Question on Springs and conservation of momentum

[ianj253]

I was working on a problem regarding a spring system and conservation of momentum which really stumped me. The problem is from TBR physics chapter 5 passage 7 question 45.

The question is as follows: "Considering the momentum and total energy of the two masses and spring, after the oscillations begin:"

The answer to the question is that momentum is not conserved but total energy is.

I understand that total energy is conserved, but I'm having difficulty understanding why momentum is not. There is no friction and as the spring is part of the system the restoring force is not an external one. So what's preventing momentum from being conserved in the defined system?

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[deleted439911]

momentum = m*v.
The mass is the same throughout the time but velocity isn't. The spring is bouncing back and forth for one and there is acceleration (which varies) over time. The restoring force is external to the masses on the spring

 

[ianj253]

Isn't the external force caused by the spring? If so it is defined as part of the system by the question.

I understand at points there is acceleration and changes in velocity, but at certain points the system has the same acceleration and velocity as it did initially, like after completing 1 cycle.