A cube of mass 10 kg with sides measuring 0.20 m in length, slides on a film of
ID: 1920243 • Letter: A
Question
A cube of mass 10 kg with sides measuring 0.20 m in length, slides on a film of SAE 20W oil at 25ï°C having a film thickness of 0.025 mm as shown in Fig. 4. (You may neglect edge effects). i) Calculate the tension in the rope if the velocity of the cube is constant at 0.5 m/s ii) Calculate the tension in the rope if the surfaces were dry (no oil) and the coefficient of sliding friction was 0.35. iii) Would the tension for part i) be different if the cube was accelerating? iv) List the information used in part i) that was not required for part ii)?
Explanation / Answer
For SAE 20w oil at 25 deg C, we have dynamic viscosity u = 0.147 Ns/m^2
a)
Shear stress = u*(dv/dy) = u*(v/h)
= 0.147*(0.5 / 0.025*10^-3)
= 2943 N/m^2
Shear force = Shear stres*area
= 2943*(0.2^2)
= 117.73 N
Tension = Shear force = 117.73 N
b)
Friction force = u*mg
= 0.35*10*9.81
= 34.335 N
c)
Yes, if it were accelerating, the tension would be different.
d)
none of the info used in part (i) was used in part(ii).
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