Answer
The average power produced by friction is 157 W.
Work Step by Step
We can find the force of friction.
$F_f = mg~mu_k$
$F_f = (20.0~kg)(9.80~m/s^2)(0.200)$
$F_f = 39.2~N$
The rock slowed down with constant acceleration from $v_0 = 8.00 m/s$ to $v = 0$. The average velocity was $v_{ave} = 4.00 m/s$
We can find the average power produced by friction.
$P_{ave} = F~ v_{ave} = (39.2~N)(4.00~m/s)$
$P_{ave} = 157~W$
The average power produced by friction is 157 W.
