A force of 8.0 N gives a 1.0 kg block an acceleration of 4.0 m/s^{2} to the right.

The force of friction acting on the block is N.

According to Newton’s second law, F_{net} = ma.

F_{net} = 1.0 kg × 4.0 m/s^{2} = 4.0 N

f = 8.0 N - 4.0 N = 4.0 N