Why does acetylation of $-NH_2$ group of aniline reduce its activating effect?

(N/A) The activating effect of the $-NH_2$ group is due to the lone pair of electrons on the nitrogen atom,which is available for donation to the benzene ring through resonance,thereby increasing the electron density at the ortho and para positions.
When aniline is acetylated to form acetanilide $(C_6H_5NHCOCH_3)$,the lone pair of electrons on the nitrogen atom participates in resonance with the carbonyl group $(C=O)$. This delocalization of the lone pair towards the oxygen atom reduces its availability for donation to the benzene ring.
Consequently,the electron-donating power of the nitrogen atom towards the benzene ring is decreased,which reduces the activating effect of the group. This is represented by the following resonance structures:
$[:N-C(=O)-CH_3 \longleftrightarrow N^+=C-O^- -CH_3]$