Answer
$(a)1785N$
$(b)339.3N$
Work Step by Step
(a) Here we use the equation of friction $F=\mu _{k}R$ where, F- friction force, $\mu_{k}$- Coefficient of kinetic friction, R- Normal force
Let's apply this equation to the disk.
$F=\mu _{k}R\space $; Plug known values into this equation.
$F=0.51\times 3.5\times 1000\space N= 1785\space N$
(b) Let's apply the equation $F=\mu _{k}R$ to the rim in rim-brake system
$F=\mu _{k}R\space$; Plug known values into this equation.
$f=0.39\times 870\space N=339.3\space N$
