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Water's density at 60-degree Fahrenheit is 62.366 pounds/ft^3. Is it inertial or gravitational mass? Actually, I am trying to calculate Potential Energy (PE) of 1 gallon of water at 1' height. Is PE = mgh = 8.35 x 32.17 x 1? or is PE = 8.35 x 1? (Here I am assuming 1 gallon of water weighs 8.35 pounds and gravity as 32.17 feet/second^2) Essentially my question is when we say 1 gallon of water weighs 8.35 pounds, is it inertial or gravitational mass?

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Essentially my question is when we say 1 gallon of water weighs 8.35 pounds, is it inertial or gravitational mass?

It depends how you measured the weight. If you used Newton ($F=ma$) to determine the mass you would call it inertial mass. If you used the the gravitational law ($F = G \frac{Mm}{r^2}$) by measuring the force between the unknown mass $m$ and the known one $M$ you would call $m$ the gravitational mass (that's what a scale does). Luckily it turns out that, as far as we know, those two are the same.

  • I am assuming we are talking about classical mechanics here.
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  • $\begingroup$ Thanks for your response. I got the water weight from Internet, so I don't have a clue what measuring method was used to get that weight. From what you are saying to calculate Potential Energy for 1 gallon of water in the equation (PE = mgh) I have to substitute m = 8.35 (i.e. m*g is not 8.35). So the PE of 1 gallon of water at 1 foot height would be 8.35 * 32.17 * 1 = 268 (approximately). $\endgroup$ – Thinker Nov 30 '16 at 18:29

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