An auto lift consists of a 55.56 cm diameter ram which slides concentrically in a 55.58 cm diameter
cylinder (see Figure 1). The cylinder engages a 2.8 m length of the ram at all times. The annular region
between the ram and cylinder is filled with viscous oil. The combined mass of the automobile, auto rack
and ram is 1750 kg. If the lift drops steadily at the speed of 0.25 m/s after being released, determine:
a) the viscosity of the oil, in Pa.s,
b) the shear stress at the surface of the inner cylinder.
The Attempt at a Solution
I can find the force which is the weight og ram+ram, also dy and dv. but i don't know how to find A. my friends told me that A = 2pi*2.8 put i don't think it was right because 2.8m is the length of the whole thing not diameter.
Please help me. I need to give my class a presentation about this and it counts as a test.