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A spring (k = 600 N/m) is placed in a vertical position with its lower end supported by a horizontal surface. The upper end is depressed 20 cm, and a 4.0 kg block is placed on the depressed spring. The system is then released from rest. How far above the point of release will the block rise?

State your answer in centimetres to 2 significant figures.

I've gotten the answer of 27cm by two different methods but my online tutorial still says it's wrong. Firstly, I used 1/2*k*x^2 where x=0.2 to get the total mechanical energy for the system (12J). At the highest point, the velocity of the block will be 0, therefore its kinetic energy will be 0. Therefore, the energy in the spring + the gravitational potential energy will = 12J. Using this i got x=7cm which means the block rose 27cm.

I then used the equation that Fnet=Fspring-Fweight

ma=-600x-mg

dv/dt=-150x-9.8

v=-75x^2-9.8x+C

subbed in @ x=-0.2, v=0

:. v=-75x^2-9.8x+1.04

Using this i got v=0 at -20cm and 7cm, which gives the same answer as above. What am I doing wrong?

Thanks