- #1
Camille
- 20
- 3
Homework Statement
The piston shown weighs 11 lbf. In its initial position, the piston is restrained from moving to the bottom of the cylinder by means of the metal stop. Assuming there is neither friction nor leakage between piston and cylinder, what volume of oil (S = 0.85) would have to be added to the 1-in. tube to cause the piston to rise 1 in. from its initial position?
Homework Equations
[itex] p = \frac{dF}{dA} [/itex]
And the basic differential equation of the fluid statics:
[itex] \frac{dp}{dz} = - γ [/itex]
Also the density or the specific weight of the oil is:
[itex] ρ = 1000 \frac{kg}{m^3} \cdot 0.85 = 850 \frac{kg}{m^3} [/itex]
[itex] γ = ρ \cdot g = 8.336 \frac{kN}{m^3} [/itex]
The Attempt at a Solution
Firstly, I have calculated what volume of oil is needed just to keep the piston "a little" above the stop, ie. to hold just the weight of the piston:
[itex] V_{oil p} = \frac{W_p}{A_p} \cdot \frac{A_o}{ρ \cdot g} [/itex] ,
where W_p is the weight of the piston,
A_p is the c-s area of the cylinder
A_o is the c-s area of the tube.
This turns out to be:
[itex] V_{oil p} = 22.39 in^3 [/itex].
Next, I have imagined, that when the piston is 1 in above the orignal level, we have to add even more oil to hold the weight of this 1-in thick layer of oil. This gave me in addition the volume of:
[itex] V_{oil o} = 3.14 in^3 [/itex].
So altogether the total amount of oil needed calculated by me was:
[itex] V_{oil} = 22.39 in^3 + 3.14 in^3 = 25.53 in^3 [/itex].
The answer should be [itex] V_{oil} = 35.7 in^3 [/itex].
I see the hole in my reasoning, because the volume of oil in the 1-in layer has to be taken from somewhere, so it's either the old oil that was in the cylinder or the old + the new oil added. However, I don't know how to solve it.
Any help will be appreciated!