$(1)$ Find the time reguired to drill a hole of $3\mathrm{.}5$ mm thick glass sheet having fracture strength $($fracture harchess of Job matcrial$)\backslash (140\backslash$mathrm$\{$~kg$\}/\backslash$mathrm$\{$mm$\}^\{2\}\backslash )$ in USM operation working at $20$ kHz frequency having stress $\backslash (25\backslash$mathrm$\{$~kg$\}/\backslash$mathrm$\{$mm$\}^\{2\}\backslash )$ using $1$ part of abrassive $($sic$)$ on $1$ part of water. Amplitude of ribration $16$ micion. Assume amplitude of vibration is half of grid size of abrasive grain.