[tex]E_{k} = \frac{mv^{2}}{2} \ \ /\cdot2\\\\mv^{2} = 2E_{k} \ \ /:v^{2}\\\\m = \frac{2E_{k}}{v^{2}}[/tex]
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[tex]E_{k} = \frac{mv^{2}}{2} \ \ /\cdot2\\\\mv^{2} = 2E_{k} \ \ /:m\\\\v^{2} = \frac{2E_{k}}{m}\\\\v = \sqrt{\frac{2E_{k}}{m}}[/tex]