Newfound Lake, a freshwater lake near Bristol, New Hampshire, has a maximum depth of 60 m, and the mean atmospheric pressure is 91 kPa. Estimate the absolute pressure in kPa at this maximum depth.

Question

Accepted Answer

• System sketch: Imagine that Fig. 2.5 is Newfound Lake, with h = 60 m and z = 0 at the surface.

• Property values: From Table 2.1, [latex]\gamma_{water}[/latex] = 9790 N/[latex]m^3[/latex]. We are given that [latex]p_{atmos}[/latex] = 91 kPa.

Table 2.1 Specific Weight of Some Common Fluids
	Specific weight [latex]\gamma[/latex] at 68°F = 20°C
Fluid	lbf/[latex]ft^3[/latex]	N/[latex]m^3[/latex]
Air (at 1 atm)	0.0752	11.8
Ethyl alcohol	49.2	7,733
SAE 30 oil	55.5	8,720
Water	62.4	9,790
Seawater	64.0	10,050
Glycerin	78.7	12,360
Carbon tetrachloride	99.1	15,570
Mercury	846	133,100

• Solution steps: Apply Eq. (2.15) to the deepest point. Use SI units, pascals, not kilopascals:

[latex]p = p_a - \gamma z[/latex]

[latex]p_{max} = p_a - \gamma z = 91,000 Pa - (9790 \frac{N}{m^3})(-60 m) = 678,400 Pa \approx 678 kPa[/latex]

• Comments: Kilopascals are awkward. Use pascals in the formula, then convert the answer.

Question 2.1: Newfound Lake, a freshwater lake near Bristol, New Hampshire...

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