高中物理题几道.（急需!跪求啊...我真得要死惨了..!）

m = 4kg,v0 = 10m/s,μ = 0.2,
f=μmg
1.
减速度a = f / m = μg =0.2 * 10 = 2m/s^2
向右运动的最大距离：x = v0^2 / (2a) = 10^2 / (2*2 ) = 25m
2.
向右运动的距离不超过10m
减速度a ≥ v0^2 / (2x) = 10^2 / (2*10) = 5m/s^2
应当再施加一个与运动方向相反的力水平方向力F
F + f = ma
F = ma - f = m (a - μg) ≥ 4 * ( 5 -0.2 * 10 ) = 12N
3.
m = 4kg,μ = 0.2,
物体在两个连续的2s内通过的位移分别为8m和12m
设两个连续的2s之间时刻速度为v,根据x=v0t+1/2at^2：
8 = v * 2 - 1/2 * a * 2^2
12 = v * 2 + 1/2 * a * 2^2
两式相减得：12 - 8 = a * 4
a = 1m/s^2
ma = F - f
F = ma + f = m (a + μg) = 4 * ( 1 + 0.2 * 10 ) = 12N
4.
t=4,x=20
根据x=1/2at^2
a = 2x / t^2 = 2 * 20 / (4^2) = 2.5m/s^2
mgsin30° - μmgcos30° = ma
μ = tan30° - a/(gcos30°) = 根号3 / 3 - 2.5 / (10*根号3 / 2) = 根号3 / 6 ≈ 0.2887