Vibrations & Waves: Solving Mass, Spring Constant & Energy

[Elvis]

Vibrations and waves
1) A 5 kg mass, m, rests on a frictionless, horizontal, wooden table top, and is attached to one end of a spring anchored at its other end . If the spring has a spring constant=13 N/cm and the mass is pulled back (in the positive direction ) 20 cm and released, to the nearest tenth of a joule, what is its total energy?
2) if the mass is 15 kg and the spring constant is 20 N/cm, to the nearest tenth of a m/s, what is its speed at x= 5 cm?

I have solved the 1st one :

1N/cm=100N/m

E = PE + KE
E = 1/2kA^2 + 1/2mv^2 At KE v=0 because the mass stops momentarily as it changes directions. Then :

E = PE
E = 1/2kA^2
E = 1/2(13*100)*(0.2)^2
E = 26J

I have problem with the second problem. please help to solve this. I have 2 hours that I am tryin to solve it . I have only the answer that is 2.2m/s.

Please help......



Lol...
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[Doc Al]

Find the total energy, just like you did for part one. Then use:

E = PE + KE
E = 1/2kA^2 + 1/2mv^2

Solve for v, when x = 5 cm.

That equation should be: E = 1/2kx^2 + 1/2mv^2. Use x, not A (which usually stands for amplitude).