Problem 2.48: Given: Find: Solution
Problem 2.48: Given: Find: Solution
Problem 2.48: Given: Find: Solution
48 [Difficulty: 2]
Solution:
Given data: Dpiston = 73⋅ mm Dtube = 75⋅ mm Mass = 2 ⋅ kg L = 100 ⋅ mm SG Al = 2.64
kg
Reference data: ρwater = 1000⋅ (maximum density of water)
3
m
N⋅ s
From Fig. A.2:, the dynamic viscosity of SAE 10W-30 oil at 25oC is: μ = 0.13⋅
2
m
The terminal velocity of the mass m is equivalent to the terminal velocity of the piston. At that terminal speed, the acceleration of
the piston is zero. Therefore, all forces acting on the piston must be balanced. This means that the force driving the motion
(i.e. the weight of mass m and the piston) balances the viscous forces acting on the surface of the piston. Thus, at r = Rpiston:
⎡⎢ ⎛⎜ π⋅ D 2 ⎞⎤
piston ⋅ L ⎥ ⎛ μ⋅ d V ⎞ ⋅ π⋅ D
⎢Mass + SGAl⋅ ρwater⋅ ⎜ ⎥ ⋅ g = τrz⋅ A = ⎜ z ( piston⋅ L)
⎣ ⎝ 4 ⎠⎦ ⎝ dr ⎠
d V
Therefore Vz =
dr ⎛ Dtube − Dpiston ⎞
⎜
⎝ 2 ⎠
m
or V = 10.2
s