# How many lbs. of water are in the tank?

1. Dec 21, 2014

### jim1174

1. The problem statement, all variables and given/known data

2. A tank has a volume of 8000 gallons and is 50% full. If the tank contains water, which has a density of 8.33 lbs/gallon, how many lbs. of water are in the tank?

2. Relevant equations

3. The attempt at a solution
Volume occupied is 50% i.e 4000 gallons , given that the density is 8.33 lbs/gallons

Therefore, the mass occupied by water in lbs is given by
mass =volume x density = 8000gallon x 8.33/gallon33320

https://www.physicsforums.com/file:///C:/Users/Jim/AppData/Local/Temp/msohtmlclip1/01/clip_image002.gif [Broken]

Last edited by a moderator: May 7, 2017
2. Dec 21, 2014

### Staff: Mentor

What's your answer? The last thing you wrote was "8000gallon x 8.33/gallon33320", which makes no sense.

Last edited by a moderator: May 7, 2017
3. Dec 21, 2014

### PhanthomJay

well, in the US system of measure , lb or lbs is a force or weight unit, not a mass unit. Density is weight per unit volume. Weight is density x volume.

Last edited by a moderator: May 7, 2017
4. Dec 21, 2014

### jbriggs444

In standard usage in the U.S. the term "pound" can denote either a unit of force or of mass. It is most commonly used in the sense of a mass. But that is irrelevant here. The problem states as a given that water has a density of 8.33 pounds per gallon. So for purposes of the problem we may safely assume that water has a density of 8.33 pounds per gallon. Whether that "8.33 pounds" is intended to denote the gravitational downforce on a gallon of water or is intended to denote the mass of a gallon of water will not change the numerical value of the intended answer to the problem and will not change the name of the unit of measure in which that result should be reported.

But we digress. Mark44's request for clarification is appropriate.

5. Dec 21, 2014

### Staff: Mentor

As another guy who grew up with the US system, I can tell you that the clear implication is lbm (as indicated by jbriggs444).

Regarding the OPs calculation, he calculated 4000 gallons, but then still used 8000 gal in the mass calculation. Go figure.

Chet

6. Dec 21, 2014

### PhanthomJay

Oh here we go again. A lb is a force unit, sometimes denoted lbf by those unfamiliar with US units. A pound of mass, which is used by virtually no one, is designated lbm. A gallon of water weighs 8.33 pounds in the US. No one cares that it has a mass of 8.33 lbm, not in the layman's world nor in the technical world. When mass must be used technically, we take its earth weight in lbs and divide by 32 and don't give it a name, even though that mass unit is slugs.

7. Dec 22, 2014

### jim1174

is this the answer mass= volume*density= 4000gallon*8.33lb/gallon=3332

Last edited: Dec 22, 2014
8. Dec 22, 2014

### Staff: Mentor

No, it's not.

And the units of the answer would be lb.

9. Dec 22, 2014

### SteamKing

Staff Emeritus
Always double check your arithmetic, and make sure you copy down results correctly. You multiplied 4000 by 8.33 and wound up with a number which was less than 4000.

10. Dec 23, 2014

### jim1174

Volume occupied is 50% i.e. 4000 gallons, given that the density is 8.33 lbs/gallons

Therefore, the mass occupied by water in lbs is given by
mass =volume x density = 4000gallon x 8.33lbs/gallon=33320 lbs

Final Answer is = 33320 lbs