
When you multiply cm by cm, you get cm squared. You CANNOT multiply cm squared by lb/cu ft and get anything at all. That is a clue. You have to multiply by something that looks like gm/cubic cm. You are right that you need density (weight / volume), but you are not dealing in pounds and cubic feet, you are dealing in grams and cubic centimeters.

Let's try again:
Quote:
Volume = pi r^2 L
Weight = Volume x Density of water x specific gravity

You are given

Quote:

What is known :

A wire is copper.
Thickness - .2 cm
Weight - 1 kg.
Specific gravity - 8.8

Want to know:
Length of wire.


From the second equation, you compute volume then use the first equation to get length.


Volume = pi r^2 L = 3.14 x 0.1cm x 0.1cm x L = .0314 L cm^2

Notice the L's in this equation. This is what you are solving for. Now we move to the second equation.

Weight = Volume x Density of water x specific gravity
The weight is given: 1kg = 1000 gm
The density of water in the metric system is 1 gm/cm^3

1000 gm = .0314 L cm^2 x 1 gm/cm^3 x 8.8
1000 gm = .276 L gm/cm
1000 gm / 0.276 gm/cm = L
3623 cm = L or 36.2 m.

Follow the units of what I posted. Getting the units to match up is the most important part of getting the right answer here.
Randy Dandy
 
  1  
Reply Wed 4 Feb, 2015 03:19 pm
@engineer,
Thanks for the helpful info ....
0 Replies
 
 

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