Swinging, equilibrium, kinetic energy problem

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1. Feb 22, 2016

slymme

1. The problem statement, all variables and given/known data
This is a more than one part question that depends on answers from the previous parts, which I have so I will not rewrite those questions. I will write the question I am facing difficulty with:
Length = 1.95 m (From top of chains to center, its a swing)
Time Period = 2.8 s
15 complete oscillations of the swing took 42 s

To set her swinging, the girl and seat were displaced from equilibrium and released from rest. This initial displacement of the girl raised the center of mass of the girl and seat 250 mm above its lowest position. If the mass of the girl was 18 kg, what was her kinetic energy as she first passed through this lowest point?

2. Relevant equations
T = t/n
f = 1/T
Kinetic Energy (KE) = 0.5mw^2(A^2 - x^2)
where A is amplitude and x is displacement

3. The attempt at a solution
I simply subbed in the numbers in to the KE equation. I think my problem has to do with the amplitude or displacement, I might have mixed them up. But I tried swapping the numbers around but to no avail. Also, I may have quite probably interpreted the question wrong and plugged in the wrong numbers in to the A and x.I put L in as the A amd 250 mm (after conversion to meter) as the x. And yes, I changed w to 2pif.
Would greatly appreciate some help :)

2. Feb 22, 2016

Staff: Mentor

The 250 mm is a vertical displacement of the center of mass, not a horizontal displacement.

Hint: Try a conservation law approach.

3. Feb 23, 2016

slymme

I've solved the question already. You were right though, I used mgh. Thanks :)