Lab practical help!! Same as this morning but more information. The lab is calle

Question

Lab practical help!! Same as this morning but more information.

The lab is called "kiss the egg". I am pretty sure it will come up if you google it in case it does not make sense how I explain it.

I am given a random length of rubber band. The goal of the lab is to drop a bag of rocks attached to this band and have it barely touch (ie kiss) the egg that is sitting on the floor directly beneath it. We do not get the rocks until after we have figured out how to solve for the height to drop it from.

The first step is to solve for k. I did this using a 0.2kg mass on the end of the rubberband. I measured the rubber band with out the mass and then measured it with the mass. I used the euqation mg=kx to solve for k using m=0.2 g=9.8 and x= the difference between the two measured lengths.

My teacher told us to set height=0 at the floor. Given that height is zero at the floor, the mass spring system will have gravitational potnetial at the point that we release it from, kinetic energy and gravitational (i think) at the point where the band is stretched just by the mass, and only spring potential at the instant that it barely touches the egg.

If all I know is the k constant of the spring, how do we solve for the height (above the floor/egg) to drop the rocks from??? My teacher's hint was "you have to decide as a group whether you want to keep the length of the band or height constant, one or the other must be constant". My group is of no help and spent the class period playing with the mass set and rubber band. I am very confused and most likely overthinking this.

Please help me! If you could answer in detail that would be extra helpful as I will be the only person carrying out the experiment and only have one more shot to do it. Again: I will have a bag of rocks and the k constant of a band (but will not be able to pracitce with the band once i recieve the rocks). From this, I have to know how high to lift the bag of rocks after attaching it to the rubber band. I will then tie the rock bag to the rubber band, lift it the height I solved for, and let go. When I drop this bag it will have to fall down, barely hit the egg (ensuring it touches but does not smash or break the shell) and rebound back up.

Thank you so much!! (also big thanks to the person that answered my question earlier today, I am sure that you were on the right track but wanted to ask it with more information with the hopes that I would recieve an answer that showed me exactly how to carry out the experiment).

Explanation / Answer

To solve:  

say k is the spring constant and l is the unload length of the rubber band. Now u know the mass of the rock bag.

Now if x be the distance between the egg top to the beneth of the rubber band, then the rubber band have to stretch by a distane x to touch the egg.

Then the energy required to stretch the rubber band by distance x is (0.5*k*x^2). And this amount of energy will be supplied by the mass potential. Now if h be the heigth of the mass from the floor then its otential energy is = m*g*h

Now to touch the egg mgh=0.5*k*x^2

or h= (05.kx^2)/mg

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