The position, velocity and acceleration of a particle executing simple harmonic motion are found to have magnitudes of \( 4 \, \text{m} \), \( 2 \, \text{m s}^{-1} \) and \( 16 \, \text{m s}^{-2} \) at a certain instant. The amplitude of the motion is \( \sqrt{x} \) \( \text{m} \), where \( x \) is ________.
