A block weighing `20N` rests on a horizontal surface . The coefficient of static friction between the block and surface is `0.4` and the coeffiecient of kinetic frictioni `0.20`
a.How much is the friction force excerted on the block?
b. How much will the friction force be if a horizontal force of `5 N` is exerted on the block?
c. What is the minimum force that will start the block in motion ?
d. What is the minimum force that will keep the block in motion once is has been started ?
e. If the horizontal force is `10 N` what is the friction force?

Explanation

a. When `F_(ext) = 0`,the block has no tendency to slip so
`F_("friction") = 0`

b. First, we calculate maximum friction force,
`F_("max") = mu_(s) N = (0.40 xx 20) = 8 N`
Since in this case `p lt F_("max") ` the block is in static equation i,e

` F = p = 5 N` c. `p_("min") = F _("max") = mu_(s) N = 8 N`
d. When the block is in motion
`F = mu_(k)N = 0.2 xx 20 = 4 N `
`P_("min") = F_("kinetic") = mu N = 4N`
e. Since in this case `Pgt mu_(s) N` the block acceleration and the friction force will be kinetic ` = 4N`