473,

a, A felső szakaszon:

`cancel(m)*cancel(g)*sinalpha` = `mu*cancel(m)*cancel(g)*cosalpha`

`mu=sinalpha/cosalpha` = `tan alpha` = tan 20° = 0,364

b, Az alsó szakaszon:

v = `a*t` `rightarrow` t = `v/a`

s = `a/2*t^2` = `a/2*(v/a)^2` = `v^2/(2*a)` `rightarrow` a = `v^2/(2*s)` = `2^2/(2*3)` = `2/3` `m/s^2` (itt tulajdonképpen lassulásról van szó, ezért a = `-2/3` `m/s^2`

`cancel(m)*g*sinalpha-mu*cancel(m)*g*cosalpha` = `cancel(m)*a`

`mu` = `(g*sinalpha-a)/(g*cosalpha)` = `(10*sin20-(-2/3))/(10*cos20)` = 0,435

474,

m = 200 kg `rightarrow` `F_(ny)` = `m*g` = 2000 N

`E_m` = 400 J

`E_m` = `1/2*m*v^2` `rightarrow` `v=root()((2*E_m)/m)` = `root()((2*400)/200)` = `root()(4)` = 2 `m/s`

P = `F*v` `rightarrow` F = `P/v` = `1200/2` = 600 N

`F = F_(ny)*mu` `rightarrow` `mu=F/F_(ny)` = `600/2000` = 0,3