How Is Hydraulic Lift Calculated in Cheerleader and Football Players Scenario?

[bcjochim07]

Homework Statement

A 55kg cheerleader uses an oil-filled hydraulic lift to hold four 110 kg football players at a height of 1 m. If her piston is 16 cm in diameter, what is the diameter of the football players' piston?

Homework Equations

The Attempt at a Solution

(55kg)(9.80)/(.02 m^2) = (440kg)(9.80)/A2
A2 = .16 m^2
.16 m^2 = pi * r^2
r=.2257 m
D= .45 m = 45 cm

Is this correct? I am a little bit confused as to the difference between these formulas:

p0 + F1/A1 = p0 + F2/A2 + ro*gh This is the one I used because she is just holding them in position and no work is being done.

for the formula DeltaF = ro*g (A1+A2)d2 You should use this one when the problem involves lifting something (doing work)

 

[bcjochim07]

any thoughts?