Here, `m=500g=0.5kg, u= 20m//s, v=0, s=0.1km= 100m, F=?`
From `v^(2)-u^(2)= 2as`
`0-20^(2)= 2a(100)`
`a=(-400)/(200)= -2m//s^(2)`
From `F=ma=(0.5(-2)= -N`
Negative sign shows that force of friction is in a direction opposite to the direction of motion.