system
Given the following post, respond as if you're commenting on Reddit:
human
At what speed does glass break?

 If I were the throw a rock through a window, how fast would the glass break from the point of contact (with the rock) to the outer edge of the windowpane?

Also, what defines what path the fracture will take from the contactpoint to the outer edge of the window?
gpt
The speed at which waves travel through a material is the same as the speed of sound in that material.  The speed of sound in a material can be estimated by knowing that material's density and bulk modulus.  The formula is 


c=(K/p)^(1/2)


where c is speed of propagation, K is the modulus of elasticity, and p is the material's density.  One may also use a materials shear modulus in place of the modulus of elasticity.  The type of breaking that occurs likely dictates how fast the break will propagate and hence, which modulus should be used in the above formula.  

If one applies the above formula to glass, (taking the shear modulus of 26.2 GPa and a density of 2.5*10^3 kg/m^3) we get a speed of sound of ~3200 m/s.