In a ballistics test, a bullet moving horizontally with a speed of $500$ m$/$s strikes a sandbag and penetrates a distance of $10\mathrm{.}0$ cm$.($a$)$ What is the magnitude of the average acceleration of the bullet in the sandbag? $1\mathrm{.}25\backslash$times $10\backslash$deg m$/$s$($b$)$ How many milliseconds does it take the bullet to come to rest in the sandbag? $0\mathrm{.}400$ m$/$s