In the three figure shown, find acceleration of block and force of friction on it in each case.

Explanation

Figure (a) `N = mg = 40N`
`mu N = 24N`
Since `F lt mu_(s)N`
Block will remain stationary and
`f = F = 20N`
Figure (b) `N = mg = 20 N`
`mu_(S) N = 12N`
and `mu_(K) N = 8 N`
Since, `F gt mu_(S) N`, block will side and kinetic friction `(= 8N)` will act.
`a = (F - f)/(m) = (20 - 8)/(2) = 6 m//s^(2)`