How can energy conservation be applied to a spring connecting two masses?

[kikifast4u]

Homework Statement

Two discs, each of mass m are connected by a stiff spring. Can you press the top disc down so far that when released it will spring back and lift the bottom disc off the table? Discuss the application of energy conservation to this problem.

2. The attempt at a solution
Well, I'm really stuck at this one and I really don't know how to do it.

I thought about using energy. When you push the spring, you get potential energy, maximum (k*h^2)/2, where h is the distance between the two discs. When you release the top disc, some of the potential energy is converted into gravitational potential and some into kinetic. I am able to find the velocity of the first disc when it comes back to its original position, but that doesn't help.

Since it's a stiff string, it won't extend when the first mass will go beyond it's original point (if it does), isn't it?

The hint I am given is "How big must the tension in the spring be if the lower plate is about to ‘lift off’?" I know the tension must be bigger than the weight of the second disc.

So I should find what the tention is. How do I do that?

 

[Delphi51]

potential energy, maximum (k*h^2)/2, where h is the distance between the two discs

Actually h is the distance the spring is compressed, greater than the distance between the masses because the spring can't be compressed to zero length.

Since it's a stiff string, it won't extend when the first mass will go beyond it's original point (if it does), isn't it?

It will extend beyond its original length. "Stiff" just means it has a large value of the spring constant, k. Say the extension is y. Then I think the tension is mg = ky when the lower mass just begins to lift off.

You could give an account of the forces and energies for each phase of the motion, telling what energy conversions take place as the upper mass is pushed down, then as it comes back up to its original position, then as it goes beyond the original position.

 

[kikifast4u]

http://img5.imagebanana.com/img/1lujnz38/thumb/pb.png
That's what I mean by h. If you are able to push top disc so much, then potential energy in spring is (kh^2)/2.

Since k is very big, I think we can assume it will not extend when mass from below will start to lift and will actually act as a cable (otherwise they wouldn't mention tension, it would we elastic force and not tension). But I still don't know how to give an answer. What must happen so that the mass from below is lifted?