$(1)($a$)$ What is the angular momentum of a $2\mathrm{.}8-kg$ uniform cylindrical grinding wheel of radius $28$ cm when rotating at $1300$ rpm $?($b$)$ How much torque is required to stop it in $6\mathrm{.}0s?$

$(1)$ What is the angular momentum of a $0\mathrm{.}270-kg$ ball revolving on the end of a thin string in a circle of radius $1\mathrm{.}35$ m at an angular speed of $10\mathrm{.}4ra\frac{d}{s}?$