PX155 - A5 - static and dynamic friction

contact forces: on macroscopic scale : a combination of normal reaction and frictional force

static friction

'free body diagram'
- vertically, there is no acceleration $∴ N = W$ by PX155 - A1 - newton's first law , and

F = F_{s}

- experimentally, static fiction can oppose $F$ up to some maximum value at which the point block accelerates
- Amonton's Law:

m a x (F_{s}) = μ_{s} N

where, $μ_{s} =$ coefficient of static friction
- so, $F_{s} \leq μ_{s} N$
$μ_{s} s t e e l / s t e e l = 0.3$
$μ_{s} t e f l o n / t e f l o n = 0.04$
$μ_{s} r u b b e r / r u b b e r = 1.2$

kinetic friction

once in motion, friction is still roughly proportional to the normal force but the coefficient changes:
- $F_{k} = μ_{k} N$
  - $μ_{k} =$ coefficient of kinetic/dynamic friction
$μ_{k}$ is usually less than $μ_{s}$ (because if it is more, the resistance to motion will increase and it might not move) but they're usually similar in size
- a force big enough to overcome $m a x (F_{s})$ will be greater than $μ_{k} N$ so, object that slips will keep slipping