Solving a Spring-Mass System Energy Problem

[Dooga Blackrazor]

I have 3 questions. I will give you what I think is the answer and you can either give me the answer or lead me in the right direction. I appreciate both equally. With direction I can find the answer, and with the answer I can find the direction, so I am cool either way.

When the compression of a spring is reduced to half it's original value, the potential energy stored in the spring is:

1/8 original value.
1/4 original value.
The same as above.

I think it is the same because, if I remmber right, E = 1/2mv^2 and isn't affected by compression.

The total energy of a spring undergoing SHM could be expressed as:
E = 1/2mv^2, correct?

This is the one I am clueless on:

A 2.5 kg object is attached to a spring of force constant k = 4.5 kN/m. The spring is streched 10 cm from the equilibrium and released. What is the kinetic energy of the mass-spring system when the mass is 5 cm from tits equilibrium position?

14 J
11 J
17 J

No clue. I think I am using the wrong formulas or factoring in A and Xmax incorrectly or something. Any help would be great on this one.

Thanks

 

[Päällikkö]

Potential energy of a spring: (1/2)kx².
E = K + U, where K is kinetic energy and U potential energy, so E = (1/2)mv² is incorrect.

Can you now solve the problem?