How Do You Calculate Energy Needed to Break Safety Glass?

[dvsumosize]

Homework Statement

This happened in real life, rock breaks through single pane safety glass and travels 9 meters, safety glass is 3 meters high. rock was 1 kg. I need to find the rough KE before the rock hit the glass. The MPa of the glass i am guessing is 50, google has yielded me little on the MPa of single pane safety glass.

So after calculations the rocks KE was 66.15 joules and after incorporating F=.65*A*v^2 (I guestimated A) as negative energy done, i got 72.47 J of KE right after glass broke. How do i find energy needed to break the 50 MPa singe pane mylar coated safety glass. I have no idea whatsoever on how to do this. I then can add this to the 72.47 J and get KE before impact. Answer is due in 12 hours, never have done anything like this. He wants us to learn from our community so we can become better at working with other people. He says our answers are going to vary greatly because its real world and too many variables we had to guestimate independently on. He just wants us to arrive at a logical answer.

Homework Equations

No clue, never attempted, learned, or saw anything like this.

The Attempt at a Solution

well i calculated time it took to fall, then velocity from that, then KE from frictionless environment which was about 66.15 J then i added the KE that the air friction which was 6.27J and i added them together so i have 72.47 J of energy right after glass breaks. If i can find the energy required to break the glass i can add that to my 72.47J and then i will have the KE just before impact, which is what i am looking for.

 

[dvsumosize]

i just found the tensile strength roughly of safety glass and it is 120 to 200 N/sq .mm. How do i find the energy required to break through that?

 

[dvsumosize]

bah i figured it out i said the rock had a radius of 3 inches converted that into mm^2 and multiplied by 120 to get N then * that by 6 mm to get work done over that interval and then added my previous result.