A spring-block system is on a horizontal, frictionless surface. The spring constant is 300 N/m. The mass of the block is

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A spring-block system is on a horizontal, frictionless surface.
The spring constant is 300 N/m. The mass of the block is unknown
whereas the spring’s mass is very small. The block is connected
permanently to the spring. The block is stretched out to 20.0 cm
from its equilibrium position and is released at time t=0.00s.
Determine the kinetic energy (J) of the block-spring system when
the velocity of the block reaches 1/3 its maximum value. Give
answer in J and to two places to the right of the decimal.