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## Homework Statement

A system has a period of 3 seconds. The 4 kg mass oscillates with an amplitude of 0.7 meters. At what position is 50% of its energy in the form of kinetic energy and what is its velocity then?## Homework Equations

work = Potential energy + Kinetic energy

(1/2)kA^2 = (1/2)kx^2 + (1/2)mv^2

## The Attempt at a Solution

I found k = 17.55 Newton meters.

So, I was thinking of two possible approaches:

a.) (1/2)kA^2 - (1/2)kx^2 = (1/2)mv^2

b.) kx^2 = mv^2

I'm pretty sure I'd use my second idea. If that's right, what would I plug in for x? The amplitude?