Question:
A body slipping on a rough horizontal plane moves with a deceleration of $4.0 \mathrm{~m} / \mathrm{s}^{2}$. What is the coefficient of kinetic friction between the block and the plane?
Solution:
Vertical equilibrium
$\mathrm{N}=\mathrm{mg}$
For horizontal; ${ }^{\text {Net }}={ }_{\text {ma }}$
$0-f f=m a$
$-\mu N=m a$
$-\mu m g=m a$
$-\mu g=\alpha$
$-\mu g=(-4)$
$\mu=0.4 \mathrm{~m} / \mathrm{s}^{2}$