Need help with part b

A vacuum$-$cup gripper is to be designed to handle flat plate glass in an automobile windshield plant. Each plate weighs $28lb.$ A single suction cup will be used and the diameter of the suction cup is $6\mathrm{.}0in.$

weight $=1\mathrm{.}5**28=42lbs$

Area $=\frac{}{4}**6^2=9$

$($a$)$ Determine the negative pressure required $($compared to atmospheric pressure of $14\mathrm{.}7$ b$/$in$\mathrm{.}2)$ to lift each plate. Use safety factor of $1\mathrm{.}5$ in your calculations.

$p=\frac{42}{9}=1\mathrm{.}485$

$($b$)$ The pressure obtained in $($a$)$ is a negative pressure and should not exceed $14\mathrm{.}7.$ Why?