A needle 35 mm long rests on a water surface at 20°C. What force over and above the needle's weight is required to lift the needle from contact with the water surface?

Question

Accepted Answer

From Table 1C,

σ=0.0728 N/m

σ=F/L

0.0728=F/(2×0.035)

F=0.00510 N

Table 1C,

MECHANICAL PROPERTIES OF WATER AT ATMOSPHERIC PRESSURE

Temperature °F	Density [latex]slug/ft^{3}[/latex]	Specific Wt. lb/ft[latex]^{3}[/latex]	Dynamic Viscosity lb-sec/ft[latex]^{2}[/latex]	Surface Tension* lb/ft	Vapor Pressure lb/in[latex]^{2}[/latex] abs.	Elastic Modulus lb/in[latex]^{2}[/latex]
32	1.94	62.4	3.75×10[latex]^{-5}[/latex]	0.00518	0.08	287000
40	1.94	62.4	3.24×10[latex]^{-5}[/latex]	0.00514	0.12	296000
50	1.94	62.4	2.74×10[latex]^{-5}[/latex]	0.00508	0.17	305000
60	1.94	62.4	2.36×10[latex]^{-5}[/latex]	0.00504	0.26	313000
70	1.94	62.3	2.04×10[latex]^{-5}[/latex]	0.00497	0.36	319000
80	1.93	62.2	1.80×10[latex]^{-5}[/latex]	0.00492	0.51	325000
90	1.93	62.1	1.59×10[latex]^{-5}[/latex]	0.00486	0.70	329000
100	1.93	62.0	1.42×10[latex]^{-5}[/latex]	0.00479	0.96	331000
120	1.92	61.7	1.17×10[latex]^{-5}[/latex]	0.00466	1.70	332000

Temperature °C	Density kg/m[latex]^{3}[/latex]	Specific Wt. kN/m[latex]^{3}[/latex]	Dynamic Viscosity N.s/m[latex]^{2}[/latex]	Surface Tension* N/m	Vapor Pressure kPa	Elastic Modulus Gpa
0	1000	9.81	1.75×10[latex]^{-3}[/latex]	0.0756	0.611	2.02
10	1000	9.81	1.30×10[latex]^{-3}[/latex]	0.0742	1.23	2.10
20	998	9.79	1.02×10[latex]^{-3}[/latex]	0.0728	2.34	2.18
30	996	9.77	8.00×10[latex]^{-4}[/latex]	0.0712	4.24	2.25
40	992	9.73	6.51×10[latex]^{-4}[/latex]	0.0696	7.38	2.28
50	988	9.69	5.41×10[latex]^{-4}[/latex]	0.0679	12.3	2.29
60	984	9.65	4.60×10[latex]^{-4}[/latex]	0.0662	19.9	2.28
70	978	9.59	4.02×10[latex]^{-4}[/latex]	0.0644	31.2	2.25
80	971	9.53	3.50×10[latex]^{-4}[/latex]	0.0626	47.4	2.20
90	965	9.47	3.11×10[latex]^{-4}[/latex]	0.0608	70.1	2.14
100	958	9.40	2.82×10[latex]^{-4}[/latex]	0.0589	101.3	2.07

* In contact with air.

Question 1.15: A needle 35 mm long rests on a water surface at 20°C. What f...

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