The train, approaching the station at a speed of 72 km / h, begins uniform braking

The train, approaching the station at a speed of 72 km / h, begins uniform braking. Determine the braking time until the train stops completely, safe for passengers (passengers do not fall off the shelf), if the coefficient of friction of the passenger on the shelf is 0.2.

Given:
V0 = 72 km / h = 72 * 1000 m / 3600 s = 20 m / s – initial train speed
V = 0 m / s
m = 0.2 – coefficient of friction
t -?
Decision:
uniformly slow motion: V = V0 – a * t, where a is the acceleration of the train
Only the friction force acts on the train: F = Ftr
m * a = m * N, N – support reaction force N = m * g
m * a = m * g * m, g = 9.8 m / s ^ 2 – acceleration of gravity
we reduce masses a = m * g
a = 0.2 * 9.8 = 2 m / s ^ 2
Let’s express the time:
V0 – V = a * t
t = (V0 – V) / a
t = (20 – 0) / 2 = 20/2 = 10 s